Debian Reference Osamu Aoki Copyright © 2013-2024 Osamu Aoki This Debian Reference (version 2.122) (2024-04-10 23:08:27 UTC)     is intended to provide a broad overview of the Debian system as a post-installation user's guide. It covers many aspects of system administration through shell-command examples for non-developers. Abstract This book is free; you may redistribute it and/or modify it under the terms of the GNU General Public License of any version compliant to the Debian Free Software Guidelines (DFSG). --------------------------------------------------------------------- Table of Contents Preface 1. Disclaimer 2. What is Debian 3. About this document 3.1. Guiding rules 3.2. Prerequisites 3.3. Conventions 3.4. The popcon 3.5. The package size 3.6. Bug reports on this document 4. Reminders for new users 5. Some quotes for new users 1. GNU/Linux tutorials 1.1. Console basics 1.1.1. The shell prompt 1.1.2. The shell prompt under GUI 1.1.3. The root account 1.1.4. The root shell prompt 1.1.5. GUI system administration tools 1.1.6. Virtual consoles 1.1.7. How to leave the command prompt 1.1.8. How to shutdown the system 1.1.9. Recovering a sane console 1.1.10. Additional package suggestions for the newbie 1.1.11. An extra user account 1.1.12. sudo configuration 1.1.13. Play time 1.2. Unix-like filesystem 1.2.1. Unix file basics 1.2.2. Filesystem internals 1.2.3. Filesystem permissions 1.2.4. Control of permissions for newly created files: umask 1.2.5. Permissions for groups of users (group) 1.2.6. Timestamps 1.2.7. Links 1.2.8. Named pipes (FIFOs) 1.2.9. Sockets 1.2.10. Device files 1.2.11. Special device files 1.2.12. procfs and sysfs 1.2.13. tmpfs 1.3. Midnight Commander (MC) 1.3.1. Customization of MC 1.3.2. Starting MC 1.3.3. File manager in MC 1.3.4. Command-line tricks in MC 1.3.5. The internal editor in MC 1.3.6. The internal viewer in MC 1.3.7. Auto-start features of MC 1.3.8. Virtual filesystem of MC 1.4. The basic Unix-like work environment 1.4.1. The login shell 1.4.2. Customizing bash 1.4.3. Special key strokes 1.4.4. Mouse operations 1.4.5. The pager 1.4.6. The text editor 1.4.7. Setting a default text editor 1.4.8. Using vim 1.4.9. Recording the shell activities 1.4.10. Basic Unix commands 1.5. The simple shell command 1.5.1. Command execution and environment variable 1.5.2. The "$LANG" variable 1.5.3. The "$PATH" variable 1.5.4. The "$HOME" variable 1.5.5. Command line options 1.5.6. Shell glob 1.5.7. Return value of the command 1.5.8. Typical command sequences and shell redirection 1.5.9. Command alias 1.6. Unix-like text processing 1.6.1. Unix text tools 1.6.2. Regular expressions 1.6.3. Replacement expressions 1.6.4. Global substitution with regular expressions 1.6.5. Extracting data from text file table 1.6.6. Script snippets for piping commands 2. Debian package management 2.1. Debian package management prerequisites 2.1.1. Debian package management system 2.1.2. Package configuration 2.1.3. Basic precautions 2.1.4. Life with eternal upgrades 2.1.5. Debian archive basics 2.1.6. Debian is 100% free software 2.1.7. Package dependencies 2.1.8. The event flow of the package management 2.1.9. First response to package management troubles 2.1.10. How to pick Debian packages 2.1.11. How to cope with conflicting requirements 2.2. Basic package management operations 2.2.1. apt vs. apt-get / apt-cache vs. aptitude 2.2.2. Basic package management operations with the commandline 2.2.3. Interactive use of aptitude 2.2.4. Key bindings of aptitude 2.2.5. Package views under aptitude 2.2.6. Search method options with aptitude 2.2.7. The aptitude regex formula 2.2.8. Dependency resolution of aptitude 2.2.9. Package activity logs 2.3. Examples of aptitude operations 2.3.1. Seeking interesting packages 2.3.2. Listing packages with regex matching on package names 2.3.3. Browsing with the regex matching 2.3.4. Purging removed packages for good 2.3.5. Tidying auto/manual install status 2.3.6. System wide upgrade 2.4. Advanced package management operations 2.4.1. Advanced package management operations with commandline 2.4.2. Verification of installed package files 2.4.3. Safeguarding for package problems 2.4.4. Searching on the package meta data 2.5. Debian package management internals 2.5.1. Archive meta data 2.5.2. Top level "Release" file and authenticity 2.5.3. Archive level "Release" files 2.5.4. Fetching of the meta data for the package 2.5.5. The package state for APT 2.5.6. The package state for aptitude 2.5.7. Local copies of the fetched packages 2.5.8. Debian package file names 2.5.9. The dpkg command 2.5.10. The update-alternatives command 2.5.11. The dpkg-statoverride command 2.5.12. The dpkg-divert command 2.6. Recovery from a broken system 2.6.1. Failed installation due to missing dependencies 2.6.2. Caching errors of the package data 2.6.3. Incompatibility with old user configuration 2.6.4. Different packages with overlapped files 2.6.5. Fixing broken package script 2.6.6. Rescue with the dpkg command 2.6.7. Recovering package selection data 2.7. Tips for the package management 2.7.1. Who uploaded the package? 2.7.2. Limiting download bandwidth for APT 2.7.3. Automatic download and upgrade of packages 2.7.4. Updates and Backports 2.7.5. External package archives 2.7.6. Packages from mixed source of archives without apt-pinning 2.7.7. Tweaking candidate version with apt-pinning 2.7.8. Blocking packages installed by "Recommends" 2.7.9. Tracking testing with some packages from unstable 2.7.10. Tracking unstable with some packages from experimental 2.7.11. Emergency downgrading 2.7.12. The equivs package 2.7.13. Porting a package to the stable system 2.7.14. Proxy server for APT 2.7.15. More readings for the package management 3. The system initialization 3.1. An overview of the boot strap process 3.1.1. Stage 1: the UEFI 3.1.2. Stage 2: the boot loader 3.1.3. Stage 3: the mini-Debian system 3.1.4. Stage 4: the normal Debian system 3.2. Systemd 3.2.1. Systemd init 3.2.2. Systemd login 3.3. The kernel message 3.4. The system message 3.5. System management 3.6. Other system monitors 3.7. System configuration 3.7.1. The hostname 3.7.2. The filesystem 3.7.3. Network interface initialization 3.7.4. Cloud system initialization 3.7.5. Customization example to tweak sshd service 3.8. The udev system 3.9. The kernel module initialization 4. Authentication and access controls 4.1. Normal Unix authentication 4.2. Managing account and password information 4.3. Good password 4.4. Creating encrypted password 4.5. PAM and NSS 4.5.1. Configuration files accessed by PAM and NSS 4.5.2. The modern centralized system management 4.5.3. "Why GNU su does not support the wheel group" 4.5.4. Stricter password rule 4.6. Security of authentication 4.6.1. Secure password on the Internet 4.6.2. Secure Shell 4.6.3. Extra security measures for the Internet 4.6.4. Securing the root password 4.7. Other access controls 4.7.1. Access control lists (ACLs) 4.7.2. sudo 4.7.3. PolicyKit 4.7.4. Restricting access to some server services 4.7.5. Linux security features 5. Network setup 5.1. The basic network infrastructure 5.1.1. The hostname resolution 5.1.2. The network interface name 5.1.3. The network address range for the LAN 5.1.4. The network device support 5.2. The modern network configuration for desktop 5.2.1. GUI network configuration tools 5.3. The modern network configuration without GUI 5.4. The modern network configuration for cloud 5.4.1. The modern network configuration for cloud with DHCP 5.4.2. The modern network configuration for cloud with static IP 5.4.3. The modern network configuration for cloud with Network Manger 5.5. The low level network configuration 5.5.1. Iproute2 commands 5.5.2. Safe low level network operations 5.6. Network optimization 5.6.1. Finding optimal MTU 5.6.2. WAN TCP optimization 5.7. Netfilter infrastructure 6. Network applications 6.1. Web browsers 6.1.1. Spoofing the User-Agent string 6.1.2. Browser extension 6.2. The mail system 6.2.1. Email basics 6.2.2. Modern mail service limitation 6.2.3. Historic mail service expectation 6.2.4. Mail transport agent (MTA) 6.3. The remote access server and utilities (SSH) 6.3.1. Basics of SSH 6.3.2. User name on the remote host 6.3.3. Connecting without remote passwords 6.3.4. Dealing with alien SSH clients 6.3.5. Setting up ssh-agent 6.3.6. Sending a mail from a remote host 6.3.7. Port forwarding for SMTP/POP3 tunneling 6.3.8. How to shutdown the remote system on SSH 6.3.9. Troubleshooting SSH 6.4. The print server and utilities 6.5. Other network application servers 6.6. Other network application clients 6.7. The diagnosis of the system daemons 7. GUI System 7.1. GUI desktop environment 7.2. GUI communication protocol 7.3. GUI infrastructure 7.4. GUI applications 7.5. User directories 7.6. Fonts 7.6.1. Basic fonts 7.6.2. Font rasterization 7.7. Sandbox 7.8. Remote desktop 7.9. X server connection 7.9.1. X server local connection 7.9.2. X server remote connection 7.9.3. X server chroot connection 7.10. Clipboard 8. I18N and L10N 8.1. The locale 8.1.1. Rationale for UTF-8 locale 8.1.2. The reconfiguration of the locale 8.1.3. Filename encoding 8.1.4. Localized messages and translated documentation 8.1.5. Effects of the locale 8.2. The keyboard input 8.2.1. The keyboard input for Linux console and X Window 8.2.2. The keyboard input for Wayland 8.2.3. The input method support with IBus 8.2.4. An example for Japanese 8.3. The display output 8.4. East Asian Ambiguous Character Width Characters 9. System tips 9.1. The console tips 9.1.1. Recording the shell activities cleanly 9.1.2. The screen program 9.1.3. Navigating around directories 9.1.4. Readline wrapper 9.1.5. Scanning the source code tree 9.2. Customizing vim 9.2.1. Customizing vim with internal features 9.2.2. Customizing vim with external packages 9.3. Data recording and presentation 9.3.1. The log daemon 9.3.2. Log analyzer 9.3.3. Customized display of text data 9.3.4. Customized display of time and date 9.3.5. Colorized shell echo 9.3.6. Colorized commands 9.3.7. Recording the editor activities for complex repeats 9.3.8. Recording the graphics image of an X application 9.3.9. Recording changes in configuration files 9.4. Monitoring, controlling, and starting program activities 9.4.1. Timing a process 9.4.2. The scheduling priority 9.4.3. The ps command 9.4.4. The top command 9.4.5. Listing files opened by a process 9.4.6. Tracing program activities 9.4.7. Identification of processes using files or sockets 9.4.8. Repeating a command with a constant interval 9.4.9. Repeating a command looping over files 9.4.10. Starting a program from GUI 9.4.11. Customizing program to be started 9.4.12. Killing a process 9.4.13. Scheduling tasks once 9.4.14. Scheduling tasks regularly 9.4.15. Scheduling tasks on event 9.4.16. Alt-SysRq key 9.5. System maintenance tips 9.5.1. Who is on the system? 9.5.2. Warning everyone 9.5.3. Hardware identification 9.5.4. Hardware configuration 9.5.5. System and hardware time 9.5.6. The terminal configuration 9.5.7. The sound infrastructure 9.5.8. Disabling the screen saver 9.5.9. Disabling beep sounds 9.5.10. Memory usage 9.5.11. System security and integrity check 9.6. Data storage tips 9.6.1. Disk space usage 9.6.2. Disk partition configuration 9.6.3. Accessing partition using UUID 9.6.4. LVM2 9.6.5. Filesystem configuration 9.6.6. Filesystem creation and integrity check 9.6.7. Optimization of filesystem by mount options 9.6.8. Optimization of filesystem via superblock 9.6.9. Optimization of hard disk 9.6.10. Optimization of solid state drive 9.6.11. Using SMART to predict hard disk failure 9.6.12. Specify temporary storage directory via $TMPDIR 9.6.13. Expansion of usable storage space via LVM 9.6.14. Expansion of usable storage space by mounting another partition 9.6.15. Expansion of usable storage space by bind-mounting another directory 9.6.16. Expansion of usable storage space by overlay-mounting another directory 9.6.17. Expansion of usable storage space using symlink 9.7. The disk image 9.7.1. Making the disk image file 9.7.2. Writing directly to the disk 9.7.3. Mounting the disk image file 9.7.4. Cleaning a disk image file 9.7.5. Making the empty disk image file 9.7.6. Making the ISO9660 image file 9.7.7. Writing directly to the CD/DVD-R/RW 9.7.8. Mounting the ISO9660 image file 9.8. The binary data 9.8.1. Viewing and editing binary data 9.8.2. Manipulating files without mounting disk 9.8.3. Data redundancy 9.8.4. Data file recovery and forensic analysis 9.8.5. Splitting a large file into small files 9.8.6. Clearing file contents 9.8.7. Dummy files 9.8.8. Erasing an entire hard disk 9.8.9. Erasing unused area of an hard disk 9.8.10. Undeleting deleted but still open files 9.8.11. Searching all hardlinks 9.8.12. Invisible disk space consumption 9.9. Data encryption tips 9.9.1. Removable disk encryption with dm-crypt/LUKS 9.9.2. Mounting encrypted disk with dm-crypt/LUKS 9.10. The kernel 9.10.1. Kernel parameters 9.10.2. Kernel headers 9.10.3. Compiling the kernel and related modules 9.10.4. Compiling the kernel source: Debian Kernel Team recommendation 9.10.5. Hardware drivers and firmware 9.11. Virtualized system 9.11.1. Virtualization and emulation tools 9.11.2. Virtualization work flow 9.11.3. Mounting the virtual disk image file 9.11.4. Chroot system 9.11.5. Multiple desktop systems 10. Data management 10.1. Sharing, copying, and archiving 10.1.1. Archive and compression tools 10.1.2. Copy and synchronization tools 10.1.3. Idioms for the archive 10.1.4. Idioms for the copy 10.1.5. Idioms for the selection of files 10.1.6. Archive media 10.1.7. Removable storage device 10.1.8. Filesystem choice for sharing data 10.1.9. Sharing data via network 10.2. Backup and recovery 10.2.1. Backup and recovery policy 10.2.2. Backup utility suites 10.2.3. Backup tips 10.3. Data security infrastructure 10.3.1. Key management for GnuPG 10.3.2. Using GnuPG on files 10.3.3. Using GnuPG with Mutt 10.3.4. Using GnuPG with Vim 10.3.5. The MD5 sum 10.3.6. Password keyring 10.4. Source code merge tools 10.4.1. Extracting differences for source files 10.4.2. Merging updates for source files 10.4.3. Interactive merge 10.5. Git 10.5.1. Configuration of Git client 10.5.2. Basic Git commands 10.5.3. Git tips 10.5.4. Git references 10.5.5. Other version control systems 11. Data conversion 11.1. Text data conversion tools 11.1.1. Converting a text file with iconv 11.1.2. Checking file to be UTF-8 with iconv 11.1.3. Converting file names with iconv 11.1.4. EOL conversion 11.1.5. TAB conversion 11.1.6. Editors with auto-conversion 11.1.7. Plain text extraction 11.1.8. Highlighting and formatting plain text data 11.2. XML data 11.2.1. Basic hints for XML 11.2.2. XML processing 11.2.3. The XML data extraction 11.2.4. The XML data lint 11.3. Type setting 11.3.1. roff typesetting 11.3.2. TeX/LaTeX 11.3.3. Pretty print a manual page 11.3.4. Creating a manual page 11.4. Printable data 11.4.1. Ghostscript 11.4.2. Merge two PS or PDF files 11.4.3. Printable data utilities 11.4.4. Printing with CUPS 11.5. The mail data conversion 11.5.1. Mail data basics 11.6. Graphic data tools 11.6.1. Graphic data tools (metapackage) 11.6.2. Graphic data tools (GUI) 11.6.3. Graphic data tools (CLI) 11.7. Miscellaneous data conversion 12. Programming 12.1. The shell script 12.1.1. POSIX shell compatibility 12.1.2. Shell parameters 12.1.3. Shell conditionals 12.1.4. Shell loops 12.1.5. Shell environment variables 12.1.6. The shell command-line processing sequence 12.1.7. Utility programs for shell script 12.2. Scripting in interpreted languages 12.2.1. Debugging interpreted language codes 12.2.2. GUI program with the shell script 12.2.3. Custom actions for GUI filer 12.2.4. Perl short script madness 12.3. Coding in compiled languages 12.3.1. C 12.3.2. Simple C program (gcc) 12.3.3. Flex — a better Lex 12.3.4. Bison — a better Yacc 12.4. Static code analysis tools 12.5. Debug 12.5.1. Basic gdb execution 12.5.2. Debugging the Debian package 12.5.3. Obtaining backtrace 12.5.4. Advanced gdb commands 12.5.5. Check dependency on libraries 12.5.6. Dynamic call tracing tools 12.5.7. Debugging X Errors 12.5.8. Memory leak detection tools 12.5.9. Disassemble binary 12.6. Build tools 12.6.1. Make 12.6.2. Autotools 12.6.3. Meson 12.7. Web 12.8. The source code translation 12.9. Making Debian package A. Appendix A.1. The Debian maze A.2. Copyright history A.3. Document format List of Tables 1.1. List of interesting text-mode program packages 1.2. List of informative documentation packages 1.3. List of usage of key directories 1.4. List of the first character of "ls -l" output 1.5. The numeric mode for file permissions in chmod(1) commands 1.6. The umask value examples 1.7. List of notable system-provided groups for file access 1.8. List of notable system provided groups for particular command executions 1.9. List of types of timestamps 1.10. List of special device files 1.11. The key bindings of MC 1.12. The reaction to the enter key in MC 1.13. List of shell programs 1.14. List of key bindings for bash 1.15. List of mouse operations and related key actions on Debian 1.16. List of basic Vim key strokes 1.17. List of basic Unix commands 1.18. The 3 parts of locale value 1.19. List of locale recommendations 1.20. List of "$HOME" values 1.21. Shell glob patterns 1.22. Command exit codes 1.23. Shell command idioms 1.24. Predefined file descriptors 1.25. Metacharacters for BRE and ERE 1.26. The replacement expression 1.27. List of script snippets for piping commands 2.1. List of Debian package management tools 2.2. List of Debian archive sites 2.3. List of Debian archive area 2.4. The relationship between suite and codename 2.5. List of key web site to resolving problems with a specific package 2.6. Basic package management operations with the commandline using apt(8), aptitude(8) and apt-get(8) /apt-cache(8) 2.7. Notable command options for aptitude(8) 2.8. List of key bindings for aptitude 2.9. List of views for aptitude 2.10. The categorization of standard package views 2.11. List of the aptitude regex formula 2.12. The log files for package activities 2.13. List of advanced package management operations 2.14. The content of the Debian archive meta data 2.15. The name structure of Debian packages 2.16. The usable characters for each component in the Debian package names 2.17. The notable files created by dpkg 2.18. List of notable Pin-Priority values for apt-pinning technique. 2.19. List of the proxy tools specially for Debian archive 3.1. List of boot loaders 3.2. The meaning of the menu entry of the above part of /boot/grub/ grub.cfg 3.3. List of boot utilities for the Debian system 3.4. List of kernel error levels 3.5. List of typical journalctl command snippets 3.6. List of typical systemctl command snippets 3.7. List of other monitoring command snippets under systemd 4.1. 3 important configuration files for pam_unix(8) 4.2. The second entry content of "/etc/passwd" 4.3. List of commands to manage account information 4.4. List of tools to generate password 4.5. List of notable PAM and NSS systems 4.6. List of configuration files accessed by PAM and NSS 4.7. List of insecure and secure services and ports 4.8. List of tools to provide extra security measures 5.1. List of network configuration tools 5.2. List of network address ranges 5.3. Translation table from obsolete net-tools commands to new iproute2 commands 5.4. List of low level network commands 5.5. List of network optimization tools 5.6. Basic guide lines of the optimal MTU value 5.7. List of firewall tools 6.1. List of web browsers 6.2. List of mail user agent (MUA) 6.3. List of basic mail transport agent related packages 6.4. List of important postfix manual pages 6.5. List of mail address related configuration files 6.6. List of basic MTA operation 6.7. List of remote access server and utilities 6.8. List of SSH configuration files 6.9. List of SSH client startup examples 6.10. List of free SSH clients for other platforms 6.11. List of print servers and utilities 6.12. List of other network application servers 6.13. List of network application clients 6.14. List of popular RFCs 7.1. List of desktop environment 7.2. List of notable GUI infrastructure packages 7.3. List of notable GUI applications 7.4. List of notable TrueType and OpenType fonts 7.5. List of notable font environment and related packages 7.6. List of notable sandbox environment and related packages 7.7. List of notable remote access server 7.8. List of connection methods to the X server 7.9. List of programs related to manipulating character clipboard 8.1. List of IBus and its engine packages 9.1. List of programs to support console activities 9.2. List of key bindings for screen 9.3. Information on the initialization of vim 9.4. List of system log analyzers 9.5. Display examples of time and date for the "ls -l" command with the time style value 9.6. List of graphics image manipulation tools 9.7. List of packages which can record configuration history 9.8. List of tools for monitoring and controlling program activities 9.9. List of nice values for the scheduling priority 9.10. List of ps command styles 9.11. List of frequently used signals for kill command 9.12. List of notable SAK command keys 9.13. List of hardware identification tools 9.14. List of hardware configuration tools 9.15. List of sound packages 9.16. List of commands for disabling the screen saver 9.17. List of memory sizes reported 9.18. List of tools for system security and integrity check 9.19. List of disk partition management packages 9.20. List of filesystem management packages 9.21. List of packages which view and edit binary data 9.22. List of packages to manipulate files without mounting disk 9.23. List of tools to add data redundancy to files 9.24. List of packages for data file recovery and forensic analysis 9.25. List of data encryption utilities 9.26. List of key packages to be installed for the kernel recompilation on the Debian system 9.27. List of virtualization tools 10.1. List of archive and compression tools 10.2. List of copy and synchronization tools 10.3. List of filesystem choices for removable storage devices with typical usage scenarios 10.4. List of the network service to chose with the typical usage scenario 10.5. List of backup suite utilities 10.6. List of data security infrastructure tools 10.7. List of GNU Privacy Guard commands for the key management 10.8. List of the meaning of the trust code 10.9. List of GNU Privacy Guard commands on files 10.10. List of source code merge tools 10.11. List of git related packages and commands 10.12. Main Git commands 10.13. Git tips 10.14. List of other version control system tools 11.1. List of text data conversion tools 11.2. List of encoding values and their usage 11.3. List of EOL styles for different platforms 11.4. List of TAB conversion commands from bsdmainutils and coreutils packages 11.5. List of tools to extract plain text data 11.6. List of tools to highlight plain text data 11.7. List of predefined entities for XML 11.8. List of XML tools 11.9. List of DSSSL tools 11.10. List of XML data extraction tools 11.11. List of XML pretty print tools 11.12. List of type setting tools 11.13. List of packages to help creating the manpage 11.14. List of Ghostscript PostScript interpreters 11.15. List of printable data utilities 11.16. List of packages to help mail data conversion 11.17. List of graphics data tools (metapackage) 11.18. List of graphics data tools (GUI) 11.19. List of graphics data tools (CLI) 11.20. List of miscellaneous data conversion tools 12.1. List of typical bashisms 12.2. List of shell parameters 12.3. List of shell parameter expansions 12.4. List of key shell parameter substitutions 12.5. List of file comparison operators in the conditional expression 12.6. List of string comparison operators in the conditional expression 12.7. List of packages containing small utility programs for shell scripts 12.8. List of interpreter related packages 12.9. List of dialog programs 12.10. List of compiler related packages 12.11. List of Yacc-compatible LALR parser generators 12.12. List of tools for static code analysis 12.13. List of debug packages 12.14. List of advanced gdb commands 12.15. List of memory leak detection tools 12.16. List of build tool packages 12.17. List of make automatic variables 12.18. List of make variable expansions 12.19. List of source code translation tools Preface Table of Contents 1. Disclaimer 2. What is Debian 3. About this document 3.1. Guiding rules 3.2. Prerequisites 3.3. Conventions 3.4. The popcon 3.5. The package size 3.6. Bug reports on this document 4. Reminders for new users 5. Some quotes for new users This Debian Reference (version 2.122) (2024-04-10 23:08:27 UTC) is intended to provide a broad overview of the Debian system administration as a post-installation user guide. The target reader is someone who is willing to learn shell scripts but who is not ready to read all the C sources to figure out how the GNU/Linux system works. For installation instructions, see: * Debian GNU/Linux Installation Guide for current stable system * Debian GNU/Linux Installation Guide for current testing system 1. Disclaimer     All warranties are disclaimed. All trademarks are property of their respective trademark owners. The Debian system itself is a moving target. This makes its documentation difficult to be current and correct. Although the     current testing version of the Debian system was used as the basis for writing this, some contents may be already outdated by the time you read this. Please treat this document as the secondary reference. This     document does not replace any authoritative guides. The author and contributors do not take responsibility for consequences of errors, omissions or ambiguity in this document. 2. What is Debian The Debian Project is an association of individuals who have made     common cause to create a free operating system. It's distribution is characterized by the following. * Commitment to the software freedom: Debian Social Contract and Debian Free Software Guidelines (DFSG) * Internet based distributed unpaid volunteer effort: https:// www.debian.org * Large number of pre-compiled high quality software packages     * Focus on stability and security with easy access to the security updates * Focus on smooth upgrade to the latest software packages in the testing archives * Large number of supported hardware architectures Free Software pieces in Debian come from GNU, Linux, BSD, X, ISC, Apache, Ghostscript, Common Unix Printing System , Samba, GNOME, KDE, Mozilla, LibreOffice, Vim, TeX, LaTeX, DocBook, Perl,     Python, Tcl, Java, Ruby, PHP, Berkeley DB, MariaDB, PostgreSQL, SQLite, Exim, Postfix, Mutt, FreeBSD, OpenBSD, Plan 9 and many more independent free software projects. Debian integrates this diversity of Free Software into one system. 3. About this document 3.1. Guiding rules     Following guiding rules are followed while compiling this document. * Provide overview and skip corner cases. (Big Picture) * Keep It Short and Simple. (KISS)     * Do not reinvent the wheel. (Use pointers to the existing references) * Focus on non-GUI tools and consoles. (Use shell examples) * Be objective. (Use popcon etc.) Tip     I tried to elucidate hierarchical aspects and lower levels of the system. 3.2. Prerequisites Warning     You are expected to make good efforts to seek answers by yourself beyond this documentation. This document only gives efficient starting points.     You must seek solution by yourself from primary sources. * The Debian site at https://www.debian.org for the general information * The documentation under the "/usr/share/doc/package_name" directory * The Unix style manpage: "dpkg -L package_name |grep '/man/ man.*/'" * The GNU style info page: "dpkg -L package_name |grep '/info/ '"     * The bug report: https://bugs.debian.org/package_name * The Debian Wiki at https://wiki.debian.org/ for the moving and specific topics * The Single UNIX Specification from the Open Group's The UNIX System Home Page * The free encyclopedia from Wikipedia at https:// www.wikipedia.org/ * The Debian Administrator's Handbook * The HOWTOs from The Linux Documentation Project (TLDP) Note     For detailed documentation, you may need to install the corresponding documentation package named with "-doc" as its suffix. 3.3. Conventions This document provides information through the following     simplified presentation style with bash(1) shell command examples.     # command-in-root-account $ command-in-user-account These shell prompts distinguish account used and correspond to     set environment variables as: "PS1='\$'" and "PS2=' '". These values are chosen for the sake of readability of this document and are not typical on actual installed system. All command examples are run under the English locale "LANG= en_US.UTF8". Please don't expect the placeholder strings such as     command-in-root-account and command-in-user-account to be translated in command examples. This is an intentional choice to keep all translated examples to be up-to-date. Note     See the meaning of the "$PS1" and "$PS2" environment variables in bash(1). Action required by the system administrator is written in the     imperative sentence, e.g. "Type Enter-key after typing each command string to the shell." The description column and similar ones in the table may contain a noun phrase following the package short description convention which drops leading articles such as "a" and "the". They may alternatively contain an infinitive phrase as a noun phrase     without leading "to" following the short command description convention in manpages. These may look funny to some people but are my intentional choices of style to keep this documentation as simple as possible. These Noun phrases do not capitalize their starting nor end with periods following these short description convention. Note     Proper nouns including command names keeps their case irrespective of their location. A command snippet quoted in a text paragraph is referred by the     typewriter font between double quotation marks, such as "aptitude safe-upgrade". A text data from a configuration file quoted in a text paragraph     is referred by the typewriter font between double quotation marks, such as "deb-src". A command is referred by its name in the typewriter font     optionally followed by its manpage section number in parenthesis, such as bash(1). You are encouraged to obtain information by typing the following.     $ man 1 bash A manpage is referred by its name in the typewriter font followed     by its manpage section number in parenthesis, such as sources.list(5). You are encouraged to obtain information by typing the following.     $ man 5 sources.list An info page is referred by its command snippet in the typewriter     font between double quotation marks, such as "info make". You are encouraged to obtain information by typing the following.     $ info make A filename is referred by the typewriter font between double     quotation marks, such as "/etc/passwd". For configuration files, you are encouraged to obtain information by typing the following.     $ sensible-pager "/etc/passwd" A directory name is referred by the typewriter font between     double quotation marks, such as "/etc/apt/". You are encouraged to explore its contents by typing the following.     $ mc "/etc/apt/" A package name is referred by its name in the typewriter font,     such as vim. You are encouraged to obtain information by typing the following. $ dpkg -L vim     $ apt-cache show vim $ aptitude show vim A documentation may indicate its location by the filename in the typewriter font between double quotation marks, such as "/usr/     share/doc/base-passwd/users-and-groups.txt.gz" and "/usr/share/ doc/base-passwd/users-and-groups.html"; or by its URL, such as https://www.debian.org. You are encouraged to read the documentation by typing the following. $ zcat "/usr/share/doc/base-passwd/users-and-groups.txt.gz" | sensible-pager     $ sensible-browser "/usr/share/doc/base-passwd/users-and-groups.html" $ sensible-browser "https://www.debian.org" An environment variable is referred by its name with leading "$"     in the typewriter font between double quotation marks, such as "$TERM". You are encouraged to obtain its current value by typing the following.     $ echo "$TERM" 3.4. The popcon The popcon data is presented as the objective measure for the     popularity of each package. It was downloaded on 2024-03-03 11:51:37 UTC and contains the total submission of 237734 reports over 198687 binary packages and 27 architectures. Note     Please note that the amd64 unstable archive contains only 74165 packages currently. The popcon data contains reports from many old system installations. The popcon number preceded with "V:" for "votes" is calculated by     "1000 * (the popcon submissions for the package executed recently on the PC)/(the total popcon submissions)". The popcon number preceded with "I:" for "installs" is calculated     by "1000 * (the popcon submissions for the package installed on the PC)/(the total popcon submissions)". Note The popcon figures should not be considered as absolute measures     of the importance of packages. There are many factors which can skew statistics. For example, some system participating popcon may have mounted directories such as "/usr/bin" with "noatime" option for system performance improvement and effectively disabled "vote" from such system. 3.5. The package size The package size data is also presented as the objective measure for each package. It is based on the "Installed-Size:" reported     by "apt-cache show" or "aptitude show" command (currently on amd64 architecture for the unstable release). The reported size is in KiB (Kibibyte = unit for 1024 bytes). Note A package with a small numerical package size may indicate that     the package in the unstable release is a dummy package which installs other packages with significant contents by the dependency. The dummy package enables a smooth transition or split of the package. Note     A package size followed by "(*)" indicates that the package in the unstable release is missing and the package size for the experimental release is used instead. 3.6. Bug reports on this document Please file bug reports on the debian-reference package using     reportbug(1) if you find any issues on this document. Please include correction suggestion by "diff -u" to the plain text version or to the source. 4. Reminders for new users     Here are some reminders for new users: * Backup your data + See Section 10.2, “Backup and recovery”. * Secure your password and security keys * KISS (keep it simple stupid) + Don't over-engineer your system * Read your log files + The FIRST error is the one that counts * RTFM (read the fine manual)     * Search the Internet before asking questions * Don't be root when you don't have to be * Don't mess with the package management system * Don't type anything you don't understand * Don't change the file permissions (before the full security review) * Don't leave your root shell until you TEST your changes * Always have an alternative boot media (USB memory stick, CD, …) 5. Some quotes for new users     Here are some interesting quotes from the Debian mailing list which may help enlighten new users. * "This is Unix. It gives you enough rope to hang yourself." --- Miquel van Smoorenburg     * "Unix IS user friendly… It's just selective about who its friends are." --- Tollef Fog Heen     Wikipedia has article "Unix philosophy" which lists interesting quotes. Chapter 1. GNU/Linux tutorials I think learning a computer system is like learning a new foreign     language. Although tutorial books and documentation are helpful, you have to practice it yourself. In order to help you get started smoothly, I elaborate a few basic points. The powerful design of Debian GNU/Linux comes from the Unix     operating system, i.e., a multiuser, multitasking operating system. You must learn to take advantage of the power of these features and similarities between Unix and GNU/Linux.     Don't shy away from Unix oriented texts and don't rely solely on GNU/Linux texts, as this robs you of much useful information. Note     If you have been using any Unix-like system for a while with command line tools, you probably know everything I explain here. Please use this as a reality check and refresher. 1.1. Console basics 1.1.1. The shell prompt Upon starting the system, you are presented with the character     based login screen if you did not install any GUI environment such as GNOME or KDE desktop system. Suppose your hostname is foo, the login prompt looks as follows. If you installed a GUI environment, then you can still get to the     character based login prompt by Ctrl-Alt-F3, and you can return to the GUI environment via Ctrl-Alt-F2 (see Section 1.1.6, “Virtual consoles” below for more).     foo login: At the login prompt, you type your username, e.g. penguin, and     press the Enter-key, then type your password and press the Enter-key again. Note Following the Unix tradition, the username and password of the     Debian system are case sensitive. The username is usually chosen only from the lowercase. The first user account is usually created during the installation. Additional user accounts can be created with adduser(8) by root.     The system starts with the greeting message stored in "/etc/motd" (Message Of The Day) and presents a command prompt. Debian GNU/Linux 12 foo tty3 foo login: penguin Password: Linux foo 6.5.0-0.deb12.4-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.5.10-1~bpo12+1 (2023-11-23) x86_64     The programs included with the Debian GNU/Linux system are free software; the exact distribution terms for each program are described in the individual files in /usr/share/doc/*/copyright. Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law. Last login: Wed Dec 20 09:39:00 JST 2023 on tty3 foo:~$     Now you are in the shell. The shell interprets your commands. 1.1.2. The shell prompt under GUI If you installed a GUI environment during the installation, you are presented with the graphical login screen upon starting your     system. You type your username and your password to login to the non-privileged user account. Use tab to navigate between username and password, or use the primary click of the mouse. You can gain the shell prompt under GUI environment by starting a x-terminal-emulator program such as gnome-terminal(1), rxvt(1) or     xterm(1). Under the GNOME desktop environment, press SUPER-key (Windows-key) and typing in "terminal" to the search prompt does the trick. Under some other Desktop systems (like fluxbox), there may be no     obvious starting point for the menu. If this happens, just try (right) clicking the background of the desktop screen and hope for a menu to pop-up. 1.1.3. The root account The root account is also called superuser or privileged user.     From this account, you can perform the following system administration tasks. * Read, write, and remove any files on the system irrespective of their file permissions     * Set file ownership and permissions of any files on the system * Set the password of any non-privileged users on the system * Login to any accounts without their passwords     This unlimited power of root account requires you to be considerate and responsible when using it. Warning     Never share the root password with others. Note File permissions of a file (including hardware devices such as CD-ROM etc. which are just another file for the Debian system)     may render it unusable or inaccessible by non-root users. Although the use of root account is a quick way to test this kind of situation, its resolution should be done through proper setting of file permissions and user's group membership (see Section 1.2.3, “Filesystem permissions”). 1.1.4. The root shell prompt     Here are a few basic methods to gain the root shell prompt by using the root password. * Type root at the character based login prompt. * Type "su -l" from any user shell prompt. + This does not preserve the environment of the current     user. * Type "su" from any user shell prompt. + This preserves some of the environment of the current user. 1.1.5. GUI system administration tools When your desktop menu does not start GUI system administration tools automatically with the appropriate privilege, you can start     them from the root shell prompt of the terminal emulator, such as gnome-terminal(1), rxvt(1), or xterm(1). See Section 1.1.4, “The root shell prompt” and Section 7.9, “X server connection”. Warning Never start the GUI display/session manager under the root account by typing in root to the prompt of the display manager     such as gdm3(1). Never run untrusted remote GUI program under X Window when critical information is displayed since it may eavesdrop your X screen. 1.1.6. Virtual consoles In the default Debian system, there are six switchable VT100-like character consoles available to start the command shell directly on the Linux host. Unless you are in a GUI environment, you can     switch between the virtual consoles by pressing the Left-Alt-key and one of the F1 — F6 keys simultaneously. Each character console allows independent login to the account and offers the multiuser environment. This multiuser environment is a great Unix feature, and very addictive. If you are in the GUI environment, you gain access to the character console 3 by pressing Ctrl-Alt-F3 key, i.e., the     left-Ctrl-key, the left-Alt-key, and the F3-key are pressed together. You can get back to the GUI environment, normally running on the virtual console 2, by pressing Alt-F2.     You can alternatively change to another virtual console, e.g. to the console 3, from the commandline.     # chvt 3 1.1.7. How to leave the command prompt You type Ctrl-D, i.e., the left-Ctrl-key and the d-key pressed together, at the command prompt to close the shell activity. If you are at the character console, you return to the login prompt     with this. Even though these control characters are referred as "control D" with the upper case, you do not need to press the Shift-key. The short hand expression, ^D, is also used for Ctrl-D. Alternately, you can type "exit".     If you are at x-terminal-emulator(1), you can close x-terminal-emulator window with this. 1.1.8. How to shutdown the system Just like any other modern OS where the file operation involves caching data in memory for improved performance, the Debian system needs the proper shutdown procedure before power can safely be turned off. This is to maintain the integrity of files,     by forcing all changes in memory to be written to disk. If the software power control is available, the shutdown procedure automatically turns off power of the system. (Otherwise, you may have to press power button for few seconds after the shutdown procedure.)     You can shutdown the system under the normal multiuser mode from the commandline.     # shutdown -h now     You can shutdown the system under the single-user mode from the commandline.     # poweroff -i -f     See Section 6.3.8, “How to shutdown the remote system on SSH”. 1.1.9. Recovering a sane console When the screen goes berserk after doing some funny things such     as "cat some-binary-file", type "reset" at the command prompt. You may not be able to see the command echoed as you type. You may also issue "clear" to clean up the screen. 1.1.10. Additional package suggestions for the newbie Although even the minimal installation of the Debian system without any desktop environment tasks provides the basic Unix     functionality, it is a good idea to install few additional commandline and curses based character terminal packages such as mc and vim with apt-get(8) for beginners to get started by the following. # apt-get update     ... # apt-get install mc vim sudo aptitude ...     If you already had these packages installed, no new packages are installed. Table 1.1. List of interesting text-mode program packages +---------------------------------------------------------------+ | package |popcon|size | description | |---------+------+-----+----------------------------------------| |mc |V:50, |1542 |A text-mode full-screen file manager | | |I:209 | | | |---------+------+-----+----------------------------------------| |sudo |V:688,|6550 |A program to allow limited root | | |I:841 | |privileges to users | |---------+------+-----+----------------------------------------| | |V:95, | |Unix text editor Vi IMproved, a | |vim |I:369 |3743 |programmers text editor (standard | | | | |version) |     |---------+------+-----+----------------------------------------| | |V:58, | |Unix text editor Vi IMproved, a | |vim-tiny |I:975 |1722 |programmers text editor (compact | | | | |version) | |---------+------+-----+----------------------------------------| |emacs-nox|V:4, |39647|GNU project Emacs, the Lisp based | | |I:16 | |extensible text editor | |---------+------+-----+----------------------------------------| |w3m |V:15, |2837 |Text-mode WWW browsers | | |I:187 | | | |---------+------+-----+----------------------------------------| |gpm |V:10, |521 |The Unix style cut-and-paste on the text| | |I:12 | |console (daemon) | +---------------------------------------------------------------+     It may be a good idea to read some informative documentations. Table 1.2. List of informative documentation packages +---------------------------------------------------------------+ | package |popcon|size | description | |--------------------+------+-----+-----------------------------| | | | |Debian Project documentation,| |doc-debian |I:867 |187 |(Debian FAQ) and other | | | | |documents | |--------------------+------+-----+-----------------------------| |debian-policy |I:14 |4659 |Debian Policy Manual and |     | | | |related documents | |--------------------+------+-----+-----------------------------| |developers-reference|V:0, |2601 |Guidelines and information | | |I:5 | |for Debian developers | |--------------------+------+-----+-----------------------------| |debmake-doc |I:0 |11701|Guide for Debian Maintainers | |--------------------+------+-----+-----------------------------| |debian-history |I:0 |4692 |History of the Debian Project| |--------------------+------+-----+-----------------------------| |debian-faq |I:865 |790 |Debian FAQ | +---------------------------------------------------------------+     You can install some of these packages by the following.     # apt-get install package_name 1.1.11. An extra user account If you do not want to use your main user account for the     following training activities, you can create a training user account, e.g. fish by the following.     # adduser fish     Answer all questions. This creates a new account named as fish. After your practice,     you can remove this user account and its home directory by the following.     # deluser --remove-home fish On non-Debian and specialized Debian systems, above activities     need to use lower level useradd(8) and userdel(8) utilities, instead. 1.1.12. sudo configuration For the typical single user workstation such as the desktop Debian system on the laptop PC, it is common to deploy simple     configuration of sudo(8) as follows to let the non-privileged user, e.g. penguin, to gain administrative privilege just with his user password but without the root password.     # echo "penguin ALL=(ALL) ALL" >> /etc/sudoers Alternatively, it is also common to do as follows to let the     non-privileged user, e.g. penguin, to gain administrative privilege without any password.     # echo "penguin ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers     This trick should only be used for the single user workstation which you administer and where you are the only user. Warning     Do not set up accounts of regular users on multiuser workstation like this because it would be very bad for system security. Caution The password and the account of the penguin in the above example requires as much protection as the root password and the root account.     Administrative privilege in this context belongs to someone authorized to perform the system administration task on the workstation. Never give some manager in the Admin department of your company or your boss such privilege unless they are authorized and capable. Note For providing access privilege to limited devices and limited files, you should consider to use group to provide limited access instead of using the root privilege via sudo(8).     With more thoughtful and careful configuration, sudo(8) can grant limited administrative privileges to other users on a shared system without sharing the root password. This can help with accountability with hosts with multiple administrators so you can tell who did what. On the other hand, you might not want anyone else to have such privileges. 1.1.13. Play time     Now you are ready to play with the Debian system without risks as long as you use the non-privileged user account. This is because the Debian system is, even after the default installation, configured with proper file permissions which     prevent non-privileged users from damaging the system. Of course, there may still be some holes which can be exploited but those who worry about these issues should not be reading this section but should be reading Securing Debian Manual.     We learn the Debian system as a Unix-like system with the following. * Section 1.2, “Unix-like filesystem” (basic concept) * Section 1.3, “Midnight Commander (MC)” (survival method) * Section 1.4, “The basic Unix-like work environment” (basic     method) * Section 1.5, “The simple shell command” (shell mechanism) * Section 1.6, “Unix-like text processing” (text processing method) 1.2. Unix-like filesystem In GNU/Linux and other Unix-like operating systems, files are organized into directories. All files and directories are     arranged in one big tree rooted at "/". It's called a tree because if you draw the filesystem, it looks like a tree but it is upside down. These files and directories can be spread out over several devices. mount(8) serves to attach the filesystem found on some device to the big file tree. Conversely, umount(8) detaches it     again. On recent Linux kernels, mount(8) with some options can bind part of a file tree somewhere else or can mount filesystem as shared, private, slave, or unbindable. Supported mount options for each filesystem are available in "/usr/share/doc/linux-doc-*/ Documentation/filesystems/". Directories on Unix systems are called folders on some other systems. Please also note that there is no concept for drive such     as "A:" on any Unix system. There is one filesystem, and everything is included. This is a huge advantage compared to Windows. 1.2.1. Unix file basics     Here are some Unix file basics. * Filenames are case sensitive. That is, "MYFILE" and "MyFile" are different files. * The root directory means root of the filesystem referred as simply "/". Don't confuse this with the home directory for the root user: "/root". * Every directory has a name which can contain any letters or symbols except "/". The root directory is an exception; its name is "/" (pronounced "slash" or "the root directory") and it cannot be renamed. * Each file or directory is designated by a fully-qualified filename, absolute filename, or path, giving the sequence of directories which must be passed through to reach it. The three terms are synonymous. * All fully-qualified filenames begin with the "/" directory, and there's a "/" between each directory or file in the filename. The first "/" is the top level directory, and the other "/"'s separate successive subdirectories, until we reach the last entry which is the name of the actual file. The words used here can be confusing. Take the following fully-qualified filename as an example: "/usr/share/keytables /us.map.gz". However, people also refers to its basename     "us.map.gz" alone as a filename. * The root directory has a number of branches, such as "/etc/" and "/usr/". These subdirectories in turn branch into still more subdirectories, such as "/etc/systemd/" and "/usr/local/ ". The whole thing viewed collectively is called the directory tree. You can think of an absolute filename as a route from the base of the tree ("/") to the end of some branch (a file). You also hear people talk about the directory tree as if it were a family tree encompassing all direct descendants of a single figure called the root directory ("/"): thus subdirectories have parents, and a path shows the complete ancestry of a file. There are also relative paths that begin somewhere other than the root directory. You should remember that the directory "../" refers to the parent directory. This terminology also applies to other directory like structures, such as hierarchical data structures. * There's no special directory path name component that corresponds to a physical device, such as your hard disk. This differs from RT-11, CP/M, OpenVMS, MS-DOS, AmigaOS, and Microsoft Windows, where the path contains a device name such as "C:\". (However, directory entries do exist that refer to physical devices as a part of the normal filesystem. See Section 1.2.2, “Filesystem internals”.) Note While you can use almost any letters or symbols in a file name, in practice it is a bad idea to do so. It is better to avoid any     characters that often have special meanings on the command line, including spaces, tabs, newlines, and other special characters: { } ( ) [ ] ' ` " \ / > < | ; ! # & ^ * % @ $ . If you want to separate words in a name, good choices are the period, hyphen, and underscore. You could also capitalize each word, "LikeThis". Experienced Linux users tend to avoid spaces in filenames. Note     The word "root" can mean either "root user" or "root directory". The context of their usage should make it clear. Note     The word path is used not only for fully-qualified filename as above but also for the command search path. The intended meaning is usually clear from the context. The detailed best practices for the file hierarchy are described     in the Filesystem Hierarchy Standard ("/usr/share/doc/ debian-policy/fhs/fhs-2.3.txt.gz" and hier(7)). You should remember the following facts as the starter. Table 1.3. List of usage of key directories +---------------------------------------------------------------+ |directory| usage of the directory | |---------+-----------------------------------------------------| |/ |the root directory |     |---------+-----------------------------------------------------| |/etc/ |system wide configuration files | |---------+-----------------------------------------------------| |/var/log/|system log files | |---------+-----------------------------------------------------| |/home/ |all the home directories for all non-privileged users| +---------------------------------------------------------------+ 1.2.2. Filesystem internals Following the Unix tradition, the Debian GNU/Linux system provides the filesystem under which physical data on hard disks     and other storage devices reside, and the interaction with the hardware devices such as console screens and remote serial consoles are represented in an unified manner under "/dev/". Each file, directory, named pipe (a way two programs can share data), or physical device on a Debian GNU/Linux system has a data structure called an inode which describes its associated attributes such as the user who owns it (owner), the group that     it belongs to, the time last accessed, etc. The idea of representing just about everything in the filesystem was a Unix innovation, and modern Linux kernels have developed this idea ever further. Now, even information about processes running in the computer can be found in the filesystem. This abstract and unified representation of physical entities and internal processes is very powerful since this allows us to use     the same command for the same kind of operation on many totally different devices. It is even possible to change the way the kernel works by writing data to special files that are linked to running processes. Tip     If you need to identify the correspondence between the file tree and the physical entity, execute mount(8) with no arguments. 1.2.3. Filesystem permissions     Filesystem permissions of Unix-like system are defined for three categories of affected users. * The user who owns the file (u)     * Other users in the group which the file belongs to (g) * All other users (o) also referred to as "world" and "everyone"     For the file, each corresponding permission allows following actions. * The read (r) permission allows owner to examine contents of the file.     * The write (w) permission allows owner to modify the file. * The execute (x) permission allows owner to run the file as a command.     For the directory, each corresponding permission allows following actions. * The read (r) permission allows owner to list contents of the directory.     * The write (w) permission allows owner to add or remove files in the directory. * The execute (x) permission allows owner to access files in the directory. Here, the execute permission on a directory means not only to     allow reading of files in that directory but also to allow viewing their attributes, such as the size and the modification time. ls(1) is used to display permission information (and more) for     files and directories. When it is invoked with the "-l" option, it displays the following information in the order given. * Type of file (first character) * Access permission of the file (nine characters, consisting of three characters each for user, group, and other in this order) * Number of hard links to the file     * Name of the user who owns the file * Name of the group which the file belongs to * Size of the file in characters (bytes) * Date and time of the file (mtime) * Name of the file Table 1.4. List of the first character of "ls -l" output +-------------------------------+ |character| meaning | |---------+---------------------| |- |normal file | |---------+---------------------| |d |directory | |---------+---------------------|     |l |symlink | |---------+---------------------| |c |character device node| |---------+---------------------| |b |block device node | |---------+---------------------| |p |named pipe | |---------+---------------------| |s |socket | +-------------------------------+ chown(1) is used from the root account to change the owner of the file. chgrp(1) is used from the file's owner or root account to     change the group of the file. chmod(1) is used from the file's owner or root account to change file and directory access permissions. Basic syntax to manipulate a foo file is the following. # chown newowner foo     # chgrp newgroup foo # chmod [ugoa][+-=][rwxXst][,...] foo     For example, you can make a directory tree to be owned by a user foo and shared by a group bar by the following. # cd /some/location/     # chown -R foo:bar . # chmod -R ug+rwX,o=rX .     There are three more special permission bits. * The set user ID bit (s or S instead of user's x)     * The set group ID bit (s or S instead of group's x) * The sticky bit (t or T instead of other's x)     Here the output of "ls -l" for these bits is capitalized if execution bits hidden by these outputs are unset. Setting set user ID on an executable file allows a user to execute the executable file with the owner ID of the file (for     example root). Similarly, setting set group ID on an executable file allows a user to execute the executable file with the group ID of the file (for example root). Because these settings can cause security risks, enabling them requires extra caution. Setting set group ID on a directory enables the BSD-like file     creation scheme where all files created in the directory belong to the group of the directory. Setting the sticky bit on a directory prevents a file in the directory from being removed by a user who is not the owner of the file. In order to secure contents of a file in world-writable     directories such as "/tmp" or in group-writable directories, one must not only reset the write permission for the file but also set the sticky bit on the directory. Otherwise, the file can be removed and a new file can be created with the same name by any user who has write access to the directory.     Here are a few interesting examples of file permissions. $ ls -l /etc/passwd /etc/shadow /dev/ppp /usr/sbin/exim4 crw------T 1 root root 108, 0 Oct 16 20:57 /dev/ppp -rw-r--r-- 1 root root 2761 Aug 30 10:38 /etc/passwd -rw-r----- 1 root shadow 1695 Aug 30 10:38 /etc/shadow -rwsr-xr-x 1 root root 973824 Sep 23 20:04 /usr/sbin/exim4     $ ls -ld /tmp /var/tmp /usr/local /var/mail /usr/src drwxrwxrwt 14 root root 20480 Oct 16 21:25 /tmp drwxrwsr-x 10 root staff 4096 Sep 29 22:50 /usr/local drwxr-xr-x 10 root root 4096 Oct 11 00:28 /usr/src drwxrwsr-x 2 root mail 4096 Oct 15 21:40 /var/mail drwxrwxrwt 3 root root 4096 Oct 16 21:20 /var/tmp There is an alternative numeric mode to describe file permissions     with chmod(1). This numeric mode uses 3 to 4 digit wide octal (radix=8) numbers. Table 1.5. The numeric mode for file permissions in chmod(1) commands +---------------------------------------------------------------+ | digit | meaning | |-------------+-------------------------------------------------| |1st optional |sum of set user ID (=4), set group ID (=2), and | |digit |sticky bit (=1) |     |-------------+-------------------------------------------------| |2nd digit |sum of read (=4), write (=2), and execute (=1) | | |permissions for user | |-------------+-------------------------------------------------| |3rd digit |ditto for group | |-------------+-------------------------------------------------| |4th digit |ditto for other | +---------------------------------------------------------------+ This sounds complicated but it is actually quite simple. If you look at the first few (2-10) columns from "ls -l" command output     and read it as a binary (radix=2) representation of file permissions ("-" being "0" and "rwx" being "1"), the last 3 digit of the numeric mode value should make sense as an octal (radix=8) representation of file permissions to you.     For example, try the following $ touch foo bar $ chmod u=rw,go=r foo     $ chmod 644 bar $ ls -l foo bar -rw-r--r-- 1 penguin penguin 0 Oct 16 21:39 bar -rw-r--r-- 1 penguin penguin 0 Oct 16 21:35 foo Tip     If you need to access information displayed by "ls -l" in shell script, you should use pertinent commands such as test(1), stat (1) and readlink(1). The shell builtin such as "[" or "test" may be used too. 1.2.4. Control of permissions for newly created files: umask What permissions are applied to a newly created file or directory     is restricted by the umask shell builtin command. See dash(1), bash(1), and builtins(7).     (file permissions) = (requested file permissions) & ~(umask value) Table 1.6. The umask value examples +---------------------------------------------------------------+ |umask|file permissions | directory | usage | | | created |permissions created | | |-----+-----------------+--------------------+------------------|     |0022 |-rw-r--r-- |-rwxr-xr-x |writable only by | | | | |the user | |-----+-----------------+--------------------+------------------| |0002 |-rw-rw-r-- |-rwxrwxr-x |writable by the | | | | |group | +---------------------------------------------------------------+ The Debian system uses a user private group (UPG) scheme as its default. A UPG is created whenever a new user is added to the system. A UPG has the same name as the user for which it was     created and that user is the only member of the UPG. UPG scheme makes it safe to set umask to 0002 since every user has their own private group. (In some Unix variants, it is quite common to setup all normal users belonging to a single users group and is a good idea to set umask to 0022 for security in such cases.) Tip     Enable UPG by putting "umask 002" in the ~/.bashrc file. 1.2.5. Permissions for groups of users (group) Warning     Please make sure to save unsaved changes before doing reboot or similar actions.     You can add a user penguin to a group bird in two steps: * Change group configuration using one of following: + Execute "sudo usermod -aG bird penguin". + Execute "sudo adduser penguin bird". (only on typical Debian systems) + Execute "sudo vigr" for /etc/group and "sudo vigr -s" for     /etc/gshadow to append penguin in the line for bird. * Apply configuration using one of following: + Cold reboot and login. (Best option) + Execute "kill -TERM -1" and do some fix-up actions such as "systemctl restart NetworkManager.service". + Logout via GUI menu and login.     You can remove a user penguin from a group bird in two steps: * Change group configuration using one of following: + Execute "sudo usermod -rG bird penguin". + Execute "sudo deluser penguin bird". (only on typical Debian systems) + Execute "sudo vigr" for /etc/group and "sudo vigr -s" for     /etc/gshadow to remove penguin in the line for bird. * Apply configuration using one of following: + Cold reboot and login. (Best option) + Execute "kill -TERM -1" and do some fix-up actions such as "systemctl restart NetworkManager.service". + Logout via GUI menu is not an option for Gnome Desktop.     Any warm reboot attempts are fragile replacements of the real cold reboot under the modern desktop system. Note     Alternatively, you may dynamically add users to groups during the authentication process by adding "auth optional pam_group.so" line to "/etc/pam.d/common-auth" and setting "/etc/security/ group.conf". (See Chapter 4, Authentication and access controls.) The hardware devices are just another kind of file on the Debian     system. If you have problems accessing devices such as CD-ROM and USB memory stick from a user account, you should make that user a member of the relevant group.     Some notable system-provided groups allow their members to access particular files and devices without root privilege. Table 1.7. List of notable system-provided groups for file access +---------------------------------------------------------------+ | group | description for accessible files and devices | |-------+-------------------------------------------------------| |dialout|full and direct access to serial ports ("/dev/ttyS[0-3]| | |") | |-------+-------------------------------------------------------| |dip |limited access to serial ports for Dialup IP connection| | |to trusted peers | |-------+-------------------------------------------------------| |cdrom |CD-ROM, DVD+/-RW drives |     |-------+-------------------------------------------------------| |audio |audio device | |-------+-------------------------------------------------------| |video |video device | |-------+-------------------------------------------------------| |scanner|scanner(s) | |-------+-------------------------------------------------------| |adm |system monitoring logs | |-------+-------------------------------------------------------| |staff |some directories for junior administrative work: "/usr/| | |local", "/home" | +---------------------------------------------------------------+ Tip You need to belong to the dialout group to reconfigure modem,     dial anywhere, etc. But if root creates pre-defined configuration files for trusted peers in "/etc/ppp/peers/", you only need to belong to the dip group to create Dialup IP connection to those trusted peers using pppd(8), pon(1), and poff(1) commands.     Some notable system-provided groups allow their members to execute particular commands without root privilege. Table 1.8. List of notable system provided groups for particular command executions +---------------------------------------------------------------+ | group | accessible commands |     |-------+-------------------------------------------------------| |sudo |execute sudo without their password | |-------+-------------------------------------------------------| |lpadmin|execute commands to add, modify, and remove printers | | |from printer databases | +---------------------------------------------------------------+ For the full listing of the system provided users and groups, see     the recent version of the "Users and Groups" document in "/usr/ share/doc/base-passwd/users-and-groups.html" provided by the base-passwd package. See passwd(5), group(5), shadow(5), newgrp(1), vipw(8), vigr(8),     and pam_group(8) for management commands of the user and group system. 1.2.6. Timestamps     There are three types of timestamps for a GNU/Linux file. Table 1.9. List of types of timestamps +------------------------------------------+ |type | meaning (historic Unix definition) | |-----+------------------------------------|     |mtime|the file modification time (ls -l) | |-----+------------------------------------| |ctime|the file status change time (ls -lc)| |-----+------------------------------------| |atime|the last file access time (ls -lu) | +------------------------------------------+ Note     ctime is not file creation time. Note     The actual value of atime on GNU/Linux system may be different from that of the historic Unix definition. * Overwriting a file changes all of the mtime, ctime, and atime attributes of the file. * Changing ownership or permission of a file changes the ctime and atime attributes of the file. * Reading a file changes the atime attribute of the file on the historic Unix system. * Reading a file changes the atime attribute of the file on the     GNU/Linux system if its filesystem is mounted with "strictatime". * Reading a file for the first time or after one day changes the atime attribute of the file on the GNU/Linux system if its filesystem is mounted with "relatime". (default behavior since Linux 2.6.30) * Reading a file doesn't change the atime attribute of the file on the GNU/Linux system if its filesystem is mounted with "noatime". Note The "noatime" and "relatime" mount options are introduced to     improve the filesystem read performance under the normal use cases. Simple file read operation under the "strictatime" option accompanies the time-consuming write operation to update the atime attribute. But the atime attribute is rarely used except for the mbox(5) file. See mount(8).     Use touch(1) command to change timestamps of existing files.     For timestamps, the ls command outputs localized strings under non-English locale ("fr_FR.UTF-8"). $ LANG=C ls -l foo -rw-rw-r-- 1 penguin penguin 0 Oct 16 21:35 foo     $ LANG=en_US.UTF-8 ls -l foo -rw-rw-r-- 1 penguin penguin 0 Oct 16 21:35 foo $ LANG=fr_FR.UTF-8 ls -l foo -rw-rw-r-- 1 penguin penguin 0 oct. 16 21:35 foo Tip     See Section 9.3.4, “Customized display of time and date” to customize "ls -l" output. 1.2.7. Links     There are two methods of associating a file "foo" with a different filename "bar". * Hard link + Duplicate name for an existing file + "ln foo bar"     * Symbolic link or symlink + Special file that points to another file by name + "ln -s foo bar"     See the following example for changes in link counts and the subtle differences in the result of the rm command. $ umask 002 $ echo "Original Content" > foo $ ls -li foo 1449840 -rw-rw-r-- 1 penguin penguin 17 Oct 16 21:42 foo $ ln foo bar # hard link $ ln -s foo baz # symlink $ ls -li foo bar baz 1449840 -rw-rw-r-- 2 penguin penguin 17 Oct 16 21:42 bar 1450180 lrwxrwxrwx 1 penguin penguin 3 Oct 16 21:47 baz -> foo     1449840 -rw-rw-r-- 2 penguin penguin 17 Oct 16 21:42 foo $ rm foo $ echo "New Content" > foo $ ls -li foo bar baz 1449840 -rw-rw-r-- 1 penguin penguin 17 Oct 16 21:42 bar 1450180 lrwxrwxrwx 1 penguin penguin 3 Oct 16 21:47 baz -> foo 1450183 -rw-rw-r-- 1 penguin penguin 12 Oct 16 21:48 foo $ cat bar Original Content $ cat baz New Content     The hardlink can be made within the same filesystem and shares the same inode number which the "-i" option with ls(1) reveals. The symlink always has nominal file access permissions of     "rwxrwxrwx", as shown in the above example, with the effective access permissions dictated by permissions of the file that it points to. Caution     It is generally a good idea not to create complicated symbolic links or hardlinks at all unless you have a very good reason. It may cause nightmares where the logical combination of the symbolic links results in loops in the filesystem. Note     It is generally preferable to use symbolic links rather than hardlinks unless you have a good reason for using a hardlink. The "." directory links to the directory that it appears in, thus     the link count of any new directory starts at 2. The ".." directory links to the parent directory, thus the link count of the directory increases with the addition of new subdirectories. If you are just moving to Linux from Windows, it soon becomes clear how well-designed the filename linking of Unix is, compared     with the nearest Windows equivalent of "shortcuts". Because it is implemented in the filesystem, applications can't see any difference between a linked file and the original. In the case of hardlinks, there really is no difference. 1.2.8. Named pipes (FIFOs) A named pipe is a file that acts like a pipe. You put something     into the file, and it comes out the other end. Thus it's called a FIFO, or First-In-First-Out: the first thing you put in the pipe is the first thing to come out the other end. If you write to a named pipe, the process which is writing to the pipe doesn't terminate until the information being written is read from the pipe. If you read from a named pipe, the reading process waits until there is nothing to read before terminating.     The size of the pipe is always zero --- it does not store data, it just links two processes like the functionality offered by the shell "|" syntax. However, since this pipe has a name, the two processes don't have to be on the same command line or even be run by the same user. Pipes were a very influential innovation of Unix.     For example, try the following $ cd; mkfifo mypipe $ echo "hello" >mypipe & # put into background [1] 8022 $ ls -l mypipe prw-rw-r-- 1 penguin penguin 0 Oct 16 21:49 mypipe     $ cat mypipe hello [1]+ Done echo "hello" >mypipe $ ls mypipe mypipe $ rm mypipe 1.2.9. Sockets Sockets are used extensively by all the Internet communication, databases, and the operating system itself. It is similar to the named pipe (FIFO) and allows processes to exchange information even between different computers. For the socket, those processes     do not need to be running at the same time nor to be running as the children of the same ancestor process. This is the endpoint for the inter process communication (IPC). The exchange of information may occur over the network between different hosts. The two most common ones are the Internet socket and the Unix domain socket. Tip     "netstat -an" provides a very useful overview of sockets that are open on a given system. 1.2.10. Device files Device files refer to physical or virtual devices on your system,     such as your hard disk, video card, screen, or keyboard. An example of a virtual device is the console, represented by "/dev/ console".     There are 2 types of device files. * Character device + Accessed one character at a time + 1 character = 1 byte + E.g. keyboard device, serial port, …     * Block device + accessed in larger units called blocks + 1 block > 1 byte + E.g. hard disk, … You can read and write device files, though the file may well contain binary data which may be an incomprehensible-to-humans gibberish. Writing data directly to these files is sometimes     useful for the troubleshooting of hardware connections. For example, you can dump a text file to the printer device "/dev/ lp0" or send modem commands to the appropriate serial port "/dev/ ttyS0". But, unless this is done carefully, it may cause a major disaster. So be cautious. Note     For the normal access to a printer, use lp(1).     The device node number are displayed by executing ls(1) as the following. $ ls -l /dev/sda /dev/sr0 /dev/ttyS0 /dev/zero brw-rw---T 1 root disk 8, 0 Oct 16 20:57 /dev/sda     brw-rw---T+ 1 root cdrom 11, 0 Oct 16 21:53 /dev/sr0 crw-rw---T 1 root dialout 4, 64 Oct 16 20:57 /dev/ttyS0 crw-rw-rw- 1 root root 1, 5 Oct 16 20:57 /dev/zero * "/dev/sda" has the major device number 8 and the minor device number 0. This is read/write accessible by users belonging to the disk group. * "/dev/sr0" has the major device number 11 and the minor device number 0. This is read/write accessible by users     belonging to the cdrom group. * "/dev/ttyS0" has the major device number 4 and the minor device number 64. This is read/write accessible by users belonging to the dialout group. * "/dev/zero" has the major device number 1 and the minor device number 5. This is read/write accessible by anyone.     On the modern Linux system, the filesystem under "/dev/" is automatically populated by the udev(7) mechanism. 1.2.11. Special device files     There are some special device files. Table 1.10. List of special device files +---------------------------------------------------------------+ |device |action| description of response | | file | | | |-------+------+------------------------------------------------| |/dev/ |read |return "end-of-file (EOF) character" | |null | | | |-------+------+------------------------------------------------| |/dev/ |write |return nothing (a bottomless data dump pit) | |null | | | |-------+------+------------------------------------------------|     |/dev/ |read |return "the \0 (NUL) character" (not the same as| |zero | |the number zero ASCII) | |-------+------+------------------------------------------------| |/dev/ |read |return random characters from a true random | |random | |number generator, delivering real entropy (slow)| |-------+------+------------------------------------------------| |/dev/ | |return random characters from a | |urandom|read |cryptographically secure pseudorandom number | | | |generator | |-------+------+------------------------------------------------| |/dev/ |write |return the disk-full (ENOSPC) error | |full | | | +---------------------------------------------------------------+ These are frequently used in conjunction with the shell     redirection (see Section 1.5.8, “Typical command sequences and shell redirection”). 1.2.12. procfs and sysfs The procfs and sysfs mounted on "/proc" and "/sys" are the pseudo-filesystem and expose internal data structures of the     kernel to the userspace. In other word, these entries are virtual, meaning that they act as a convenient window into the operation of the operating system. The directory "/proc" contains (among other things) one subdirectory for each process running on the system, which is     named after the process ID (PID). System utilities that access process information, such as ps(1), get their information from this directory structure. The directories under "/proc/sys/" contain interfaces to change     certain kernel parameters at run time. (You may do the same through the specialized sysctl(8) command or its preload/ configuration file "/etc/sysctl.conf".) People frequently panic when they notice one file in particular - "/proc/kcore" - which is generally huge. This is (more or less) a     copy of the content of your computer's memory. It's used to debug the kernel. It is a virtual file that points to computer memory, so don't worry about its size. The directory under "/sys" contains exported kernel data     structures, their attributes, and their linkages between them. It also contains interfaces to change certain kernel parameters at run time. See "proc.txt(.gz)", "sysfs.txt(.gz)" and other related documents     in the Linux kernel documentation ("/usr/share/doc/linux-doc-*/ Documentation/filesystems/*") provided by the linux-doc-* package. 1.2.13. tmpfs The tmpfs is a temporary filesystem which keeps all files in the     virtual memory. The data of the tmpfs in the page cache on memory may be swapped out to the swap space on disk as needed. The directory "/run" is mounted as the tmpfs in the early boot process. This enables writing to it even when the directory "/"     is mounted as read-only. This is the new location for the storage of transient state files and replaces several locations described in the Filesystem Hierarchy Standard version 2.3: * "/var/run" → "/run"     * "/var/lock" → "/run/lock" * "/dev/shm" → "/run/shm" See "tmpfs.txt(.gz)" in the Linux kernel documentation ("/usr/     share/doc/linux-doc-*/Documentation/filesystems/*") provided by the linux-doc-* package. 1.3. Midnight Commander (MC) Midnight Commander (MC) is a GNU "Swiss army knife" for the Linux     console and other terminal environments. This gives newbie a menu driven console experience which is much easier to learn than standard Unix commands.     You may need to install the Midnight Commander package which is titled "mc" by the following.     $ sudo apt-get install mc Use the mc(1) command to explore the Debian system. This is the     best way to learn. Please explore few interesting locations just using the cursor keys and Enter key. * "/etc" and its subdirectories * "/var/log" and its subdirectories     * "/usr/share/doc" and its subdirectories * "/usr/sbin" and "/usr/bin" 1.3.1. Customization of MC In order to make MC to change working directory upon exit and cd     to the directory, I suggest to modify "~/.bashrc" to include a script provided by the mc package.     . /usr/lib/mc/mc.sh See mc(1) (under the "-P" option) for the reason. (If you do not     understand what exactly I am talking here, you can do this later.) 1.3.2. Starting MC     MC can be started by the following.     $ mc MC takes care of all file operations through its menu, requiring     minimal user effort. Just press F1 to get the help screen. You can play with MC just by pressing cursor-keys and function-keys. Note     In some consoles such as gnome-terminal(1), key strokes of function-keys may be stolen by the console program. You can disable these features in "Preferences" → "General" and "Shortcuts" menu for gnome-terminal. If you encounter character encoding problem which displays     garbage characters, adding "-a" to MC's command line may help prevent problems.     If this doesn't clear up your display problems with MC, see Section 9.5.6, “The terminal configuration”. 1.3.3. File manager in MC The default is two directory panels containing file lists. Another useful mode is to set the right window to "information"     to see file access privilege information, etc. Following are some essential keystrokes. With the gpm(8) daemon running, one can use a mouse on Linux character consoles, too. (Make sure to press the shift-key to obtain the normal behavior of cut and paste in MC.) Table 1.11. The key bindings of MC +---------------------------------------------------------------+ | key | key binding | |--------------+------------------------------------------------| |F1 |help menu | |--------------+------------------------------------------------| |F3 |internal file viewer | |--------------+------------------------------------------------| |F4 |internal editor | |--------------+------------------------------------------------| |F9 |activate pull down menu |     |--------------+------------------------------------------------| |F10 |exit Midnight Commander | |--------------+------------------------------------------------| |Tab |move between two windows | |--------------+------------------------------------------------| |Insert or |mark file for a multiple-file operation such as | |Ctrl-T |copy | |--------------+------------------------------------------------| |Del |delete file (be careful---set MC to safe delete | | |mode) | |--------------+------------------------------------------------| |Cursor keys |self-explanatory | +---------------------------------------------------------------+ 1.3.4. Command-line tricks in MC * cd command changes the directory shown on the selected screen. * Ctrl-Enter or Alt-Enter copies a filename to the command line. Use this with cp(1) and mv(1) commands together with command-line editing. * Alt-Tab shows shell filename expansion choices.     * One can specify the starting directory for both windows as arguments to MC; for example, "mc /etc /root". * Esc + n-key → Fn (i.e., Esc + 1 → F1, etc.; Esc + 0 → F10) * Pressing Esc before the key has the same effect as pressing the Alt and the key together.; i.e., type Esc + c for Alt-C. Esc is called meta-key and sometimes noted as "M-". 1.3.5. The internal editor in MC The internal editor has an interesting cut-and-paste scheme. Pressing F3 marks the start of a selection, a second F3 marks the end of selection and highlights the selection. Then you can move     your cursor. If you press F6, the selected area is moved to the cursor location. If you press F5, the selected area is copied and inserted at the cursor location. F2 saves the file. F10 gets you out. Most cursor keys work intuitively.     This editor can be directly started on a file using one of the following commands.     $ mc -e filename_to_edit     $ mcedit filename_to_edit This is not a multi-window editor, but one can use multiple Linux consoles to achieve the same effect. To copy between windows, use     Alt-Fn keys to switch virtual consoles and use "File→Insert file" or "File→Copy to file" to move a portion of a file to another file.     This internal editor can be replaced with any external editor of choice. Also, many programs use the environment variables "$EDITOR" or     "$VISUAL" to decide which editor to use. If you are uncomfortable with vim(1) or nano(1) initially, you may set these to "mcedit" by adding the following lines to "~/.bashrc".     export EDITOR=mcedit export VISUAL=mcedit     I do recommend setting these to "vim" if possible.     If you are uncomfortable with vim(1), you can keep using mcedit (1) for most system maintenance tasks. 1.3.6. The internal viewer in MC MC is a very smart viewer. This is a great tool for searching words in documents. I always use this for files in the "/usr/     share/doc" directory. This is the fastest way to browse through masses of Linux information. This viewer can be directly started using one of the following commands.     $ mc -v path/to/filename_to_view     $ mcview path/to/filename_to_view 1.3.7. Auto-start features of MC Press Enter on a file, and the appropriate program handles the     content of the file (see Section 9.4.11, “Customizing program to be started”). This is a very convenient MC feature. Table 1.12. The reaction to the enter key in MC +---------------------------------------------------------------+ | file type | reaction to enter key | |--------------------------+------------------------------------| |executable file |execute command | |--------------------------+------------------------------------|     |man file |pipe content to viewer software | |--------------------------+------------------------------------| |html file |pipe content to web browser | |--------------------------+------------------------------------| |"*.tar.gz" and "*.deb" |browse its contents as if | |file |subdirectory | +---------------------------------------------------------------+ In order to allow these viewer and virtual file features to     function, viewable files should not be set as executable. Change their status using chmod(1) or via the MC file menu. 1.3.8. Virtual filesystem of MC MC can be used to access files over the Internet. Go to the menu by pressing F9, "Enter" and "h" to activate the Shell filesystem.     Enter a URL in the form "sh://[user@]machine[:options]/ [remote-dir]", which retrieves a remote directory that appears like a local one using ssh. 1.4. The basic Unix-like work environment Although MC enables you to do almost everything, it is very     important for you to learn how to use the command line tools invoked from the shell prompt and become familiar with the Unix-like work environment. 1.4.1. The login shell Since the login shell may be used by some system initialization     programs, it is prudent to keep it as bash(1) and avoid switching the login shell with chsh(1). If you want to use a different interactive shell prompt, set it     from GUI terminal emulator configuration or start it from ~ /.bashrc, e.g., by placing "exec /usr/bin/zsh -i -l" or "exec / usr/bin/fish -i -l" in it. Table 1.13. List of shell programs +---------------------------------------------------------------+ | package |popcon|size|POSIX| description | | | | |shell| | |---------------+------+----+-----+-----------------------------| |bash |V:838,|7175|Yes |Bash: the GNU Bourne Again | | |I:999 | | |SHell (de facto standard) | |---------------+------+----+-----+-----------------------------| |bash-completion|V:32, |1454|N/A |programmable completion for | | |I:933 | | |the bash shell | |---------------+------+----+-----+-----------------------------| |dash |V:884,|191 |Yes |Debian Almquist Shell, good | | |I:997 | | |for shell script | |---------------+------+----+-----+-----------------------------| |zsh |V:40, |2463|Yes |Z shell: the standard shell | | |I:73 | | |with many enhancements | |---------------+------+----+-----+-----------------------------| |tcsh |V:6, |1355|No |TENEX C Shell: an enhanced | | |I:20 | | |version of Berkeley csh |     |---------------+------+----+-----+-----------------------------| |mksh |V:6, |1579|Yes |A version of the Korn shell | | |I:11 | | | | |---------------+------+----+-----+-----------------------------| |csh |V:1, |339 |No |OpenBSD C Shell, a version of| | |I:6 | | |Berkeley csh | |---------------+------+----+-----+-----------------------------| | |V:0, | | |Stand-alone shell with | |sash |I:5 |1157|Yes |builtin commands (Not meant | | | | | |for standard "/usr/bin/sh") | |---------------+------+----+-----+-----------------------------| |ksh |V:1, |61 |Yes |the real, AT&T version of the| | |I:10 | | |Korn shell | |---------------+------+----+-----+-----------------------------| |rc |V:0, |178 |No |implementation of the AT&T | | |I:1 | | |Plan 9 rc shell | |---------------+------+----+-----+-----------------------------| |posh |V:0, |190 |Yes |Policy-compliant Ordinary | | |I:0 | | |SHell (pdksh derivative) | +---------------------------------------------------------------+ Tip     Although POSIX-like shells share the basic syntax, they can differ in behavior for things as basic as shell variables and glob expansions. Please check their documentation for details.     In this tutorial chapter, the interactive shell always means bash. 1.4.2. Customizing bash     You can customize bash(1) behavior by "~/.bashrc".     For example, try the following. # enable bash-completion if ! shopt -oq posix; then if [ -f /usr/share/bash-completion/bash_completion ]; then . /usr/share/bash-completion/bash_completion elif [ -f /etc/bash_completion ]; then . /etc/bash_completion fi fi # CD upon exiting MC . /usr/lib/mc/mc.sh     # set CDPATH to a good one CDPATH=.:/usr/share/doc:~:~/Desktop:~ export CDPATH PATH="${PATH+$PATH:}/usr/sbin:/sbin" # set PATH so it includes user's private bin if it exists if [ -d ~/bin ] ; then PATH="~/bin${PATH+:$PATH}" fi export PATH EDITOR=vim export EDITOR Tip     You can find more bash customization tips, such as Section 9.3.6, “Colorized commands”, in Chapter 9, System tips. Tip     The bash-completion package enables programmable completion for bash. 1.4.3. Special key strokes In the Unix-like environment, there are few key strokes which     have special meanings. Please note that on a normal Linux character console, only the left-hand Ctrl and Alt keys work as expected. Here are few notable key strokes to remember. Table 1.14. List of key bindings for bash +---------------------------------------------------------------+ | key | description of key binding | |------------------------+--------------------------------------| |Ctrl-U |erase line before cursor | |------------------------+--------------------------------------| |Ctrl-H |erase a character before cursor | |------------------------+--------------------------------------| |Ctrl-D |terminate input (exit shell if you are| | |using shell) | |------------------------+--------------------------------------| |Ctrl-C |terminate a running program | |------------------------+--------------------------------------| |Ctrl-Z |temporarily stop program by moving it | | |to the background job | |------------------------+--------------------------------------| |Ctrl-S |halt output to screen | |------------------------+--------------------------------------|     |Ctrl-Q |reactivate output to screen | |------------------------+--------------------------------------| |Ctrl-Alt-Del |reboot/halt the system, see inittab(5)| |------------------------+--------------------------------------| |Left-Alt-key | | |(optionally, |meta-key for Emacs and the similar UI | |Windows-key) | | |------------------------+--------------------------------------| |Up-arrow |start command history search under | | |bash | |------------------------+--------------------------------------| |Ctrl-R |start incremental command history | | |search under bash | |------------------------+--------------------------------------| |Tab |complete input of the filename to the | | |command line under bash | |------------------------+--------------------------------------| |Ctrl-V Tab |input Tab without expansion to the | | |command line under bash | +---------------------------------------------------------------+ Tip     The terminal feature of Ctrl-S can be disabled using stty(1). 1.4.4. Mouse operations     Mouse operations for text on Debian system mix 2 styles with some twists: * Traditional Unix style mouse operations: + use 3 buttons (click) + use PRIMARY + used by X applications such as xterm and text     applications in Linux console * Modern GUI style mouse operations: + use 2 buttons (drag + click) + use PRIMARY and CLIPBOARD + used in Modern GUI applications such as gnome-terminal Table 1.15. List of mouse operations and related key actions on Debian +---------------------------------------------------------------+ | action | response | |--------------------------+------------------------------------| |Left-click-and-drag mouse |select range as PRIMARY selection | |--------------------------+------------------------------------| |Left-click |select the start of range for | | |PRIMARY selection | |--------------------------+------------------------------------| |Right-click (traditional) |select the end of range for PRIMARY | | |selection |     |--------------------------+------------------------------------| |Right-click (modern) |context dependent menu (cut/copy/ | | |paste) | |--------------------------+------------------------------------| |Middle-click or Shift-Ins |insert PRIMARY selection at the | | |cursor | |--------------------------+------------------------------------| |Ctrl-X |cut PRIMARY selection to CLIPBOARD | |--------------------------+------------------------------------| |Ctrl-C (Shift-Ctrl-C in |copy PRIMARY selection to CLIPBOARD | |terminal) | | |--------------------------+------------------------------------| |Ctrl-V |paste CLIPBOARD at the cursor | +---------------------------------------------------------------+ Here, the PRIMARY selection is the highlighted text range. Within     the terminal program, Shift-Ctrl-C is used instead to avoid terminating a running program. The center wheel on the modern wheel mouse is considered middle     mouse button and can be used for middle-click. Clicking left and right mouse buttons together serves as the middle-click under the 2 button mouse system situation.     In order to use a mouse in Linux character consoles, you need to have gpm(8) running as daemon. 1.4.5. The pager The less(1) command is the enhanced pager (file content browser). It reads the file specified by its command argument or its standard input. Hit "h" if you need help while browsing with the     less command. It can do much more than more(1) and can be supercharged by executing "eval $(lesspipe)" or "eval $(lessfile) " in the shell startup script. See more in "/usr/share/doc/less/ LESSOPEN". The "-R" option allows raw character output and enables ANSI color escape sequences. See less(1). Tip     In the less command, type "h" to see the help screen, type "/" or "?" to search a string, and type "-i" to the change case sensitivity. 1.4.6. The text editor     You should become proficient in one of variants of Vim or Emacs programs which are popular in the Unix-like system. I think getting used to Vim commands is the right thing to do, since Vi-editor is always there in the Linux/Unix world.     (Actually, original vi or new nvi are programs you find everywhere. I chose Vim instead for newbie since it offers you help through F1 key while it is similar enough and more powerful.) If you chose either Emacs or XEmacs instead as your choice of the editor, that is another good choice indeed, particularly for programming. Emacs has a plethora of other features as well, including functioning as a newsreader, directory editor, mail     program, etc. When used for programming or editing shell scripts, it intelligently recognizes the format of what you are working on, and tries to provide assistance. Some people maintain that the only program they need on Linux is Emacs. Ten minutes learning Emacs now can save hours later. Having the GNU Emacs manual for reference when learning Emacs is highly recommended. All these programs usually come with tutoring program for you to learn them by practice. Start Vim by typing "vim" and press     F1-key. You should at least read the first 35 lines. Then do the online training course by moving cursor to "|tutor|" and pressing Ctrl-]. Note Good editors, such as Vim and Emacs, can handle UTF-8 and other     exotic encoding texts correctly. It is a good idea to use the GUI environment in the UTF-8 locale and to install required programs and fonts to it. Editors have options to set the file encoding independent of the GUI environment. Please refer to their documentation on multibyte text. 1.4.7. Setting a default text editor     Debian comes with a number of different editors. We recommend to install the vim package, as mentioned above. Debian provides unified access to the system default editor via     command "/usr/bin/editor" so other programs (e.g., reportbug(1)) can invoke it. You can change it by the following.     $ sudo update-alternatives --config editor     The choice "/usr/bin/vim.basic" over "/usr/bin/vim.tiny" is my recommendation for newbies since it supports syntax highlighting. Tip Many programs use the environment variables "$EDITOR" or     "$VISUAL" to decide which editor to use (see Section 1.3.5, “The internal editor in MC” and Section 9.4.11, “Customizing program to be started”). For the consistency on the Debian system, set these to "/usr/bin/editor". (Historically, "$EDITOR" was "ed" and "$VISUAL" was "vi".) 1.4.8. Using vim     The recent vim(1) starts itself in the sane "nocompatible" option and enters into the NORMAL mode.^[1] Table 1.16. List of basic Vim key strokes +---------------------------------------------------------------+ | mode | key strokes | action | |---------------+-----------------+-----------------------------| |NORMAL |:help|only |display the help file | |---------------+-----------------+-----------------------------| |NORMAL |:e filename.ext |open new buffer to edit | | | |filename.ext | |---------------+-----------------+-----------------------------| |NORMAL |:w |overwrite current buffer to | | | |the original file | |---------------+-----------------+-----------------------------| |NORMAL |:w filename.ext |write current buffer to | | | |filename.ext | |---------------+-----------------+-----------------------------| |NORMAL |:q |quit vim | |---------------+-----------------+-----------------------------| |NORMAL |:q! |force to quit vim | |---------------+-----------------+-----------------------------| |NORMAL |:only |close all other split open | | | |windows | |---------------+-----------------+-----------------------------| |NORMAL |:set |check if vim is in the sane | | |nocompatible? |nocompatible mode | |---------------+-----------------+-----------------------------| |NORMAL |:set nocompatible|set vim to the sane |     | | |nocompatible mode | |---------------+-----------------+-----------------------------| |NORMAL |i |enter the INSERT mode | |---------------+-----------------+-----------------------------| |NORMAL |R |enter the REPLACE mode | |---------------+-----------------+-----------------------------| |NORMAL |v |enter the VISUAL mode | |---------------+-----------------+-----------------------------| |NORMAL |V |enter the linewise VISUAL | | | |mode | |---------------+-----------------+-----------------------------| |NORMAL |Ctrl-V |enter the blockwise VISUAL | | | |mode | |---------------+-----------------+-----------------------------| |except |ESC-key |enter the NORMAL mode | |TERMINAL-JOB | | | |---------------+-----------------+-----------------------------| |NORMAL |:term |enter the TERMINAL-JOB mode | |---------------+-----------------+-----------------------------| |TERMINAL-NORMAL|i |enter the TERMINAL-JOB mode | |---------------+-----------------+-----------------------------| |TERMINAL-JOB |Ctrl-W N (or |enter the TERMINAL-NORMAL | | |Ctrl-\ Ctrl-N) |mode | |---------------+-----------------+-----------------------------| |TERMINAL-JOB |Ctrl-W : |enter the Ex-mode in | | | |TERMINAL-NORMAL mode | +---------------------------------------------------------------+     Please use the "vimtutor" program to learn vim through an interactive tutorial course. The vim program changes its behavior to typed key strokes based on mode. Typing in key strokes to the buffer is mostly done in     INSERT-mode and REPLACE-mode. Moving cursor is mostly done in NORMAL-mode. Interactive selection is done in VISUAL-mode. Typing ":" in NORMAL-mode changes its mode to Ex-mode. Ex-mode accepts commands. Tip     The Vim comes with the Netrw package. Netrw supports reading files, writing files, browsing directories over a network, and local browsing! Try Netrw with "vim ." (a period as the argument) and read its manual at ":help netrw".     For the advanced configuration of vim, see Section 9.2, “Customizing vim”. 1.4.9. Recording the shell activities The output of the shell command may roll off your screen and may     be lost forever. It is a good practice to log shell activities into the file for you to review them later. This kind of record is essential when you perform any system administration tasks. Tip     The new Vim (version>=8.2) can be used to record the shell activities cleanly using TERMINAL-JOB-mode. See Section 1.4.8, “Using vim”.     The basic method of recording the shell activity is to run it under script(1).     For example, try the following     $ script Script started, file is typescript     Do whatever shell commands under script.     Press Ctrl-D to exit script.     $ vim typescript     See Section 9.1.1, “Recording the shell activities cleanly” . 1.4.10. Basic Unix commands Let's learn basic Unix commands. Here I use "Unix" in its generic sense. Any Unix clone OSs usually offer equivalent commands. The     Debian system is no exception. Do not worry if some commands do not work as you wish now. If alias is used in the shell, its corresponding command outputs are different. These examples are not meant to be executed in this order.     Try all following commands from the non-privileged user account. Table 1.17. List of basic Unix commands +---------------------------------------------------------------+ | command | description | |-----------+---------------------------------------------------| |pwd |display name of current/working directory | |-----------+---------------------------------------------------| |whoami |display current user name | |-----------+---------------------------------------------------| |id |display current user identity (name, uid, gid, and | | |associated groups) | |-----------+---------------------------------------------------| |file foo |display a type of file for the file "foo" | |-----------+---------------------------------------------------| |type -p |display a file location of command "commandname" | |commandname| | |-----------+---------------------------------------------------| |which |, , | |commandname| | |-----------+---------------------------------------------------| |type |display information on command "commandname" | |commandname| | |-----------+---------------------------------------------------| |apropos |find commands related to "key-word" | |key-word | | |-----------+---------------------------------------------------| |man -k |, , | |key-word | | |-----------+---------------------------------------------------| |whatis |display one line explanation on command " | |commandname|commandname" | |-----------+---------------------------------------------------| |man -a |display explanation on command "commandname" (Unix | |commandname|style) | |-----------+---------------------------------------------------| |info |display rather long explanation on command " | |commandname|commandname" (GNU style) | |-----------+---------------------------------------------------| |ls |list contents of directory (non-dot files and | | |directories) | |-----------+---------------------------------------------------| |ls -a |list contents of directory (all files and | | |directories) | |-----------+---------------------------------------------------| |ls -A |list contents of directory (almost all files and | | |directories, i.e., skip ".." and ".") | |-----------+---------------------------------------------------| |ls -la |list all contents of directory with detail | | |information | |-----------+---------------------------------------------------| |ls -lai |list all contents of directory with inode number | | |and detail information | |-----------+---------------------------------------------------| |ls -d |list all directories under the current directory | |-----------+---------------------------------------------------| |tree |display file tree contents | |-----------+---------------------------------------------------| |lsof foo |list open status of file "foo" | |-----------+---------------------------------------------------| |lsof -p pid|list files opened by the process ID: "pid" | |-----------+---------------------------------------------------| |mkdir foo |make a new directory "foo" in the current directory| |-----------+---------------------------------------------------| |rmdir foo |remove a directory "foo" in the current directory | |-----------+---------------------------------------------------| | |change directory to the directory "foo" in the | |cd foo |current directory or in the directory listed in the| | |variable "$CDPATH" | |-----------+---------------------------------------------------| |cd / |change directory to the root directory | |-----------+---------------------------------------------------| |cd |change directory to the current user's home | | |directory | |-----------+---------------------------------------------------| |cd /foo |change directory to the absolute path directory "/ | | |foo" | |-----------+---------------------------------------------------| |cd .. |change directory to the parent directory | |-----------+---------------------------------------------------| |cd ~foo |change directory to the home directory of the user | | |"foo" | |-----------+---------------------------------------------------| |cd - |change directory to the previous directory | |-----------+---------------------------------------------------| ||of "README.gz" | |foo | | |-----------+---------------------------------------------------| |zcat |append the decompressed content of "README.gz" to | |README.gz |the end of the file "foo" (if it does not exist, | |>> foo |create it first) | +---------------------------------------------------------------+ Note Unix has a tradition to hide filenames which start with ".". They are traditionally files that contain configuration information and user preferences. For cd command, see builtins(7). The default pager of the bare bone Debian system is more(1) which     cannot scroll back. By installing the less package using command line "apt-get install less", less(1) becomes default pager and you can scroll back with cursor keys. The "[" and "]" in the regular expression of the "ps aux | grep -e "[e]xim4*"" command above enable grep to avoid matching itself. The "4*" in the regular expression means 0 or more repeats of character "4" thus enables grep to match both "exim" and "exim4". Although "*" is used in the shell filename glob and the regular expression, their meanings are different. Learn the regular expression from grep(1). Please traverse directories and peek into the system using the     above commands as training. If you have questions on any of console commands, please make sure to read the manual page.     For example, try the following $ man man $ man bash     $ man builtins $ man grep $ man ls The style of man pages may be a little hard to get used to,     because they are rather terse, particularly the older, very traditional ones. But once you get used to it, you come to appreciate their succinctness. Please note that many Unix-like commands including ones from GNU     and BSD display brief help information if you invoke them in one of the following ways (or without any arguments in some cases).     $ commandname --help $ commandname -h 1.5. The simple shell command Now you have some feel on how to use the Debian system. Let's     look deep into the mechanism of the command execution in the Debian system. Here, I have simplified reality for the newbie. See bash(1) for the exact explanation.     A simple command is a sequence of components. 1. Variable assignments (optional) 2. Command name     3. Arguments (optional) 4. Redirections (optional: > , >> , < , << , etc.) 5. Control operator (optional: && , || , newline , ; , & , ( , ) ) 1.5.1. Command execution and environment variable     The values of some environment variables change the behavior of some Unix commands. Default values of environment variables are initially set by the     PAM system and then some of them may be reset by some application programs. * The PAM system such as pam_env may set environment variables by /etc/pam.conf", "/etc/environment" and "/etc/default/ locale".     * The display manager such as gdm3 may reset environment variables for GUI session by "~/.profile". * The user specific program initialization may reset environment variables by "~/.profile", "~/.bash_profile" and "~/.bashrc". 1.5.2. The "$LANG" variable The default locale is defined in the "$LANG" environment variable     and is configured as "LANG=xx_YY.UTF-8" by the installer or by the subsequent GUI configuration, e.g., "Settings" → "Region & Language" → "Language" / "Formats" for GNOME. Note     I recommend you to configure the system environment just by the "$LANG" variable for now and to stay away from "$LC_*" variables unless it is absolutely needed.     The full locale value given to "$LANG" variable consists of 3 parts: "xx_YY.ZZZZ". Table 1.18. The 3 parts of locale value +-------------------------------------------------------------+ |locale value| meaning | |------------+------------------------------------------------|     |xx |ISO 639 language codes (lower case) such as "en"| |------------+------------------------------------------------| |YY |ISO 3166 country codes (upper case) such as "US"| |------------+------------------------------------------------| |ZZZZ |codeset, always set to "UTF-8" | +-------------------------------------------------------------+ Table 1.19. List of locale recommendations +------------------------------------------------+ |locale recommendation| Language (area) | |---------------------+--------------------------| |en_US.UTF-8 |English (USA) | |---------------------+--------------------------| |en_GB.UTF-8 |English (Great Britain) | |---------------------+--------------------------| |fr_FR.UTF-8 |French (France) | |---------------------+--------------------------| |de_DE.UTF-8 |German (Germany) | |---------------------+--------------------------| |it_IT.UTF-8 |Italian (Italy) | |---------------------+--------------------------| |es_ES.UTF-8 |Spanish (Spain) | |---------------------+--------------------------|     |ca_ES.UTF-8 |Catalan (Spain) | |---------------------+--------------------------| |sv_SE.UTF-8 |Swedish (Sweden) | |---------------------+--------------------------| |pt_BR.UTF-8 |Portuguese (Brazil) | |---------------------+--------------------------| |ru_RU.UTF-8 |Russian (Russia) | |---------------------+--------------------------| |zh_CN.UTF-8 |Chinese (P.R. of China) | |---------------------+--------------------------| |zh_TW.UTF-8 |Chinese (Taiwan R.O.C.) | |---------------------+--------------------------| |ja_JP.UTF-8 |Japanese (Japan) | |---------------------+--------------------------| |ko_KR.UTF-8 |Korean (Republic of Korea)| |---------------------+--------------------------| |vi_VN.UTF-8 |Vietnamese (Vietnam) | +------------------------------------------------+     Typical command execution uses a shell line sequence as the following. $ echo $LANG en_US.UTF-8     $ date -u Wed 19 May 2021 03:18:43 PM UTC $ LANG=fr_FR.UTF-8 date -u mer. 19 mai 2021 15:19:02 UTC     Here, the program date(1) is executed with different locale values. * For the first command, "$LANG" is set to the system default locale value "en_US.UTF-8".     * For the second command, "$LANG" is set to the French UTF-8 locale value "fr_FR.UTF-8". Most command executions usually do not have preceding environment     variable definition. For the above example, you can alternatively execute as the following. $ LANG=fr_FR.UTF-8     $ date -u mer. 19 mai 2021 15:19:24 UTC Tip     When filing a bug report, running and checking the command under "en_US.UTF-8" locale is a good idea if you use non-English environment.     For fine details of the locale configuration, see Section 8.1, “The locale”. 1.5.3. The "$PATH" variable When you type a command into the shell, the shell searches the     command in the list of directories contained in the "$PATH" environment variable. The value of the "$PATH" environment variable is also called the shell's search path. In the default Debian installation, the "$PATH" environment variable of user accounts may not include "/usr/sbin" and "/usr/     sbin". For example, the ifconfig command needs to be issued with full path as "/usr/sbin/ifconfig". (Similar ip command is located in "/usr/bin".)     You can change the "$PATH" environment variable of Bash shell by "~/.bash_profile" or "~/.bashrc" files. 1.5.4. The "$HOME" variable Many commands stores user specific configuration in the home     directory and changes their behavior by their contents. The home directory is identified by the environment variable "$HOME". Table 1.20. List of "$HOME" values +---------------------------------------------------------------+ |value of "$HOME"| program execution situation | |----------------+----------------------------------------------| |/ |program run by the init process (daemon) | |----------------+----------------------------------------------| |/root |program run from the normal root shell | |----------------+----------------------------------------------| |/home/ |program run from the normal user shell |     |normal_user | | |----------------+----------------------------------------------| |/home/ |program run from the normal user GUI desktop | |normal_user |menu | |----------------+----------------------------------------------| |/home/ |program run as root with "sudo program" | |normal_user | | |----------------+----------------------------------------------| |/root |program run as root with "sudo -H program" | +---------------------------------------------------------------+ Tip     Shell expands "~/" to current user's home directory, i.e., "$HOME /". Shell expands "~foo/" to foo's home directory, i.e., "/home/ foo/".     See Section 12.1.5, “Shell environment variables” if $HOME isn't available for your program. 1.5.5. Command line options     Some commands take arguments. Arguments starting with "-" or "--" are called options and control the behavior of the command. $ date     Thu 20 May 2021 01:08:08 AM JST $ date -R Thu, 20 May 2021 01:08:12 +0900     Here the command-line argument "-R" changes date(1) behavior to output RFC2822 compliant date string. 1.5.6. Shell glob Often you want a command to work with a group of files without     typing all of them. The filename expansion pattern using the shell glob, (sometimes referred as wildcards), facilitate this need. Table 1.21. Shell glob patterns +---------------------------------------------------------------+ |shell glob | description of match rule | | pattern | | |-----------+---------------------------------------------------| |* |filename (segment) not started with "." | |-----------+---------------------------------------------------| |.* |filename (segment) started with "." | |-----------+---------------------------------------------------|     |? |exactly one character | |-----------+---------------------------------------------------| |[…] |exactly one character with any character enclosed | | |in brackets | |-----------+---------------------------------------------------| |[a-z] |exactly one character with any character between | | |"a" and "z" | |-----------+---------------------------------------------------| |[^…] |exactly one character other than any character | | |enclosed in brackets (excluding "^") | +---------------------------------------------------------------+     For example, try the following $ mkdir junk; cd junk; touch 1.txt 2.txt 3.c 4.h .5.txt ..6.txt $ echo *.txt 1.txt 2.txt $ echo * 1.txt 2.txt 3.c 4.h $ echo *.[hc]     3.c 4.h $ echo .* . .. .5.txt ..6.txt $ echo .*[^.]* .5.txt ..6.txt $ echo [^1-3]* 4.h $ cd ..; rm -rf junk     See glob(7). Note     Unlike normal filename expansion by the shell, the shell pattern "*" tested in find(1) with "-name" test etc., matches the initial "." of the filename. (New POSIX feature) Note     BASH can be tweaked to change its glob behavior with its shopt builtin options such as "dotglob", "noglob", "nocaseglob", "nullglob", "extglob", etc. See bash(1). 1.5.7. Return value of the command     Each command returns its exit status (variable: "$?") as the return value. Table 1.22. Command exit codes +-------------------------------------------------------------+ |command exit status|numeric return value|logical return value|     |-------------------+--------------------+--------------------| |success |zero, 0 |TRUE | |-------------------+--------------------+--------------------| |error |non-zero, -1 |FALSE | +-------------------------------------------------------------+     For example, try the following. $ [ 1 = 1 ] ; echo $?     0 $ [ 1 = 2 ] ; echo $? 1 Note     Please note that, in the logical context for the shell, success is treated as the logical TRUE which has 0 (zero) as its value. This is somewhat non-intuitive and needs to be reminded here. 1.5.8. Typical command sequences and shell redirection     Let's try to remember following shell command idioms typed in one line as a part of shell command. Table 1.23. Shell command idioms +---------------------------------------------------------------+ | command | description | | idiom | | |----------+----------------------------------------------------| |command & |background execution of command in the subshell | |----------+----------------------------------------------------| |command1 ||pipe the standard output of command1 to the standard| |command2 |input of command2 (concurrent execution) | |----------+----------------------------------------------------| |command1 |pipe both standard output and standard error of | |2>&1 | |command1 to the standard input of command2 ( | |command2 |concurrent execution) | |----------+----------------------------------------------------| |command1 ;|execute command1 and command2 sequentially | |command2 | | |----------+----------------------------------------------------| |command1 &|execute command1; if successful, execute command2 | |& command2|sequentially (return success if both command1 and | | |command2 are successful) | |----------+----------------------------------------------------| |command1 ||execute command1; if not successful, execute | || command2|command2 sequentially (return success if command1 or| | |command2 are successful) | |----------+----------------------------------------------------|     |command > |redirect standard output of command to a file foo | |foo |(overwrite) | |----------+----------------------------------------------------| |command 2>|redirect standard error of command to a file foo | |foo |(overwrite) | |----------+----------------------------------------------------| |command >>|redirect standard output of command to a file foo | |foo |(append) | |----------+----------------------------------------------------| |command |redirect standard error of command to a file foo | |2>> foo |(append) | |----------+----------------------------------------------------| |command > |redirect both standard output and standard error of | |foo 2>&1 |command to a file foo | |----------+----------------------------------------------------| |command < |redirect standard input of command to a file foo | |foo | | |----------+----------------------------------------------------| |command <<|redirect standard input of command to the following | |delimiter |lines until "delimiter" is met (here document) | |----------+----------------------------------------------------| |command |redirect standard input of command to the following | |<<- |lines until "delimiter" is met (here document, the | |delimiter |leading tab characters are stripped from input | | |lines) | +---------------------------------------------------------------+ The Debian system is a multi-tasking system. Background jobs allow users to run multiple programs in a single shell. The     management of the background process involves the shell builtins: jobs, fg, bg, and kill. Please read sections of bash(1) under "SIGNALS", and "JOB CONTROL", and builtins(1).     For example, try the following     $ foo $ exec 3bar # open files     $ cat <&3 >&4 # redirect stdin to 3, stdout to 4 $ exec 3<&- 4>&- # close files $ cat bar Hello     The file descriptor 0-2 are predefined. Table 1.24. Predefined file descriptors +--------------------------------------+ |device| description |file descriptor| |------+---------------+---------------|     |stdin |standard input |0 | |------+---------------+---------------| |stdout|standard output|1 | |------+---------------+---------------| |stderr|standard error |2 | +--------------------------------------+ 1.5.9. Command alias     You can set an alias for the frequently used command.     For example, try the following     $ alias la='ls -la'     Now, "la" works as a short hand for "ls -la" which lists all files in the long listing format.     You can list any existing aliases by alias (see bash(1) under "SHELL BUILTIN COMMANDS"). $ alias     ... alias la='ls -la'     You can identity exact path or identity of the command by type (see bash(1) under "SHELL BUILTIN COMMANDS").     For example, try the following $ type ls ls is hashed (/bin/ls) $ type la     la is aliased to ls -la $ type echo echo is a shell builtin $ type file file is /usr/bin/file Here ls was recently searched while "file" was not, thus "ls" is     "hashed", i.e., the shell has an internal record for the quick access to the location of the "ls" command. Tip     See Section 9.3.6, “Colorized commands”. 1.6. Unix-like text processing In Unix-like work environment, text processing is done by piping     text through chains of standard text processing tools. This was another crucial Unix innovation. 1.6.1. Unix text tools     There are few standard text processing tools which are used very often on the Unix-like system. * No regular expression is used: + cat(1) concatenates files and outputs the whole content. + tac(1) concatenates files and outputs in reverse. + cut(1) selects parts of lines and outputs. + head(1) outputs the first part of files. + tail(1) outputs the last part of files. + sort(1) sorts lines of text files. + uniq(1) removes duplicate lines from a sorted file. + tr(1) translates or deletes characters. + diff(1) compares files line by line. * Basic regular expression (BRE) is used as default: + ed(1) is a primitive line editor.     + sed(1) is a stream editor. + grep(1) matches text with patterns. + vim(1) is a screen editor. + emacs(1) is a screen editor. (somewhat extended BRE) * Extended regular expression (ERE) is used: + awk(1) does simple text processing. + egrep(1) matches text with patterns. + tcl(3tcl) can do every conceivable text processing: See re_syntax(3). Often used with tk(3tk). + perl(1) can do every conceivable text processing. See perlre(1). + pcregrep(1) from the pcregrep package matches text with Perl Compatible Regular Expressions (PCRE) pattern. + python(1) with the re module can do every conceivable text processing. See "/usr/share/doc/python/html/ index.html".     If you are not sure what exactly these commands do, please use "man command" to figure it out by yourself. Note     Sort order and range expression are locale dependent. If you wish to obtain traditional behavior for a command, use C locale or C.UTF-8 locale instead of normal UTF-8 ones (see Section 8.1, “The locale”). Note     Perl regular expressions (perlre(1)), Perl Compatible Regular Expressions (PCRE), and Python regular expressions offered by the re module have many common extensions to the normal ERE. 1.6.2. Regular expressions Regular expressions are used in many text processing tools. They     are analogous to the shell globs, but they are more complicated and powerful.     The regular expression describes the matching pattern and is made up of text characters and metacharacters. A metacharacter is just a character with a special meaning. There     are 2 major styles, BRE and ERE, depending on the text tools as described above. Table 1.25. Metacharacters for BRE and ERE +---------------------------------------------------------------+ | BRE | ERE | description of the regular expression | |---------+------+----------------------------------------------| |\ . [ ] ^|\ . [ | | |$ * |] ^ $ |common metacharacters | | |* | | |---------+------+----------------------------------------------| |\+ \? \( | | | |\) \{ \} |  |BRE only "\" escaped metacharacters | |\| | | | |---------+------+----------------------------------------------| | |+ ? ( | | |  |) { } |ERE only non-"\" escaped metacharacters | | || | | |---------+------+----------------------------------------------| |c |c |match non-metacharacter "c" | |---------+------+----------------------------------------------| |\c |\c |match a literal character "c" even if "c" is | | | |metacharacter by itself | |---------+------+----------------------------------------------| |. |. |match any character including newline | |---------+------+----------------------------------------------| |^ |^ |position at the beginning of a string | |---------+------+----------------------------------------------|     |$ |$ |position at the end of a string | |---------+------+----------------------------------------------| |\< |\< |position at the beginning of a word | |---------+------+----------------------------------------------| |\> |\> |position at the end of a word | |---------+------+----------------------------------------------| |[abc…] |[abc…]|match any characters in "abc…" | |---------+------+----------------------------------------------| |[^abc…] |[^ |match any characters except in "abc…" | | |abc…] | | |---------+------+----------------------------------------------| |r* |r* |match zero or more regular expressions | | | |identified by "r" | |---------+------+----------------------------------------------| |r\+ |r+ |match one or more regular expressions | | | |identified by "r" | |---------+------+----------------------------------------------| |r\? |r? |match zero or one regular expressions | | | |identified by "r" | |---------+------+----------------------------------------------| |r1\|r2 |r1|r2 |match one of the regular expressions | | | |identified by "r1" or "r2" | |---------+------+----------------------------------------------| |\(r1\|r2 |(r1| |match one of the regular expressions | |\) |r2) |identified by "r1" or "r2" and treat it as a | | | |bracketed regular expression | +---------------------------------------------------------------+ The regular expression of emacs is basically BRE but has been     extended to treat "+"and "?" as the metacharacters as in ERE. Thus, there are no needs to escape them with "\" in the regular expression of emacs.     grep(1) can be used to perform the text search using the regular expression.     For example, try the following $ egrep 'GNU.*LICENSE|Yoyodyne' /usr/share/common-licenses/GPL     GNU GENERAL PUBLIC LICENSE GNU GENERAL PUBLIC LICENSE Yoyodyne, Inc., hereby disclaims all copyright interest in the program Tip     See Section 9.3.6, “Colorized commands”. 1.6.3. Replacement expressions     For the replacement expression, some characters have special meanings. Table 1.26. The replacement expression +---------------------------------------------------------------+ | replacement | description of the text to replace the | | expression | replacement expression | |---------------+-----------------------------------------------|     |& |what the regular expression matched (use \& in | | |emacs) | |---------------+-----------------------------------------------| |\n |what the n-th bracketed regular expression | | |matched ("n" being number) | +---------------------------------------------------------------+     For Perl replacement string, "$&" is used instead of "&" and "$n" is used instead of "\n".     For example, try the following $ echo zzz1abc2efg3hij4 | \ sed -e 's/\(1[a-z]*\)[0-9]*\(.*\)$/=&=/' zzz=1abc2efg3hij4= $ echo zzz1abc2efg3hij4 | \ sed -E -e 's/(1[a-z]*)[0-9]*(.*)$/=&=/' zzz=1abc2efg3hij4= $ echo zzz1abc2efg3hij4 | \ perl -pe 's/(1[a-z]*)[0-9]*(.*)$/=$&=/'     zzz=1abc2efg3hij4= $ echo zzz1abc2efg3hij4 | \ sed -e 's/\(1[a-z]*\)[0-9]*\(.*\)$/\2===\1/' zzzefg3hij4===1abc $ echo zzz1abc2efg3hij4 | \ sed -E -e 's/(1[a-z]*)[0-9]*(.*)$/\2===\1/' zzzefg3hij4===1abc $ echo zzz1abc2efg3hij4 | \ perl -pe 's/(1[a-z]*)[0-9]*(.*)$/$2===$1/' zzzefg3hij4===1abc Here please pay extra attention to the style of the bracketed     regular expression and how the matched strings are used in the text replacement process on different tools.     These regular expressions can be used for cursor movements and text replacement actions in some editors too. The back slash "\" at the end of line in the shell commandline     escapes newline as a white space character and continues shell command line input to the next line.     Please read all the related manual pages to learn these commands. 1.6.4. Global substitution with regular expressions     The ed(1) command can replace all instances of "FROM_REGEX" with "TO_TEXT" in "file". $ ed file < input to main shell Sat 23 Aug 2003 08 30 36 +0200 $ unset IFS # reset IFS to the default $ echo $(date -R) Sat, 23 Aug 2003 08:30:50 +0200 1.6.6. Script snippets for piping commands     The following scripts do nice things as a part of a pipe. Table 1.27. List of script snippets for piping commands +---------------------------------------------------------------+ |script snippet (type | effect of command | | in one line) | | |---------------------+-----------------------------------------| |find /usr -print |find all files under "/usr" | |---------------------+-----------------------------------------| |seq 1 100 |print 1 to 100 | |---------------------+-----------------------------------------| || xargs -n 1 command |run command repeatedly with each item | | |from pipe as its argument | |---------------------+-----------------------------------------| || xargs -n 1 echo |split white-space-separated items from | | |pipe into lines | |---------------------+-----------------------------------------| || xargs echo |merge all lines from pipe into a line | |---------------------+-----------------------------------------| || grep -e |extract lines from pipe containing | |regex_pattern |regex_pattern | |---------------------+-----------------------------------------| || grep -v -e |extract lines from pipe not containing | |regex_pattern |regex_pattern | |---------------------+-----------------------------------------| || cut -d: -f3 - |extract third field from pipe separated | | |by ":" (passwd file etc.) | |---------------------+-----------------------------------------| || awk '{ print $3 }' |extract third field from pipe separated |     | |by whitespaces | |---------------------+-----------------------------------------| || awk -F'\t' '{ print|extract third field from pipe separated | |$3 }' |by tab | |---------------------+-----------------------------------------| || col -bx |remove backspace and expand tabs to | | |spaces | |---------------------+-----------------------------------------| || expand - |expand tabs | |---------------------+-----------------------------------------| || sort| uniq |sort and remove duplicates | |---------------------+-----------------------------------------| || tr 'A-Z' 'a-z' |convert uppercase to lowercase | |---------------------+-----------------------------------------| || tr -d '\n' |concatenate lines into one line | |---------------------+-----------------------------------------| || tr -d '\r' |remove CR | |---------------------+-----------------------------------------| || sed 's/^/# /' |add "#" to the start of each line | |---------------------+-----------------------------------------| || sed 's/\.ext//g' |remove ".ext" | |---------------------+-----------------------------------------| || sed -n -e 2p |print the second line | |---------------------+-----------------------------------------| || head -n 2 - |print the first 2 lines | |---------------------+-----------------------------------------| || tail -n 2 - |print the last 2 lines | +---------------------------------------------------------------+ A one-line shell script can loop over many files using find(1)     and xargs(1) to perform quite complicated tasks. See Section 10.1.5, “Idioms for the selection of files” and Section 9.4.9, “Repeating a command looping over files”. When using the shell interactive mode becomes too complicated,     please consider to write a shell script (see Section 12.1, “The shell script”). ---------------------------------------------------------------------     ^[1] Even the older vim can starts in the sane "nocompatible" mode by starting it with the "-N" option. Chapter 2. Debian package management Note This chapter is written assuming the latest stable release is codename: bookworm.     The data source of the APT system is collectively refered as the source list in this document . This can be defined anywhere in the "/etc/apt/sources.list" file, "/etc/apt/sources.list.d/ *.list" files, or "/etc/apt/sources.list.d/*.source" files. 2.1. Debian package management prerequisites 2.1.1. Debian package management system Debian is a volunteer organization which builds consistent     distributions of pre-compiled binary packages of free software and distributes them from its archive. The Debian archive is offered by many remote mirror sites for     access through HTTP and FTP methods. It is also available as CD-ROM/DVD.     The current Debian package management system which can utilize all these resources is Advanced Packaging Tool (APT). The Debian package management system, when used properly, offers     the user to install consistent sets of binary packages to the system from the archive. Currently, there are 74165 packages available for the amd64 architecture. The Debian package management system has a rich history and many     choices for the front end user program and back end archive access method to be used. Currently, we recommend the following. * apt(8) for all interactive command line operations, including package installation, removal and dist-upgrades. * apt-get(8) for calling Debian package management system from     scripts. It is also a fallback option when apt is not available (often with older Debian systems). * aptitude(8) for an interactive text interface to manage the installed packages and to search the available packages. Table 2.1. List of Debian package management tools +---------------------------------------------------------------+ | package |popcon|size| description | |-------------------+------+----+-------------------------------| |dpkg |V:912,|6388|low level package management | | |I:999 | |system for Debian (file based) | |-------------------+------+----+-------------------------------| | |V:865,| |APT front-end to manage | |apt |I:999 |4318|packages with CLI: apt/apt-get/| | | | |apt-cache | |-------------------+------+----+-------------------------------| | |V:48, | |APT front-end to interactively | |aptitude |I:253 |4389|manage packages with full | | | | |screen console: aptitude(8) | |-------------------+------+----+-------------------------------| |tasksel |V:34, |347 |APT front-end to install | | |I:980 | |selected tasks: tasksel(8) | |-------------------+------+----+-------------------------------| | |V:182,| |enhancement package for APT to | |unattended-upgrades|I:278 |301 |enable automatic installation | | | | |of security upgrades | |-------------------+------+----+-------------------------------|     |gnome-software |V:153,|3085|Software Center for GNOME (GUI | | |I:263 | |APT front-end) | |-------------------+------+----+-------------------------------| |synaptic |V:46, |7627|graphical package manager (GTK | | |I:375 | |APT front-end) | |-------------------+------+----+-------------------------------| | | | |APT utility programs: | |apt-utils |V:379,|1065|apt-extracttemplates(1), | | |I:998 | |apt-ftparchive(1), and | | | | |apt-sortpkgs(1) | |-------------------+------+----+-------------------------------| |apt-listchanges |V:358,|398 |package change history | | |I:872 | |notification tool | |-------------------+------+----+-------------------------------| |apt-listbugs |V:6, |477 |lists critical bugs before each| | |I:8 | |APT installation | |-------------------+------+----+-------------------------------| |apt-file |V:17, |89 |APT package searching utility —| | |I:67 | |command-line interface | |-------------------+------+----+-------------------------------| |apt-rdepends |V:0, |39 |recursively lists package | | |I:5 | |dependencies | +---------------------------------------------------------------+ 2.1.2. Package configuration     Here are some key points for package configuration on the Debian system. * The manual configuration by the system administrator is respected. In other words, the package configuration system makes no intrusive configuration for the sake of convenience. * Each package comes with its own configuration script with standardized user interface called debconf(7) to help initial installation process of the package. * Debian Developers try their best to make your upgrade experience flawless with package configuration scripts.     * Full functionalities of packaged software are available to the system administrator. But ones with security risks are disabled in the default installation. * If you manually activate a service with some security risks, you are responsible for the risk containment. * Esoteric configuration may be manually enabled by the system administrator. This may create interference with popular generic helper programs for the system configuration. 2.1.3. Basic precautions Warning     Do not install packages from random mixture of suites. It probably breaks the package consistency which requires deep system management knowledge, such as compiler ABI, library version, interpreter features, etc. The newbie Debian system administrator should stay with the     stable release of Debian while applying only security updates. Until you understand the Debian system very well, you should follow the following precautions. * Do not include testing or unstable in the source list. * Do not mix standard Debian with other non-Debian archives such as Ubuntu in the source list . * Do not create "/etc/apt/preferences". * Do not change default behavior of package management tools through configuration files without knowing their full impacts.     * Do not install random packages by "dpkg -i random_package". * Do not ever install random packages by "dpkg --force-all -i random_package". * Do not erase or alter files in "/var/lib/dpkg/". * Do not overwrite system files by installing software programs directly compiled from source. + Install them into "/usr/local" or "/opt", if needed. The non-compatible effects caused by violating above precautions     to the Debian package management system may leave your system unusable.     The serious Debian system administrator who runs mission critical servers, should use extra precautions. * Do not install any packages including security updates from Debian without thoroughly testing them with your particular configuration under safe conditions.     + You as the system administrator are responsible for your system in the end. + The long stability history of the Debian system is no guarantee by itself. 2.1.4. Life with eternal upgrades Caution     For your production server, the stable suite with the security updates is recommended. The same can be said for desktop PCs on which you can spend limited administration efforts.     Despite my warnings above, I know many readers of this document may wish to run the newer testing or unstable suites.     Enlightenment with the following saves a person from the eternal karmic struggle of upgrade hell and let him reach Debian nirvana.     This list is targeted for the self-administered Desktop environment. * Use the testing suite since it is practically the rolling release automatically managed by the Debian archive QA infrastructure such as the Debian continuous integration, the source only upload practices, and the library transition tracking. The packages in the testing suite are updated frequently enough to offer all the latest features.     * Set the codename corresponding to the testing suite ("trixie" during the bookworm-as-stable release cycle) in the source list. * Manually update this codename in the source list to the new one only after assessing situation by yourself for about a month after the major suite release. The Debian user and developer mailing list are good sources of information for this, too. The use of the unstable suite isn't recommended. The unstable suite is good for debugging packages as a developer but tends to     expose you to unnecessary risks for the normal Desktop usage. Even though the unstable suite of the Debian system looks very stable for most of the times, there have been some package problems and a few of them were not so trivial to resolve.     Here are some basic precautionary measure ideas to ensure quick and easy recovery from bugs in Debian packages. * Make the system dual bootable by installing the stable suite of the Debian system to another partition * Make the installation CD handy for the rescue boot     * Consider installing apt-listbugs to check the Debian Bug Tracking System (BTS) information before the upgrade * Learn the package system infrastructure enough to work around the problem Caution     If you can not do any one of these precautionary actions, you are probably not ready for the testing and unstable suites. 2.1.5. Debian archive basics Tip     Official policy of the Debian archive is defined at Debian Policy Manual, Chapter 2 - The Debian Archive.     Let's look into the Debian archive from a system user's perspective.     For a system user, the Debian archive is accessed using the APT system.     The APT system specifies its data source as the source list and it is described in sources.list(5).     For the bookworm system with the typical HTTP access, the source list in one-line-style as the following: deb http://deb.debian.org/debian/ bookworm main non-free-firmware contrib non-free deb-src http://deb.debian.org/debian/ bookworm main non-free-firmware contrib non-free     deb http://security.debian.org/debian-security bookworm-security main non-free-firmware contrib non-free deb-src http://security.debian.org/debian-security bookworm-security main non-free-firmware contrib non-free     Alternatively, the equivqlent source list in deb822-style as the following. Types: deb deb-src URIs: http://deb.debian.org/debian/ Suites: bookworm Components: main non-free-firmware contrib non-free     Types: deb deb-src URIs: http://security.debian.org/debian-security/ Suites: bookworm-security Components: main non-free-firmware contrib non-free     Key points of the source list are followings. * One-line-style format + It's definition files are in the "/etc/apt/sources.list" file and "/etc/apt/sources.list.d/*.list" files. + Each line defines the data source for the APT system. + The "deb" line defines for the binary packages. + The "deb-src" line defines for the source packages. + The 1st argument is the root URL of the Debian archive. + The 2nd argument is the distribution name using either the suite name or the codename. + The 3rd and following arguments are the list of valid archive area names of the Debian archive.     * Deb822-style format + It's definition files are in "/etc/apt/sources.list.d/ *.source" files. + Each block of lines separated by a blank line defines the data source for the APT system. + The "Types:" stanza defines the list of types such as "deb" and "deb-src". + The "URIs:" stanza defines the list of root URIs of the Debian archive. + The "Suites:" stanza defines the list of distribution names using either the suite name or the codename. + The "Components:" stanza defines the list of valid archive area names of the Debian archive. The definition for "deb-src" can safely be omitted if it is just     for aptitude which does not access source related meta data. It speeds up the updates of the archive meta data.     The URL can be "https://", "http://", "ftp://", "file://", ….     Lines starting with "#" are comments and ignored. Here, I tend to use codename "bookworm" or "trixie" instead of     suite name "stable" or "testing" to avoid surprises when the next stable is released. Tip If "sid" is used in the above example instead of "bookworm", the     "deb: http://security.debian.org/ …" line or its deb822 equivalent content for security updates in the source list is not required. This is because there is no security update archive for "sid" (unstable). Here is the list of URL of the Debian archive sites and suite     name or codename used in the configuration file after the bookworm release. Table 2.2. List of Debian archive sites +---------------------------------------------------------------------------------------------+ | archive URL | suite name | codename | purpose of repository | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Quasi-static stable | |deb.debian.org/ |stable |bookworm |release after extensive| |debian/ | | |checks | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Dynamic testing release| |deb.debian.org/ |testing |trixie |after decent checks and| |debian/ | | |short waits | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Dynamic unstable | |deb.debian.org/ |unstable |sid |release after minimal | |debian/ | | |checks and no waits | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Pre-release experiments| |deb.debian.org/ |experimental |N/A |by developers | |debian/ | | |(optional, only for | | | | |developer) | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Updates for the next |     |deb.debian.org/ |stable-proposed-updates|bookworm-proposed-updates|stable point release | |debian/ | | |(optional) | |-------------------+-----------------------+-------------------------+-----------------------| | | | |Subset of | |http:// | | |stable-proposed-updates| |deb.debian.org/ |stable-updates |bookworm-updates |suite needing urgent | |debian/ | | |updates such as | | | | |timezone data | | | | |(optional) | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Random collection of | |deb.debian.org/ |stable-backports |bookworm-backports |recompiled packages | |debian/ | | |mostly from the testing| | | | |release (optional) | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |Security updates for | |security.debian.org|stable-security |bookworm-security |the stable release | |/debian-security/ | | |(important) | |-------------------+-----------------------+-------------------------+-----------------------| |http:// | | |This isn't actively | |security.debian.org|testing-security |trixie-security |supported nor used by | |/debian-security/ | | |the security team | +---------------------------------------------------------------------------------------------+ Caution Only pure stable release with security updates provides the best stability. Running mostly stable release mixed with some packages     from testing or unstable release is riskier than running pure unstable release for library version mismatch etc. If you really need the latest version of some programs under stable release, please use packages from stable-updates and backports (see Section 2.7.4, “Updates and Backports”) services. These services must be used with extra care. Caution You should basically list only one of stable, testing, or unstable suites in the "deb" line. If you list any combination of     stable, testing, and unstable suites in the "deb" line, APT programs slow down while only the latest archive is effective. Multiple listing makes sense for these when the "/etc/apt/ preferences" file is used with clear objectives (see Section 2.7.7, “Tweaking candidate version with apt-pinning”). Tip     For the Debian system with the stable suite, it is a good idea to include the content with "http://security.debian.org/" in the source list to enable security updates as in the example above. Note The security bugs for the stable archive are fixed by the Debian security team. This activity has been quite rigorous and reliable. Those for the testing archive may be fixed by the     Debian testing security team. For several reasons, this activity is not as rigorous as that for stable and you may need to wait for the migration of fixed unstable packages. Those for the unstable archive are fixed by the individual maintainer. Actively maintained unstable packages are usually in a fairly good shape by leveraging latest upstream security fixes. See Debian security FAQ for how Debian handles security bugs. Table 2.3. List of Debian archive area +---------------------------------------------------------------+ | | number | | | area | of | criteria of package component | | |packages| | |-----------------+--------+------------------------------------| |main |72806 |DFSG compliant and no dependency to | | | |non-free | |-----------------+--------+------------------------------------|     | | |not DFSG compliant, firmware | |non-free-firmware|39 |required for reasonable system | | | |installation experience | |-----------------+--------+------------------------------------| |contrib |356 |DFSG compliant but having dependency| | | |to non-free | |-----------------+--------+------------------------------------| |non-free |964 |not DFSG compliant and not in | | | |non-free-firmware | +---------------------------------------------------------------+ Here the number of packages in the above is for the amd64     architecture. The main area provides the Debian system (see Section 2.1.6, “Debian is 100% free software”).     The Debian archive organization can be studied best by pointing your browser to the each archive URL appended with dists or pool. The distribution is referred by two ways, the suite or codename.     The word distribution is alternatively used as the synonym to the suite in many documentations. The relationship between the suite and the codename can be summarized as the following. Table 2.4. The relationship between suite and codename +---------------------------------------------------------------+ | Timing |suite = stable | suite = | suite = | | | | testing | unstable | |--------------------+---------------+-------------+------------|     |after the bookworm |codename = |codename = |codename = | |release |bookworm |trixie |sid | |--------------------+---------------+-------------+------------| |after the trixie |codename = |codename = |codename = | |release |trixie |forky |sid | +---------------------------------------------------------------+     The history of codenames are described in Debian FAQ: 6.2.1 Which other codenames have been used in the past? In the stricter Debian archive terminology, the word "section" is specifically used for the categorization of packages by the     application area. (Although, the word "main section" may sometimes be used to describe the Debian archive area named as "main".) Every time a new upload is done by a Debian developer (DD) to the     unstable archive (via incoming processing), the DD is required to ensure uploaded packages to be compatible with the latest set of packages in the latest unstable archive. If DD breaks this compatibility intentionally for important     library upgrade etc, there is usually announcement to the debian-devel mailing list etc. Before a set of packages are moved by the Debian archive maintenance script from the unstable archive to the testing archive, the archive maintenance script not only checks the     maturity (about 2-10 days old) and the status of the RC bug reports for the packages but also tries to ensure them to be compatible with the latest set of packages in the testing archive. This process makes the testing archive very current and usable. Through the gradual archive freeze process led by the release team, the testing archive is matured to make it completely consistent and bug free with some manual interventions. Then the     new stable release is created by assigning the codename for the old testing archive to the new stable archive and creating the new codename for the new testing archive. The initial contents of the new testing archive is exactly the same as that of the newly released stable archive.     Both the unstable and the testing archives may suffer temporary glitches due to several factors. * Broken package upload to the archive (mostly for unstable) * Delay of accepting the new packages to the archive (mostly for unstable)     * Archive synchronization timing issue (both for testing and unstable) * Manual intervention to the archive such as package removal (more for testing) etc.     So if you ever decide to use these archives, you should be able to fix or work around these kinds of glitches. Caution For about few months after a new stable release, most desktop users should use the stable archive with its security updates even if they usually use unstable or testing archives. For this transition period, both unstable and testing archives are not     good for most people. Your system is difficult to keep in good working condition with the unstable archive since it suffers surges of major upgrades for core packages. The testing archive is not useful either since it contains mostly the same content as the stable archive without its security support (Debian testing-security-announce 2008-12). After a month or so, unstable or testing archives may become useful if you are careful. Tip     When tracking the testing archive, a problem caused by a removed package is usually worked around by installing corresponding package from the unstable archive which is uploaded for bug fix.     See Debian Policy Manual for archive definitions. * "Sections" * "Priorities"     * "Base system" * "Essential packages" 2.1.6. Debian is 100% free software     Debian is 100% free software because of the followings: * Debian installs only free software by default to respect user's freedoms. * Debian provides only free software in main.     * Debian recommends running only free software from main. * No packages in main depend nor recommend packages in non-free nor non-free-firmware nor contrib.     Some people wonder if the following 2 facts contradict or not. * "Debian will remain 100% free". (First term of Debian Social Contract)     * Debian servers host some non-free-firmware, non-free and contrib packages.     These do not contradict, because of the followings. * The Debian system is 100% free and its packages are hosted by Debian servers in the main area.     * Packages outside of the Debian system are hosted by Debian servers in the non-free, non-free-firmware and contrib areas.     These are precisely explained in the 4th and 5th terms of Debian Social Contract: * Our priorities are our users and free software + We will be guided by the needs of our users and the free software community. We will place their interests first in our priorities. We will support the needs of our users for operation in many different kinds of computing environments. We will not object to non-free works that are intended to be used on Debian systems, or attempt to charge a fee to people who create or use such works. We will allow others to create distributions containing both the Debian system and other works, without any fee from us. In furtherance of these goals, we will provide an integrated system of high-quality materials with no legal restrictions that would prevent such uses of the system. * Works that do not meet our free software standards     + We acknowledge that some of our users require the use of works that do not conform to the Debian Free Software Guidelines. We have created "non-free", "non-free-firmware" and "contrib" areas in our archive for these works. The packages in these areas are not part of the Debian system, although they have been configured for use with Debian. We encourage CD manufacturers to read the licenses of the packages in these areas and determine if they can distribute the packages on their CDs. Thus, although non-free works are not a part of Debian, we support their use and provide infrastructure for non-free packages (such as our bug tracking system and mailing lists). The Debian official media may include firmware that is otherwise not part of the Debian system to enable use of Debian with hardware that requires such firmware. Note The actual text of the 5th term in the current Debian Social     Contract 1.2 is slightly different from the above text. This editorial deviation is intensional one to make this user document consistent without changing the real content of the Social Contract.     Users should be aware of the risks of using packages in the non-free, non-free-firmware and contrib areas: * lack of freedom for such software packages * lack of support from Debian on such software packages (Debian     can't support software properly without having access to its source code.) * contamination of your 100% free Debian system The Debian Free Software Guidelines are the free software standards for Debian. Debian interprets "software" in the widest     scope including document, firmware, logo, and artwork data in the package. This makes Debian's free software standards very strict ones.     Typical non-free, non-free-firmware and contrib packages include freely distributable packages of following types: * Document packages under GNU Free Documentation License with invariant sections such as ones for GCC and Make. (mostly found in the non-free/doc section.) * Firmware packages containing sourceless binary data such as     ones listed in Section 9.10.5, “Hardware drivers and firmware” as non-free-firmware. (mostly found in the non-free-firmware/kernel section.) * Game and font packages with restriction on commercial use and /or content modification. Please note that the number of non-free, non-free-firmware and contrib packages is less than 2% of that of main packages. Enabling access to the non-free, non-free-firmware and contrib     areas does not obscure the source of packages. Interactive full screen use of aptitude(8) provides you with full visibility and control over what packages are installed from which area to keep your system as free as you wish. 2.1.7. Package dependencies The Debian system offers a consistent set of binary packages     through its versioned binary dependency declaration mechanism in the control file fields. Here is a bit over simplified definition for them. * "Depends" + This declares an absolute dependency and all of the packages listed in this field must be installed at the same time or in advance. * "Pre-Depends" + This is like Depends, except that it requires completed installation of the listed packages in advance. * "Recommends" + This declares a strong, but not absolute, dependency. Most users would not want the package unless all of the packages listed in this field are installed. * "Suggests" + This declares a weak dependency. Many users of this package may benefit from installing packages listed in this field but can have reasonable functions without them.     * "Enhances" + This declares a weak dependency like Suggests but works in the opposite direction. * "Breaks" + This declares a package incompatibility usually with some version specification. Generally the resolution is to upgrade all of the packages listed in this field. * "Conflicts" + This declares an absolute incompatibility. All of the packages listed in this field must be removed to install this package. * "Replaces" + This is declared when files installed by this package replace files in the listed packages. * "Provides" + This is declared when this package provide all of the files and functionality in the listed packages. Note     Please note that defining "Provides", "Conflicts" and "Replaces" simultaneously to an virtual package is the sane configuration. This ensures that only one real package providing this virtual package can be installed at any one time. The official definition including source dependency can be found     in the Policy Manual: Chapter 7 - Declaring relationships between packages. 2.1.8. The event flow of the package management     Here is a summary of the simplified event flow of the package management by APT. * Update ("apt update", "aptitude update" or "apt-get update"): 1. Fetch archive metadata from remote archive 2. Reconstruct and update local metadata for use by APT * Upgrade ("apt upgrade" and "apt full-upgrade", or "aptitude safe-upgrade" and "aptitude full-upgrade", or "apt-get upgrade" and "apt-get dist-upgrade"): 1. Choose candidate version which is usually the latest available version for all installed packages (see Section 2.7.7, “Tweaking candidate version with apt-pinning” for exception) 2. Make package dependency resolution 3. Fetch selected binary packages from remote archive if candidate version is different from installed version 4. Unpack fetched binary packages 5. Run preinst script 6. Install binary files 7. Run postinst script * Install ("apt install …", aptitude install …" or "apt-get install …"): 1. Choose packages listed on the command line 2. Make package dependency resolution     3. Fetch selected binary packages from remote archive 4. Unpack fetched binary packages 5. Run preinst script 6. Install binary files 7. Run postinst script * Remove ("apt remove …", "aptitude remove …" or "apt-get remove …"): 1. Choose packages listed on the command line 2. Make package dependency resolution 3. Run prerm script 4. Remove installed files except configuration files 5. Run postrm script * Purge ("apt purge", "aptitude purge …" or "apt-get purge …"): 1. Choose packages listed on the command line 2. Make package dependency resolution 3. Run prerm script 4. Remove installed files including configuration files 5. Run postrm script     Here, I intentionally skipped technical details for the sake of big picture. 2.1.9. First response to package management troubles You should read the fine official documentation. The first document to read is the Debian specific "/usr/share/doc/     package_name/README.Debian". Other documentation in "/usr/share/ doc/package_name/" should be consulted too. If you set shell as Section 1.4.2, “Customizing bash”, type the following. $ cd package_name     $ pager README.Debian $ mc     You may need to install the corresponding documentation package named with "-doc" suffix for detailed information. If you are experiencing problems with a specific package, make     sure to check out the Debian bug tracking system (BTS) sites, first. Table 2.5. List of key web site to resolving problems with a specific package +---------------------------------------------------------------+ | web site | command | |-----------------------------+---------------------------------| |Home page of the Debian bug |sensible-browser "https:// |     |tracking system (BTS) |bugs.debian.org/" | |-----------------------------+---------------------------------| |The bug report of a known |sensible-browser "https:// | |package name |bugs.debian.org/package_name" | |-----------------------------+---------------------------------| |The bug report of known bug |sensible-browser "https:// | |number |bugs.debian.org/bug_number" | +---------------------------------------------------------------+     Search Google with search words including "site:debian.org", "site:wiki.debian.org", "site:lists.debian.org", etc.     When you file a bug report, please use reportbug(1) command. 2.1.10. How to pick Debian packages When you encounter more than 2 similar packages and wonder which     one to install without "trial and error" efforts, you should use some common sense. I consider following points are good indications of preferred packages. * Essential: yes > no * Area: main > contrib > non-free * Priority: required > important > standard > optional > extra * Tasks: packages listed in tasks such as "Desktop environment" * Packages selected by the dependency package (e.g., gcc-10 by gcc)     * Popcon: higher in the vote and install number * Changelog: regular updates by the maintainer * BTS: No RC bugs (no critical, no grave, and no serious bugs) * BTS: responsive maintainer to bug reports * BTS: higher number of the recently fixed bugs * BTS: lower number of remaining non-wishlist bugs Debian being a volunteer project with distributed development     model, its archive contains many packages with different focus and quality. You must make your own decision what to do with them. 2.1.11. How to cope with conflicting requirements Whatever suite of Debian system you may decide to use, you may still wish to run versions of programs which aren't available in     that suite. Even if you find binary packages of such programs in other Debian suites or in other non-Debian resources, their requirements may conflict with your current Debian system. Although you can tweak package management system with apt-pinning technique etc. as described in Section 2.7.7, “Tweaking candidate     version with apt-pinning” to instal such out-of-sync binary packages, such tweaking approaches have only limited use cases since they may break those programs and your system. Before brutally installing such out-of-sync packages, you should     seek all available alternative safer technical solutions which are compatible with your current Debian syetem. * Install such programs using corresponding sandboxed upstream binary packages (see Section 7.7, “Sandbox”). + Many mostly GUI programs such as LibreOffice and GNOME applications are available as Flatpak, Snap, or AppImage packages. * Create a chroot or similar environment and run such programs in it (see Section 9.11, “Virtualized system”).     + CLI commands can be executed easily under its compatible chroot (see Section 9.11.4, “Chroot system”). + Multiple full desktop environments can be tried easily without reboot (see Section 9.11.5, “Multiple desktop systems”). * Build desired versions of binary packages which are compatible with your current Debian syetem by yourself. + This is a non-trivial task (see Section 2.7.13, “Porting a package to the stable system”). 2.2. Basic package management operations Repository based package management operations on the Debian     system can be performed by many APT-based package management tools available on the Debian system. Here, we explain 3 basic package management tools: apt, apt-get / apt-cache and aptitude. For the package management operation which involves package     installation or updates package metadata, you need to have root privilege. 2.2.1. apt vs. apt-get / apt-cache vs. aptitude     Although aptitude is a very nice interactive tool which the author mainly uses, you should know some cautionary facts: * The aptitude command is not recommended for the release-to-release system upgrade on the stable Debian system after the new release. + The use of "apt full-upgrade" or "apt-get dist-upgrade" is recommended for it. See Bug #411280. * The aptitude command sometimes suggests mass package removals for the system upgrade on the testing or unstable Debian system.     + This situation has frightened many system administrators. Don't panic. + This seems to be caused mostly by the version skew among packages depended or recommended by a meta-package such as gnome-core. + This can be resolved by selecting "Cancel pending actions" in the aptitude command menu, exiting aptitude, and using "apt full-upgrade".     The apt-get and apt-cache commands are the most basic APT-based package management tools. * apt-get and apt-cache offer only the commandline user interface. * apt-get is most suitable for the major system upgrade between releases, etc. * apt-get offers a robust package dependency resolver.     * apt-get is less demanding on hardware resources. It consumes less memory and runs faster. * apt-cache offers a standard regex based search on the package name and description. * apt-get and apt-cache can manage multiple versions of packages using /etc/apt/preferences but it is quite cumbersome. The apt command is a high-level commandline interface for package management. It is basically a wrapper of apt-get, apt-cache and     similar commands, originally intended as an end-user interface and enables some options better suited for interactive usage by default. * apt provides a friendly progress bar when installing packages using apt install.     * apt will remove cached .deb packages by default after sucessful installation of downloaded packages. Tip     Users are recommended to use the new apt(8) command for interactive usage and use the apt-get(8) and apt-cache(8) commands in the shell script.     The aptitude command is the most versatile APT-based package management tool. * aptitude offers the fullscreen interactive text user interface. * aptitude offers the commandline user interface, too. * aptitude is most suitable for the daily interactive package management such as inspecting installed packages and searching available packages.     * aptitude is more demanding on hardware resources. It consumes more memory and runs slower. * aptitude offers an enhanced regex based search on all of the package metadata. * aptitude can manage multiple versions of packages without using /etc/apt/preferences and it is quite intuitive. 2.2.2. Basic package management operations with the commandline     Here are basic package management operations with the commandline using apt(8), aptitude(8) and apt-get(8) /apt-cache(8). Table 2.6. Basic package management operations with the commandline using apt(8), aptitude(8) and apt-get(8) /apt-cache (8) +---------------------------------------------------------------+ | | aptitude | apt-get/ | | | apt syntax | syntax | apt-cache | description | | | | syntax | | |------------------+------------+------------+------------------| |apt update |aptitude |apt-get |update package | | |update |update |archive metadata | |------------------+------------+------------+------------------| | | | |install candidate | |apt install foo |aptitude |apt-get |version of "foo" | | |install foo |install foo |package with its | | | | |dependencies | |------------------+------------+------------+------------------| | | | |install candidate | | |aptitude |apt-get |version of | |apt upgrade |safe-upgrade|upgrade |installed packages| | | | |without removing | | | | |any other packages| |------------------+------------+------------+------------------| | | | |install candidate | | | | |version of | |apt full-upgrade |aptitude |apt-get |installed packages| | |full-upgrade|dist-upgrade|while removing | | | | |other packages if | | | | |needed | |------------------+------------+------------+------------------| | | | |remove "foo" | | |aptitude |apt-get |package while | |apt remove foo |remove foo |remove foo |leaving its | | | | |configuration | | | | |files | |------------------+------------+------------+------------------| | | | |remove | |apt autoremove |N/A |apt-get |auto-installed | | | |autoremove |packages which are|     | | | |no longer required| |------------------+------------+------------+------------------| | | | |purge "foo" | |apt purge foo |aptitude |apt-get |package with its | | |purge foo |purge foo |configuration | | | | |files | |------------------+------------+------------+------------------| | | | |clear out the | | |aptitude |apt-get |local repository | |apt clean |clean |clean |of retrieved | | | | |package files | | | | |completely | |------------------+------------+------------+------------------| | | | |clear out the | | |aptitude |apt-get |local repository | |apt autoclean |autoclean |autoclean |of retrieved | | | | |package files for | | | | |outdated packages | |------------------+------------+------------+------------------| | |aptitude |apt-cache |display detailed | |apt show foo |show foo |show foo |information about | | | | |"foo" package | |------------------+------------+------------+------------------| |apt search regex |aptitude |apt-cache |search packages | | |search regex|search regex|which match regex | |------------------+------------+------------+------------------| | | | |explain the reason| |N/A |aptitude why|N/A |why regex matching| | |regex | |packages should be| | | | |installed | |------------------+------------+------------+------------------| | |aptitude | |explain the reason| |N/A |why-not |N/A |why regex matching| | |regex | |packages can not | | | | |be installed | |------------------+------------+------------+------------------| |apt list |aptitude |apt-mark |list manually | |--manual-installed|search '~i! |showmanual |installed packages| | |~M' | | | +---------------------------------------------------------------+     apt / apt-get and aptitude can be mixed without major troubles. The "aptitude why regex" can list more information by "aptitude     -v why regex". Similar information can be obtained by "apt rdepends package" or "apt-cache rdepends package". When aptitude command is started in the commandline mode and     faces some issues such as package conflicts, you can switch to the full screen interactive mode by pressing "e"-key later at the prompt. Note Although the aptitude command comes with rich features such as     its enhanced package resolver, this complexity has caused (or may still causes) some regressions such as Bug #411123, Bug #514930, and Bug #570377. In case of doubt, please use the apt, apt-get and apt-cache commands over the aptitude command.     You may provide command options right after "aptitude". Table 2.7. Notable command options for aptitude(8) +---------------------------------------------------------------+ | command | description | | option | | |----------+----------------------------------------------------|     |-s |simulate the result of the command | |----------+----------------------------------------------------| |-d |download only but no install/upgrade | |----------+----------------------------------------------------| |-D |show brief explanations before the automatic | | |installations and removals | +---------------------------------------------------------------+     See aptitude(8) and "aptitude user's manual" at "/usr/share/doc/ aptitude/README" for more. 2.2.3. Interactive use of aptitude     For the interactive package management, you start aptitude in interactive mode from the console shell prompt as follows.     $ sudo aptitude -u Password: This updates the local copy of the archive information and     display the package list in the full screen with menu. Aptitude places its configuration at "~/.aptitude/config". Tip     If you want to use root's configuration instead of user's one, use "sudo -H aptitude …" instead of "sudo aptitude …" in the above expression. Tip     Aptitude automatically sets pending actions as it is started interactively. If you do not like it, you can reset it from menu: "Action" → "Cancel pending actions". 2.2.4. Key bindings of aptitude Notable key strokes to browse status of packages and to set     "planned action" on them in this full screen mode are the following. Table 2.8. List of key bindings for aptitude +---------------------------------------------------------------+ | key | key binding | |-------------------+-------------------------------------------| |F10 or Ctrl-t |menu | |-------------------+-------------------------------------------| |? |display help for keystroke (more complete | | |listing) | |-------------------+-------------------------------------------| |F10 → Help → User's|display User's Manual | |Manual | | |-------------------+-------------------------------------------| |u |update package archive information | |-------------------+-------------------------------------------| |+ |mark the package for the upgrade or the | | |install | |-------------------+-------------------------------------------| |- |mark the package for the remove (keep | | |configuration files) | |-------------------+-------------------------------------------| |_ |mark the package for the purge (remove | | |configuration files) |     |-------------------+-------------------------------------------| |= |place the package on hold | |-------------------+-------------------------------------------| |U |mark all upgradable packages (function as | | |full-upgrade) | |-------------------+-------------------------------------------| |g |start downloading and installing selected | | |packages | |-------------------+-------------------------------------------| |q |quit current screen and save changes | |-------------------+-------------------------------------------| |x |quit current screen and discard changes | |-------------------+-------------------------------------------| |Enter |view information about a package | |-------------------+-------------------------------------------| |C |view a package's changelog | |-------------------+-------------------------------------------| |l |change the limit for the displayed packages| |-------------------+-------------------------------------------| |/ |search for the first match | |-------------------+-------------------------------------------| |\ |repeat the last search | +---------------------------------------------------------------+ The file name specification of the command line and the menu prompt after pressing "l" and "//" take the aptitude regex as     described below. Aptitude regex can explicitly match a package name using a string started by "~n" and followed by the package name. Tip     You need to press "U" to get all the installed packages upgraded to the candidate version in the visual interface. Otherwise only the selected packages and certain packages with versioned dependency to them are upgraded to the candidate version. 2.2.5. Package views under aptitude     In the interactive full screen mode of aptitude(8), packages in the package list are displayed as the next example.     idA libsmbclient -2220kB 3.0.25a-1 3.0.25a-2     Here, this line means from the left as the following. * The "current state" flag (the first letter) * The "planned action" flag (the second letter) * The "automatic" flag (the third letter)     * The Package name * The change in disk space usage attributed to "planned action" * The current version of the package * The candidate version of the package Tip     The full list of flags are given at the bottom of Help screen shown by pressing "?". The candidate version is chosen according to the current local     preferences (see apt_preferences(5) and Section 2.7.7, “Tweaking candidate version with apt-pinning”).     Several types of package views are available under the menu "Views". Table 2.9. List of views for aptitude +---------------------------------------------------------------+ | view | description of view | |---------------+-----------------------------------------------| |Package View |see Table 2.10, “The categorization of standard| | |package views” (default) | |---------------+-----------------------------------------------| |Audit |list packages which are recommended by some | |Recommendations|installed packages but not yet installed |     |---------------+-----------------------------------------------| |Flat Package |list packages without categorization (for use | |List |with regex) | |---------------+-----------------------------------------------| |Debtags Browser|list packages categorized according to their | | |debtags entries | |---------------+-----------------------------------------------| |Source Package |list packages grouped by source packages | |View | | +---------------------------------------------------------------+ Note     Please help us improving tagging packages with debtags!     The standard "Package View" categorizes packages somewhat like dselect with few extra features. Table 2.10. The categorization of standard package views +---------------------------------------------------------------+ | category | description of view | |----------------------+----------------------------------------| |Upgradable Packages |list packages organized as section → | | |area → package | |----------------------+----------------------------------------| |New Packages |, , | |----------------------+----------------------------------------| |Installed Packages |, , |     |----------------------+----------------------------------------| |Not Installed Packages|, , | |----------------------+----------------------------------------| |Obsolete and Locally |, , | |Created Packages | | |----------------------+----------------------------------------| |Virtual Packages |list packages with the same function | |----------------------+----------------------------------------| |Tasks |list packages with different functions | | |generally needed for a task | +---------------------------------------------------------------+ Tip     Tasks view can be used to cherry pick packages for your task. 2.2.6. Search method options with aptitude     Aptitude offers several options for you to search packages using its regex formula. * Shell commandline: + "aptitude search 'aptitude_regex'" to list installation status, package name and short description of matching packages + "aptitude show 'package_name'" to list detailed description of the package     * Interactive full screen mode: + "l" to limit package view to matching packages + "/" for search to a matching package + "\" for backward search to a matching package + "n" for find-next + "N" for find-next (backward) Tip     The string for package_name is treated as the exact string match to the package name unless it is started explicitly with "~" to be the regex formula. 2.2.7. The aptitude regex formula The aptitude regex formula is mutt-like extended ERE (see     Section 1.6.2, “Regular expressions”) and the meanings of the aptitude specific special match rule extensions are as follows. Table 2.11. List of the aptitude regex formula +-----------------------------------------------------------------+ |description of| | | the extended | regex formula | | match rule | | |--------------+--------------------------------------------------| |match on |~nregex_name | |package name | | |--------------+--------------------------------------------------| |match on |~dregex_description | |description | | |--------------+--------------------------------------------------| |match on task |~tregex_task | |name | | |--------------+--------------------------------------------------| |match on |~Gregex_debtag | |debtag | | |--------------+--------------------------------------------------| |match on |~mregex_maintainer | |maintainer | | |--------------+--------------------------------------------------| |match on | | |package |~sregex_section | |section | | |--------------+--------------------------------------------------| |match on | | |package |~Vregex_version | |version | | |--------------+--------------------------------------------------| |match archive |~A{bookworm,trixie,sid} | |--------------+--------------------------------------------------| |match origin |~O{debian,…} | |--------------+--------------------------------------------------| |match priority|~p{extra,important,optional,required,standard} | |--------------+--------------------------------------------------| |match | | |essential |~E | |packages | | |--------------+--------------------------------------------------| |match virtual |~v | |packages | | |--------------+--------------------------------------------------| |match new |~N | |packages | | |--------------+--------------------------------------------------| |match with |~a | |pending action|{install,upgrade,downgrade,remove,purge,hold,keep}| |--------------+--------------------------------------------------| |match | | |installed |~i | |packages | | |--------------+--------------------------------------------------| |match | | |installed | | |packages with |~M | |A-mark (auto | | |installed | | |packages) | | |--------------+--------------------------------------------------| |match | | |installed | | |packages | | |without A-mark|~i!~M | |(administrator| | |selected | | |packages) | | |--------------+--------------------------------------------------| |match | | |installed and |~U | |upgradable | | |packages | | |--------------+--------------------------------------------------| |match removed | | |but not purged|~c | |packages | | |--------------+--------------------------------------------------| |match removed,| | |purged or |~g | |can-be-removed| |     |packages | | |--------------+--------------------------------------------------| |match packages| | |declaring a |~b | |broken | | |dependency | | |--------------+--------------------------------------------------| |match packages| | |declaring | | |broken |~Btype | |dependency of | | |type | | |--------------+--------------------------------------------------| |match pattern | | |packages | | |declaring |~D[type:]pattern | |dependency of | | |type | | |--------------+--------------------------------------------------| |match pattern | | |packages | | |declaring |~DB[type:]pattern | |broken | | |dependency of | | |type | | |--------------+--------------------------------------------------| |match packages| | |to which the | | |pattern | | |matching |~R[type:]pattern | |package | | |declares | | |dependency | | |type | | |--------------+--------------------------------------------------| |match packages| | |to which the | | |pattern | | |matching | | |package |~RB[type:]pattern | |declares | | |broken | | |dependency | | |type | | |--------------+--------------------------------------------------| |match packages| | |to which some | | |other |~R~i | |installed | | |packages | | |depend on | | |--------------+--------------------------------------------------| |match packages| | |to which no | | |other |!~R~i | |installed | | |packages | | |depend on | | |--------------+--------------------------------------------------| |match packages| | |to which some | | |other | | |installed |~R~i|~Rrecommends:~i | |packages | | |depend or | | |recommend on | | |--------------+--------------------------------------------------| |match pattern | | |package with |~S filter pattern | |filtered | | |version | | |--------------+--------------------------------------------------| |match all | | |packages |~T | |(true) | | |--------------+--------------------------------------------------| |match no | | |packages |~F | |(false) | | +-----------------------------------------------------------------+ * The regex part is the same ERE as the one used in typical Unix-like text tools using "^", ".*", "$" etc. as in egrep (1), awk(1) and perl(1).     * The dependency type is one of (depends, predepends, recommends, suggests, conflicts, replaces, provides) specifying the package interrelationship. * The default dependency type is "depends". Tip     When regex_pattern is a null string, place "~T" immediately after the command.     Here are some short cuts. * "~Pterm" == "~Dprovides:term"     * "~Cterm" == "~Dconflicts:term" * "…~W term" == "(…|term)" Users familiar with mutt pick up quickly, as mutt was the     inspiration for the expression syntax. See "SEARCHING, LIMITING, AND EXPRESSIONS" in the "User's Manual" "/usr/share/doc/aptitude/ README". Note With the lenny version of aptitude(8), the new long form syntax     such as "?broken" may be used for regex matching in place for its old short form equivalent "~b". Now space character " " is considered as one of the regex terminating character in addition to tilde character "~". See "User's Manual" for the new long form syntax. 2.2.8. Dependency resolution of aptitude The selection of a package in aptitude not only pulls in packages which are defined in its "Depends:" list but also defined in the     "Recommends:" list if the menu "F10 → Options → Preferences → Dependency handling" is set accordingly. These auto installed packages are removed automatically if they are no longer needed under aptitude. The flag controlling the "auto install" behavior of the aptitude     command can also be manipulated using the apt-mark(8) command from the apt package. 2.2.9. Package activity logs     You can check package activity history in the log files. Table 2.12. The log files for package activities +---------------------------------------------------------------+ | file | content | |------------------+--------------------------------------------| |/var/log/dpkg.log |Log of dpkg level activity for all package |     | |activities | |------------------+--------------------------------------------| |/var/log/apt/ |Log of generic APT activity | |term.log | | |------------------+--------------------------------------------| |/var/log/aptitude |Log of aptitude command activity | +---------------------------------------------------------------+ In reality, it is not so easy to get meaningful understanding     quickly out from these logs. See Section 9.3.9, “Recording changes in configuration files” for easier way. 2.3. Examples of aptitude operations     Here are few examples of aptitude(8) operations. 2.3.1. Seeking interesting packages     You can seek packages which satisfy your needs with aptitude from the package description or from the list under "Tasks". 2.3.2. Listing packages with regex matching on package names     The following command lists packages with regex matching on package names. $ aptitude search '~n(pam|nss).*ldap'     p libnss-ldap - NSS module for using LDAP as a naming service p libpam-ldap - Pluggable Authentication Module allowing LDAP interfaces     This is quite handy for you to find the exact name of a package. 2.3.3. Browsing with the regex matching The regex "~dipv6" in the "New Flat Package List" view with "l"     prompt, limits view to packages with the matching description and let you browse their information interactively. 2.3.4. Purging removed packages for good     You can purge all remaining configuration files of removed packages.     Check results of the following command.     # aptitude search '~c'     If you think listed packages are OK to be purged, execute the following command.     # aptitude purge '~c'     You may want to do the similar in the interactive mode for fine grained control. You provide the regex "~c" in the "New Package View" view with     "l" prompt. This limits the package view only to regex matched packages, i.e., "removed but not purged". All these regex matched packages can be shown by pressing "[" at top level headings. Then you press "_" at top level headings such as "Not Installed     Packages". Only regex matched packages under the heading are marked to be purged by this. You can exclude some packages to be purged by pressing "=" interactively for each of them.     This technique is quite handy and works for many other command keys. 2.3.5. Tidying auto/manual install status     Here is how I tidy auto/manual install status for packages (after using non-aptitude package installer etc.). 1. Start aptitude in interactive mode as root. 2. Type "u", "U", "f" and "g" to update and upgrade package list and packages. 3. Type "l" to enter the package display limit as "~i(~R~i| ~Rrecommends:~i)" and type "M" over "Installed Packages" as auto installed. 4. Type "l" to enter the package display limit as "~prequired| ~pimportant|~pstandard|~E" and type "m" over "Installed Packages" as manual installed. 5. Type "l" to enter the package display limit as "~i!~M" and remove unused package by typing "-" over each of them after exposing them by typing "[" over "Installed Packages".     6. Type "l", to enter the package display limit as "~i"; then type "m" over "Tasks", to mark that packages as manual installed. 7. Exit aptitude. 8. Start "apt-get -s autoremove|less" as root to check what are not used. 9. Restart aptitude in interactive mode and mark needed packages as "m". 10. Restart "apt-get -s autoremove|less" as root to recheck REMOVED contain only expected packages. 11. Start "apt-get autoremove|less" as root to autoremove unused packages.     The "m" action over "Tasks" is an optional one to prevent mass package removal situation in future. 2.3.6. System wide upgrade Note When moving to a new release etc, you should consider to perform a clean installation of new system even though Debian is     upgradable as described below. This provides you a chance to remove garbages collected and exposes you to the best combination of latest packages. Of course, you should make a full backup of system to a safe place (see Section 10.2, “Backup and recovery”) before doing this. I recommend to make a dual boot configuration using different partition to have the smoothest transition. You can perform system wide upgrade to a newer release by     changing contents of the source list pointing to a new release and running the "apt update; apt dist-upgrade" command. To upgrade from stable to testing or unstable during the     bookworm-as-stable release cycle, you replace "bookworm" in the source list example of Section 2.1.5, “Debian archive basics” with "trixie" or "sid". In reality, you may face some complications due to some package transition issues, mostly due to package dependencies. The larger     the difference of the upgrade, the more likely you face larger troubles. For the transition from the old stable to the new stable after its release, you can read its new Release Notes and follow the exact procedure described in it to minimize troubles. When you decide to move from stable to testing before its formal     release, there are no Release Notes to help you. The difference between stable and testing could have grown quite large after the previous stable release and makes upgrade situation complicated. You should make precautionary moves for the full upgrade while     gathering latest information from mailing list and using common senses. 1. Read previous "Release Notes". 2. Backup entire system (especially data and configuration information). 3. Have bootable media handy for broken bootloader. 4. Inform users on the system well in advance. 5. Record upgrade activity with script(1). 6. Apply "unmarkauto" to required packages, e.g., "aptitude unmarkauto vim", to prevent removal. 7. Minimize installed packages to reduce chance of package     conflicts, e.g., remove desktop task packages. 8. Remove the "/etc/apt/preferences" file (disable apt-pinning). 9. Try to upgrade step wise: oldstable → stable → testing → unstable. 10. Update the source list to point to new archive only and run "aptitude update". 11. Install, optionally, new core packages first, e.g., "aptitude install perl". 12. Run the "apt-get -s dist-upgrade" command to assess impact. 13. Run the "apt-get dist-upgrade" command at last. Caution     It is not wise to skip major Debian release when upgrading between stable releases. Caution     In previous "Release Notes", GCC, Linux Kernel, initrd-tools, Glibc, Perl, APT tool chain, etc. have required some special attention for system wide upgrade.     For daily upgrade in unstable, see Section 2.4.3, “Safeguarding for package problems”. 2.4. Advanced package management operations 2.4.1. Advanced package management operations with commandline     Here are list of other package management operations for which aptitude is too high-level or lacks required functionalities. Table 2.13. List of advanced package management operations +---------------------------------------------------------------+ | command | action | |---------------------------------------------+-----------------| | |list status of an| |COLUMNS=120 dpkg -l package_name_pattern |installed package| | |for the bug | | |report | |---------------------------------------------+-----------------| | |list contents of | |dpkg -L package_name |an installed | | |package | |---------------------------------------------+-----------------| |dpkg -L package_name | egrep '/usr/share/man/|list manpages for| |man.*/.+' |an installed | | |package | |---------------------------------------------+-----------------| | |list installed | |dpkg -S file_name_pattern |packages which | | |have matching | | |file name | |---------------------------------------------+-----------------| | |list packages in | |apt-file search file_name_pattern |archive which | | |have matching | | |file name | |---------------------------------------------+-----------------| | |list contents of | |apt-file list package_name_pattern |matching packages| | |in archive | |---------------------------------------------+-----------------| |dpkg-reconfigure package_name |reconfigure the | | |exact package | |---------------------------------------------+-----------------| | |reconfigure the | |dpkg-reconfigure -plow package_name |exact package | | |with the most | | |detailed question| |---------------------------------------------+-----------------| | |reconfigure | |configure-debian |packages from the| | |full screen menu | |---------------------------------------------+-----------------| | |audit system for | |dpkg --audit |partially | | |installed | | |packages | |---------------------------------------------+-----------------| | |configure all | |dpkg --configure -a |partially | | |installed | | |packages | |---------------------------------------------+-----------------| | |show available | | |version, | |apt-cache policy binary_package_name |priority, and | | |archive | | |information of a | | |binary package | |---------------------------------------------+-----------------| | |show available | |apt-cache madison package_name |version, archive | | |information of a | | |package | |---------------------------------------------+-----------------| | |show source | |apt-cache showsrc binary_package_name |package | | |information of a | | |binary package | |---------------------------------------------+-----------------|     | |install required | |apt-get build-dep package_name |packages to build| | |package | |---------------------------------------------+-----------------| | |install required | |aptitude build-dep package_name |packages to build| | |package | |---------------------------------------------+-----------------| | |download a source| |apt-get source package_name |(from standard | | |archive) | |---------------------------------------------+-----------------| | |download a source| |dget URL for dsc file |packages (from | | |other archive) | |---------------------------------------------+-----------------| | |build a source | | |tree from a set | | |of source | |dpkg-source -x package_name_version- |packages | |debian.revision.dsc |("*.orig.tar.gz" | | |and | | |"*.debian.tar.gz"| | |/"*.diff.gz") | |---------------------------------------------+-----------------| | |build package(s) | |debuild binary |from a local | | |source tree | |---------------------------------------------+-----------------| | |build a kernel | |make-kpkg kernel_image |package from a | | |kernel source | | |tree | |---------------------------------------------+-----------------| | |build a kernel | | |package from a | |make-kpkg --initrd kernel_image |kernel source | | |tree with | | |initramfs enabled| |---------------------------------------------+-----------------| |dpkg -i package_name_version-debian.revision_|install a local | |arch.deb |package to the | | |system | |---------------------------------------------+-----------------| | |install a local | | |package to the | |apt install /path/to/package_filename.deb |system, meanwhile| | |try to resolve | | |dependency | | |automatically | |---------------------------------------------+-----------------| |debi package_name_version-debian.revision_ |install local | |arch.dsc |package(s) to the| | |system | |---------------------------------------------+-----------------| | |save dpkg level | |dpkg --get-selections '*' >selection.txt |package selection| | |state information| |---------------------------------------------+-----------------| | |set dpkg level | |dpkg --set-selections /etc/alternatives/vi $ sudo update-alternatives --display vi ... $ sudo update-alternatives --config vi     Selection Command ---------------------------------------------- 1 /usr/bin/vim *+ 2 /usr/bin/nvi Enter to keep the default[*], or type selection number: 1 The Debian alternatives system keeps its selection as symlinks in     "/etc/alternatives/". The selection process uses corresponding file in "/var/lib/dpkg/alternatives/". 2.5.11. The dpkg-statoverride command Stat overrides provided by the dpkg-statoverride(8) command are a     way to tell dpkg(1) to use a different owner or mode for a file when a package is installed. If "--update" is specified and file exists, it is immediately set to the new owner and mode. Caution     The direct alteration of owner or mode for a file owned by the package using chmod or chown commands by the system administrator is reset by the next upgrade of the package. Note     I use the word file here, but in reality this can be any filesystem object that dpkg handles, including directories, devices, etc. 2.5.12. The dpkg-divert command File diversions provided by the dpkg-divert(8) command are a way of forcing dpkg(1) not to install a file into its default     location, but to a diverted location. The use of dpkg-divert is meant for the package maintenance scripts. Its casual use by the system administrator is deprecated. 2.6. Recovery from a broken system     When running testing or unstable system, the administrator is expected to recover from broken package management situation. Caution     Some methods described here are high risk actions. You have been warned! 2.6.1. Failed installation due to missing dependencies If you force to install a package by "sudo dpkg -i ..." to a     system without all dependency packages installed, the package installation will fail as partially installed.     You should install all dependency packages using APT-system or "sudo dpkg -i ...".     Then, configure all partially installed packages with the following command.     # dpkg --configure -a 2.6.2. Caching errors of the package data     Caching errors of the package data cause intriguing errors, such as "GPG error: ... invalid: BADSIG ..." with APT. You should remove all cached data by "sudo rm -rf /var/lib/apt/*     " and try again. (If apt-cacher-ng is used, you should also run "sudo rm -rf /var/cache/apt-cacher-ng/* ".) 2.6.3. Incompatibility with old user configuration If a desktop GUI program experienced instability after significant upstream version upgrade, you should suspect     interference with old local configuration files created by it. If it is stable under a newly created user account, this hypothesis is confirmed. (This is a bug of packaging and usually avoided by the packager.) To recover stability, you should move corresponding local     configuration files and restart the GUI program. You may need to read old configuration file contents to recover configuration information later. (Do not erase them too quickly.) 2.6.4. Different packages with overlapped files Archive level package management systems, such as aptitude(8) or     apt-get(1), do not even try to install packages with overlapped files using package dependencies (see Section 2.1.7, “Package dependencies”). Errors by the package maintainer or deployment of inconsistently mixed source of archives (see Section 2.7.6, “Packages from mixed source of archives without apt-pinning”) by the system     administrator may create a situation with incorrectly defined package dependencies. When you install a package with overlapped files using aptitude(8) or apt-get(1) under such a situation, dpkg(1) which unpacks package ensures to return error to the calling program without overwriting existing files. Caution     The use of third party packages introduces significant system risks via maintainer scripts which are run with root privilege and can do anything to your system. The dpkg(1) command only protects against overwriting by the unpacking.     You can work around such broken installation by removing the old offending package, old-package, first.     $ sudo dpkg -P old-package 2.6.5. Fixing broken package script When a command in the package script returns error for some reason and the script exits with error, the package management     system aborts their action and ends up with partially installed packages. When a package contains bugs in its removal scripts, the package may become impossible to remove and quite nasty.     For the package script problem of "package_name", you should look into following package scripts. * "/var/lib/dpkg/info/package_name.preinst" * "/var/lib/dpkg/info/package_name.postinst"     * "/var/lib/dpkg/info/package_name.prerm" * "/var/lib/dpkg/info/package_name.postrm"     Edit the offending package script from the root using following techniques. * disable the offending line by preceding "#"     * force to return success by appending the offending line with "|| true"     Then, follow Section 2.6, “Recovery from a broken system”. 2.6.6. Rescue with the dpkg command Since dpkg is very low level package tool, it can function under     the very bad situation such as unbootable system without network connection. Let's assume foo package was broken and needs to be replaced. You may still find cached copies of older bug free version of foo package in the package cache directory: "/var/cache/apt/archives/     ". (If not, you can download it from archive of https:// snapshot.debian.org/ or copy it from package cache of a functioning machine.)     If you can boot the system, you may install it by the following command.     # dpkg -i /path/to/foo_old_version_arch.deb Tip     If system breakage is minor, you may alternatively downgrade the whole system as in Section 2.7.11, “Emergency downgrading” using the higher level APT system.     If your system is unbootable from hard disk, you should seek other ways to boot it. 1. Boot the system using the debian-installer CD in rescue mode.     2. Mount the unbootable system on the hard disk to "/target". 3. Install older version of foo package by the following.     # dpkg --root /target -i /path/to/foo_old_version_arch.deb     This example works even if the dpkg command on the hard disk is broken. Tip     Any GNU/Linux system started by another system on hard disk, live GNU/Linux CD, bootable USB-key drive, or netboot can be used similarly to rescue broken system. If attempting to install a package this way fails due to some dependency violations and you really need to do this as the last     resort, you can override dependency using dpkg's "--ignore-depends", "--force-depends" and other options. If you do this, you need to make serious effort to restore proper dependency later. See dpkg(8) for details. Note     If your system is seriously broken, you should make a full backup of system to a safe place (see Section 10.2, “Backup and recovery”) and should perform a clean installation. This is less time consuming and produces better results in the end. 2.6.7. Recovering package selection data If "/var/lib/dpkg/status" becomes corrupt for any reason, the     Debian system loses package selection data and suffers severely. Look for the old "/var/lib/dpkg/status" file at "/var/lib/dpkg/ status-old" or "/var/backups/dpkg.status.*".     Keeping "/var/backups/" in a separate partition may be a good idea since this directory contains lots of important system data. For serious breakage, I recommend to make fresh re-install after     making backup of the system. Even if everything in "/var/" is gone, you can still recover some information from directories in "/usr/share/doc/" to guide your new installation.     Reinstall minimal (desktop) system.     # mkdir -p /path/to/old/system     Mount old system at "/path/to/old/system/". # cd /path/to/old/system/usr/share/doc # ls -1 >~/ls1.txt     # cd /usr/share/doc # ls -1 >>~/ls1.txt # cd # sort ls1.txt | uniq | less     Then you are presented with package names to install. (There may be some non-package names such as "texmf".) 2.7. Tips for the package management For simplicity, the source list examples in this section are     presented as "/etc/apt/sources.list" in one-line-style after the bookworm release. 2.7.1. Who uploaded the package? Although the maintainer name listed in "/var/lib/dpkg/available" and "/usr/share/doc/package_name/changelog" provide some     information on "who is behind the packaging activity", the actual uploader of the package is somewhat obscure. who-uploads(1) in the devscripts package identifies the actual uploader of Debian source packages. 2.7.2. Limiting download bandwidth for APT If you want to limit the download bandwidth for APT to e.g.     800Kib/sec (=100kiB/sec), you should configure APT with its configuration parameter as the following.     APT::Acquire::http::Dl-Limit "800"; 2.7.3. Automatic download and upgrade of packages The apt package comes with its own cron script "/etc/cron.daily/ apt" to support the automatic download of packages. This script can be enhanced to perform the automatic upgrade of packages by     installing the unattended-upgrades package. These can be customized by parameters in "/etc/apt/apt.conf.d/02backup" and "/ etc/apt/apt.conf.d/50unattended-upgrades" as described in "/usr/ share/doc/unattended-upgrades/README". The unattended-upgrades package is mainly intended for the security upgrade for the stable system. If the risk of breaking an existing stable system by the automatic upgrade is smaller     than that of the system broken by the intruder using its security hole which has been closed by the security update, you should consider using this automatic upgrade with configuration parameters as the following. APT::Periodic::Update-Package-Lists "1";     APT::Periodic::Download-Upgradeable-Packages "1"; APT::Periodic::Unattended-Upgrade "1"; If you are running an testing or unstable system, you do not want to use the automatic upgrade since it certainly breaks system     some day. Even for such testing or unstable case, you may still want to download packages in advance to save time for the interactive upgrade with configuration parameters as the following. APT::Periodic::Update-Package-Lists "1";     APT::Periodic::Download-Upgradeable-Packages "1"; APT::Periodic::Unattended-Upgrade "0"; 2.7.4. Updates and Backports There are stable-updates ("bookworm-updates" during the     bookworm-as-stable release cycle) and backports.debian.org archives which provide upgrade packages for stable.     In order to use these archives, you list all required archives in the "/etc/apt/sources.list" file as the following. deb http://deb.debian.org/debian/ bookworm main non-free-firmware contrib non-free     deb http://security.debian.org/debian-security bookworm-security main non-free-firmware contrib non-free deb http://deb.debian.org/debian/ bookworm-updates main non-free-firmware contrib non-free deb http://deb.debian.org/debian/ bookworm-backports main non-free-firmware contrib non-free There is no need to set Pin-Priority value explicitly in the "/     etc/apt/preferences" file. When newer packages become available, the default configuration provides most reasonable upgrades (see Section 2.5.3, “Archive level "Release" files”). * All installed older packages are upgraded to newer ones from bookworm-updates.     * Only manually installed older packages from bookworm-backports are upgraded to newer ones from bookworm-backports. Whenever you wish to install a package named "package-name" with     its dependency from bookworm-backports archive manually, you use following command while switching target release with "-t" option.     $ sudo apt-get install -t bookworm-backports package-name Warning     Do not install too many packages from backports.debian.org archives. It may cause package dependency complications. See Section 2.1.11, “How to cope with conflicting requirements” for alternative solutions. 2.7.5. External package archives Warning     You should be aware that the external package gains the root priviledge to your system. You should only use the trusted external package archive. See Section 2.1.11, “How to cope with conflicting requirements” for alternative solutions. You can use secure APT with Debian-compatible external package     archive by adding it to the source list and its archive key file into the "/etc/apt/trusted.gpg.d/" directory. See sources.list (5), apt-secure(8) and apt-key(8). 2.7.6. Packages from mixed source of archives without apt-pinning Caution     Installing packages from mixed source of archives is not supported by the official Debian distribution except for officially supported particular combinations of archives such as stable with security updates and stable-updates. Here is an example of operations to include specific newer     upstream version packages found in unstable while tracking testing for single occasion. 1. Change the "/etc/apt/sources.list" file temporarily to single "unstable" entry. 2. Run "aptitude update".     3. Run "aptitude install package-name". 4. Recover the original "/etc/apt/sources.list" file for testing. 5. Run "aptitude update". You do not create the "/etc/apt/preferences" file nor need to     worry about apt-pinning with this manual approach. But this is very cumbersome. Caution When using mixed source of archives, you must ensure     compatibility of packages by yourself since the Debian does not guarantee it. If package incompatibility exists, you may break system. You must be able to judge these technical requirements. The use of mixed source of random archives is completely optional operation and its use is not something I encourage you to use.     General rules for installing packages from different archives are the following. * Non-binary packages ("Architecture: all") are safer to install. + documentation packages: no special requirements + interpreter program packages: compatible interpreter must be available * Binary packages (non "Architecture: all") usually face many     road blocks and are unsafe to install. + library version compatibility (including "libc") + related utility program version compatibility + Kernel ABI compatibility + C++ ABI compatibility + … Note     In order to make a package to be safer to install, some commercial non-free binary program packages may be provided with completely statically linked libraries. You should still check ABI compatibility issues etc. for them. Note Except to avoid broken package for a short term, installing     binary packages from non-Debian archives is generally bad idea. You should seek all available alternative safer technical solutions which are compatible with your current Debian syetem (see Section 2.1.11, “How to cope with conflicting requirements” ). 2.7.7. Tweaking candidate version with apt-pinning Warning     Use of apt-pinning by a novice user is sure call for major troubles. You must avoid using apt-pinning except when you absolutely need it. Without the "/etc/apt/preferences" file, APT system choses the latest available version as the candidate version using the version string. This is the normal state and most recommended     usage of APT system. All officially supported combinations of archives do not require the "/etc/apt/preferences" file since some archives which should not be used as the automatic source of upgrades are marked as NotAutomatic and dealt properly. Tip     The version string comparison rule can be verified with, e.g., "dpkg --compare-versions ver1.1 gt ver1.1~1; echo $?" (see dpkg (1)). When you install packages from mixed source of archives (see Section 2.7.6, “Packages from mixed source of archives without apt-pinning”) regularly, you can automate these complicated     operations by creating the "/etc/apt/preferences" file with proper entries and tweaking the package selection rule for candidate version as described in apt_preferences(5). This is called apt-pinning. When using apt-pinning, you must ensure compatibility of packages     by yourself since the Debian does not guarantee it. The apt-pinning is completely optional operation and its use is not something I encourage you to use. Archive level Release files (see Section 2.5.3, “Archive level "Release" files”) are used for the rule of apt_preferences(5). Thus apt-pinning works only with "suite" name for normal Debian     archives and security Debian archives. (This is different from Ubuntu archives.) For example, you can do "Pin: release a= unstable" but can not do "Pin: release a=sid" in the "/etc/apt/ preferences" file. When you use non-Debian archive as a part of apt-pinning, you     should check what they are intended for and also check their credibility. For example, Ubuntu and Debian are not meant to be mixed. Note Even if you do not create the "/etc/apt/preferences" file, you     can do fairly complex system operations (see Section 2.6.6, “Rescue with the dpkg command” and Section 2.7.6, “Packages from mixed source of archives without apt-pinning”) without apt-pinning.     Here is a simplified explanation of apt-pinning technique. The APT system choses the highest Pin-Priority upgrading package from available package sources defined in the "/etc/apt/     sources.list" file as the candidate version package. If the Pin-Priority of the package is larger than 1000, this version restriction for upgrading is dropped to enable downgrading (see Section 2.7.11, “Emergency downgrading”). Pin-Priority value of each package is defined by "Pin-Priority"     entries in the "/etc/apt/preferences" file or uses its default value. Table 2.18. List of notable Pin-Priority values for apt-pinning technique. +---------------------------------------------------------------+ |Pin-Priority| apt-pinning effects to the package | |------------+--------------------------------------------------| |1001 |install the package even if this constitutes a | | |downgrade of the package | |------------+--------------------------------------------------| |990 |used as the default for the target release archive| |------------+--------------------------------------------------|     |500 |used as the default for the normal archive | |------------+--------------------------------------------------| |100 |used as the default for the NotAutomatic and | | |ButAutomaticUpgrades archive | |------------+--------------------------------------------------| |100 |used for the installed package | |------------+--------------------------------------------------| |1 |used as the default for the NotAutomatic archive | |------------+--------------------------------------------------| |-1 |never install the package even if recommended | +---------------------------------------------------------------+     The target release archive can be set by the command line option, e.g., "apt-get install -t testing some-package" The NotAutomatic and ButAutomaticUpgrades archive is set by archive server having its archive level Release file (see     Section 2.5.3, “Archive level "Release" files”) containing both "NotAutomatic: yes" and "ButAutomaticUpgrades: yes". The NotAutomatic archive is set by archive server having its archive level Release file containing only "NotAutomatic: yes".     The apt-pinning situation of package from multiple archive sources is displayed by "apt-cache policy package". * A line started with "Package pin:" lists the package version of pin if association just with package is defined, e.g., "Package pin: 0.190". * No line with "Package pin:" exists if no association just with package is defined. * The Pin-Priority value associated just with package is listed     right side of all version strings, e.g., "0.181 700". * "0" is listed right side of all version strings if no association just with package is defined, e.g., "0.181 0". * The Pin-Priority values of archives (defined as "Package: *" in the "/etc/apt/preferences" file) are listed left side of all archive paths, e.g., "100 http://deb.debian.org/debian/ bookworm-backports/main Packages". 2.7.8. Blocking packages installed by "Recommends" Warning     Use of apt-pinning by a novice user is sure call for major troubles. You must avoid using apt-pinning except when you absolutely need it. If you wish not to pull in particular packages automatically by     "Recommends", you must create the "/etc/apt/preferences" file and explicitly list all those packages at the top of it as the following. Package: package-1 Pin: version * Pin-Priority: -1     Package: package-2 Pin: version * Pin-Priority: -1 2.7.9. Tracking testing with some packages from unstable Warning     Use of apt-pinning by a novice user is sure call for major troubles. You must avoid using apt-pinning except when you absolutely need it. Here is an example of apt-pinning technique to include specific     newer upstream version packages found in unstable regularly upgraded while tracking testing. You list all required archives in the "/etc/apt/sources.list" file as the following. deb http://deb.debian.org/debian/ testing main contrib non-free     deb http://deb.debian.org/debian/ unstable main contrib non-free deb http://security.debian.org/debian-security testing-security main contrib     Set the "/etc/apt/preferences" file as the following. Package: *     Pin: release a=unstable Pin-Priority: 100 When you wish to install a package named "package-name" with its     dependencies from unstable archive under this configuration, you issue the following command which switches target release with "-t" option (Pin-Priority of unstable becomes 990).     $ sudo apt-get install -t unstable package-name With this configuration, usual execution of "apt-get upgrade" and "apt-get dist-upgrade" (or "aptitude safe-upgrade" and "aptitude     full-upgrade") upgrades packages which were installed from testing archive using current testing archive and packages which were installed from unstable archive using current unstable archive. Caution     Be careful not to remove "testing" entry from the "/etc/apt/ sources.list" file. Without "testing" entry in it, APT system upgrades packages using newer unstable archive. Tip I usually edit the "/etc/apt/sources.list" file to comment out     "unstable" archive entry right after above operation. This avoids slow update process of having too many entries in the "/etc/apt/ sources.list" file although this prevents upgrading packages which were installed from unstable archive using current unstable archive. Tip If "Pin-Priority: 1" is used instead of "Pin-Priority: 100" in     the "/etc/apt/preferences" file, already installed packages having Pin-Priority value of 100 are not upgraded by unstable archive even if "testing" entry in the "/etc/apt/sources.list" file is removed. If you wish to track particular packages in unstable     automatically without initial "-t unstable" installation, you must create the "/etc/apt/preferences" file and explicitly list all those packages at the top of it as the following. Package: package-1 Pin: release a=unstable Pin-Priority: 700     Package: package-2 Pin: release a=unstable Pin-Priority: 700 These set Pin-Priority value for each specific package. For     example, in order to track the latest unstable version of this "Debian Reference" in English, you should have following entries in the "/etc/apt/preferences" file. Package: debian-reference-en Pin: release a=unstable Pin-Priority: 700     Package: debian-reference-common Pin: release a=unstable Pin-Priority: 700 Tip     This apt-pinning technique is valid even when you are tracking stable archive. Documentation packages have been always safe to install from unstable archive in my experience, so far. 2.7.10. Tracking unstable with some packages from experimental Warning     Use of apt-pinning by a novice user is sure call for major troubles. You must avoid using apt-pinning except when you absolutely need it. Here is another example of apt-pinning technique to include     specific newer upstream version packages found in experimental while tracking unstable. You list all required archives in the "/ etc/apt/sources.list" file as the following. deb http://deb.debian.org/debian/ unstable main contrib non-free     deb http://deb.debian.org/debian/ experimental main contrib non-free deb http://security.debian.org/ testing-security main contrib The default Pin-Priority value for experimental archive is always 1 (<<100) since it is NotAutomatic archive (see Section 2.5.3,     “Archive level "Release" files”). There is no need to set Pin-Priority value explicitly in the "/etc/apt/preferences" file just to use experimental archive unless you wish to track particular packages in it automatically for next upgrading. 2.7.11. Emergency downgrading Warning     Use of apt-pinning by a novice user is sure call for major troubles. You must avoid using apt-pinning except when you absolutely need it. Caution Downgrading is not officially supported by the Debian by design.     It should be done only as a part of emergency recovery process. Despite of this situation, it is known to work well in many incidents. For critical systems, you should backup all important data on the system after the recovery operation and re-install the new system from the scratch. You may be lucky to downgrade from newer archive to older archive to recover from broken system upgrade by manipulating candidate     version (see Section 2.7.7, “Tweaking candidate version with apt-pinning”). This is lazy alternative to tedious actions of many "dpkg -i broken-package_old-version.deb" commands (see Section 2.6.6, “Rescue with the dpkg command”).     Search lines in the "/etc/apt/sources.list" file tracking unstable as the following.     deb http://deb.debian.org/debian/ sid main contrib non-free     Replace it with the following to track testing.     deb http://deb.debian.org/debian/ trixie main contrib non-free     Set the "/etc/apt/preferences" file as the following. Package: *     Pin: release a=testing Pin-Priority: 1010     Run "apt-get update; apt-get dist-upgrade" to force downgrading of packages across the system.     Remove this special "/etc/apt/preferences" file after this emergency downgrading. Tip It is a good idea to remove (not purge!) as much packages to     minimize dependency problems. You may need to manually remove and install some packages to get system downgraded. Linux kernel, bootloader, udev, PAM, APT, and networking related packages and their configuration files require special attention. 2.7.12. The equivs package If you are to compile a program from source to replace the Debian     package, it is best to make it into a real local debianized package (*.deb) and use private archive. If you chose to compile a program from source and to install them     under "/usr/local" instead, you may need to use equivs as a last resort to satisfy the missing package dependency. Package: equivs Priority: optional Section: admin Description: Circumventing Debian package dependencies This package provides a tool to create trivial Debian packages. Typically these packages contain only dependency information, but they can also include normal installed files like other packages do.     . One use for this is to create a metapackage: a package whose sole purpose is to declare dependencies and conflicts on other packages so that these will be automatically installed, upgraded, or removed. . Another use is to circumvent dependency checking: by letting dpkg think a particular package name and version is installed when it isn't, you can work around bugs in other packages' dependencies. (Please do still file such bugs, though.) 2.7.13. Porting a package to the stable system Caution     There is no gurantee for the procedure descried here to work without extra manual efforts for system differences. For partial upgrades of the stable system, rebuilding a package     within its environment using the source package is desirable. This avoids massive package upgrades due to their dependencies.     Add the following entries to the "/etc/apt/sources.list" of a stable system.     deb-src http://deb.debian.org/debian unstable main contrib non-free     Install required packages for the compilation and download the source package as the following. # apt-get update # apt-get dist-upgrade     # apt-get install fakeroot devscripts build-essential # apt-get build-dep foo $ apt-get source foo $ cd foo*     Update some tool chain packages such as dpkg, and debhelper from the backport packages if they are required for the backporting.     Execute the following.     $ dch -i     Bump package version, e.g. one appended with "+bp1" in "debian/ changelog"     Build packages and install them to the system as the following. $ debuild     $ cd .. # debi foo*.changes 2.7.14. Proxy server for APT Since mirroring whole subsection of Debian archive wastes disk space and network bandwidth, deployment of a local proxy server for APT is desirable consideration when you administer many systems on LAN. APT can be configure to use generic web (http)     proxy servers such as squid (see Section 6.5, “Other network application servers”) as described in apt.conf(5) and in "/usr/ share/doc/apt/examples/configure-index.gz". The "$http_proxy" environment variable can be used to override proxy server setting in the "/etc/apt/apt.conf" file.     There are proxy tools specially for Debian archive. You should check BTS before using them. Table 2.19. List of the proxy tools specially for Debian archive +---------------------------------------------------------------+ | package |popcon|size| description | |-------------+------+----+-------------------------------------| | |V:0, | |caching proxy server for Debian | |approx |I:0 |7124|archive files (compiled OCaml | | | | |program) |     |-------------+------+----+-------------------------------------| |apt-cacher |V:0, |266 |Caching proxy for Debian package and | | |I:0 | |source files (Perl program) | |-------------+------+----+-------------------------------------| | |V:4, | |Caching proxy for distribution of | |apt-cacher-ng|I:4 |1816|software packages (compiled C++ | | | | |program) | +---------------------------------------------------------------+ Caution When Debian reorganizes its archive structure, these specialized     proxy tools tend to require code rewrites by the package maintainer and may not be functional for a while. On the other hand, generic web (http) proxy servers are more robust and easier to cope with such changes. 2.7.15. More readings for the package management     You can learn more on the package management from following documentations. * Primary documentations on the package management: + aptitude(8), dpkg(1), tasksel(8), apt(8), apt-get(8), apt-config(8), apt-secure(8), sources.list(5), apt.conf (5), and apt_preferences(5); + "/usr/share/doc/apt-doc/guide.html/index.html" and "/usr/ share/doc/apt-doc/offline.html/index.html" from the apt-doc package; and + "/usr/share/doc/aptitude/html/en/index.html" from the aptitude-doc-en package.     * Official and detailed documentations on the Debian archive: + "Debian Policy Manual Chapter 2 - The Debian Archive", + "Debian Developer's Reference, Chapter 4 - Resources for Debian Developers 4.6 The Debian archive", and + "The Debian GNU/Linux FAQ, Chapter 6 - The Debian FTP archives". * Tutorial for building of a Debian package for Debian users: + "Guide for Debian Maintainers". Chapter 3. The system initialization It is wise for you as the system administrator to know roughly how the Debian system is started and configured. Although the     exact details are in the source files of the packages installed and their documentations, it is a bit overwhelming for most of us. Here is a rough overview of the key points of the Debian system     initialization. Since the Debian system is a moving target, you should refer to the latest documentation. * Debian Linux Kernel Handbook is the primary source of information on the Debian kernel.     * bootup(7) describes the system bootup process based on systemd . (Recent Debian) * boot(7) describes the system bootup process based on UNIX System V Release 4. (Older Debian) 3.1. An overview of the boot strap process The computer system undergoes several phases of boot strap     processes from the power-on event until it offers the fully functional operating system (OS) to the user.     For simplicity, I limit discussion to the typical PC platform with the default installation.     The typical boot strap process is like a four-stage rocket. Each stage rocket hands over the system control to the next stage one. * Section 3.1.1, “Stage 1: the UEFI” * Section 3.1.2, “Stage 2: the boot loader”     * Section 3.1.3, “Stage 3: the mini-Debian system” * Section 3.1.4, “Stage 4: the normal Debian system” Of course, these can be configured differently. For example, if     you compiled your own kernel, you may be skipping the step with the mini-Debian system. So please do not assume this is the case for your system until you check it yourself. 3.1.1. Stage 1: the UEFI The Unified Extensible Firmware Interface (UEFI) defines a boot manager as part of the UEFI specification. When a computer is powered on, the boot manager is the 1st stage of the boot process     which checks the boot configuration and based on its settings, then executes the specified OS boot loader or operating system kernel (usually boot loader). The boot configuration is defined by variables stored in NVRAM, including variables that indicate the file system paths to OS loaders or OS kernels. An EFI system partition (ESP) is a data storage device partition that is used in computers adhering to the UEFI specification.     Accessed by the UEFI firmware when a computer is powered up, it stores UEFI applications and the files these applications need to run, including operating system boot loaders. (On the legacy PC system, BIOS stored in the MBR may be used instead.) 3.1.2. Stage 2: the boot loader The boot loader is the 2nd stage of the boot process which is started by the UEFI. It loads the system kernel image and the     initrd image to the memory and hands control over to them. This initrd image is the root filesystem image and its support depends on the bootloader used. The Debian system normally uses the Linux kernel as the default     system kernel. The initrd image for the current 5.x Linux kernel is technically the initramfs (initial RAM filesystem) image.     There are many boot loaders and configuration options available. Table 3.1. List of boot loaders +---------------------------------------------------------------+ | package |popcon|size| initrd |bootloader| description | |--------------+------+----+---------+----------+---------------| | | | | | |This is smart | | | | | | |enough to | | | | | | |understand disk| |grub-efi-amd64|I:339 |184 |Supported|GRUB UEFI |partitions and | | | | | | |filesystems | | | | | | |such as vfat, | | | | | | |ext4, …. (UEFI)| |--------------+------+----+---------+----------+---------------| | | | | | |This is smart | | | | | | |enough to | | |V:21, | | | |understand disk| |grub-pc |I:634 |557 |Supported|GRUB 2 |partitions and | | | | | | |filesystems | | | | | | |such as vfat, | | | | | | |ext4, …. (BIOS)| |--------------+------+----+---------+----------+---------------| | | | | | |This is GRUB 2 | | |V:0, | | | |bootable rescue| |grub-rescue-pc|I:0 |6625|Supported|GRUB 2 |images (CD and | | | | | | |floppy) (PC/ | | | | | | |BIOS version) | |--------------+------+----+---------+----------+---------------| | | | | | |This |     | | | | | |understands the| |syslinux |V:3, |344 |Supported|Isolinux |ISO9660 | | |I:36 | | | |filesystem. | | | | | | |This is used by| | | | | | |the boot CD. | |--------------+------+----+---------+----------+---------------| | | | | | |This | | | | | | |understands the| | |V:3, | | | |MSDOS | |syslinux |I:36 |344 |Supported|Syslinux |filesystem | | | | | | |(FAT). This is | | | | | | |used by the | | | | | | |boot floppy. | |--------------+------+----+---------+----------+---------------| | | | | | |New system is | |loadlin |V:0, |90 |Supported|Loadlin |started from | | |I:0 | | | |the FreeDOS/ | | | | | | |MSDOS system. | |--------------+------+----+---------+----------+---------------| | | | | | |This is free | | | | | | |software which | | | | | |MBR by |substitutes | |mbr |V:0, |47 |Not |Neil |MSDOS MBR. This| | |I:4 | |supported|Turton |only | | | | | | |understands | | | | | | |disk | | | | | | |partitions. | +---------------------------------------------------------------+ Warning     Do not play with boot loaders without having bootable rescue media (USB memory stick, CD or floppy) created from images in the grub-rescue-pc package. It makes you boot your system even without functioning bootloader on the hard disk. For UEFI system, GRUB2 first reads the ESP partition and uses     UUID specified for search.fs_uuid in "/boot/efi/EFI/debian/ grub.cfg" to determine the partition of the GRUB2 menu configuration file "/boot/grub/grub.cfg".     The key part of the GRUB2 menu configuration file looks like: menuentry 'Debian GNU/Linux' ... { load_video insmod gzio insmod part_gpt insmod ext2     search --no-floppy --fs-uuid --set=root fe3e1db5-6454-46d6-a14c-071208ebe4b1 echo 'Loading Linux 5.10.0-6-amd64 ...' linux /boot/vmlinuz-5.10.0-6-amd64 root=UUID=fe3e1db5-6454-46d6-a14c-071208ebe4b1 ro quiet echo 'Loading initial ramdisk ...' initrd /boot/initrd.img-5.10.0-6-amd64 }     For this part of /boot/grub/grub.cfg, this menu entry means the following. Table 3.2. The meaning of the menu entry of the above part of / boot/grub/grub.cfg +---------------------------------------------------------------+ | setting | value | |------------------------+--------------------------------------| |GRUB2 modules loaded |gzio, part_gpt, ext2 | |------------------------+--------------------------------------| |root file system |partition identified by UUID= | |partition used |fe3e1db5-6454-46d6-a14c-071208ebe4b1 |     |------------------------+--------------------------------------| |kernel image path in the|/boot/vmlinuz-5.10.0-6-amd64 | |root file system | | |------------------------+--------------------------------------| |kernel boot parameter |"root=UUID= | |used |fe3e1db5-6454-46d6-a14c-071208ebe4b1 | | |ro quiet" | |------------------------+--------------------------------------| |initrd image path in the|/boot/initrd.img-5.10.0-6-amd64 | |root file system | | +---------------------------------------------------------------+ Tip     You can enable to see kernel boot log messages by removing quiet in "/boot/grub/grub.cfg". For the persistent change, please edit "GRUB_CMDLINE_LINUX_DEFAULT="quiet"" line in "/etc/default/grub". Tip     You can customize GRUB splash image by setting GRUB_BACKGROUND variable in "/etc/default/grub" pointing to the image file or placing the image file itself in "/boot/grub/".     See "info grub" and grub-install(8). 3.1.3. Stage 3: the mini-Debian system The mini-Debian system is the 3rd stage of the boot process which     is started by the boot loader. It runs the system kernel with its root filesystem on the memory. This is an optional preparatory stage of the boot process. Note     The term "the mini-Debian system" is coined by the author to describe this 3rd stage boot process for this document. This system is commonly referred as the initrd or initramfs system. Similar system on the memory is used by the Debian Installer. The "/init" program is executed as the first program in this root filesystem on the memory. It is a program which initializes the     kernel in user space and hands control over to the next stage. This mini-Debian system offers flexibility to the boot process such as adding kernel modules before the main boot process or mounting the root filesystem as an encrypted one. * The "/init" program is a shell script program if initramfs was created by initramfs-tools. + You can interrupt this part of the boot process to gain root shell by providing "break=init" etc. to the kernel boot parameter. See the "/init" script for more break conditions. This shell environment is sophisticated enough to make a good inspection of your machine's hardware.     + Commands available in this mini-Debian system are stripped down ones and mainly provided by a GNU tool called busybox(1). * The "/init" program is a binary systemd program if initramfs was created by dracut. + Commands available in this mini-Debian system are stripped down systemd(1) environment. Caution     You need to use "-n" option for mount command when you are on the readonly root filesystem. 3.1.4. Stage 4: the normal Debian system The normal Debian system is the 4th stage of the boot process which is started by the mini-Debian system. The system kernel for     the mini-Debian system continues to run in this environment. The root filesystem is switched from the one on the memory to the one on the real hard disk filesystem. The init program is executed as the first program with PID=1 to perform the main boot process of starting many programs. The     default file path for the init program is "/usr/sbin/init" but it can be changed by the kernel boot parameter as "init=/path/to/ init_program".     "/usr/sbin/init" is symlinked to "/lib/systemd/systemd" after Debian 8 Jessie (released in 2015). Tip     The actual init command on your system can be verified by the "ps --pid 1 -f" command. Table 3.3. List of boot utilities for the Debian system +---------------------------------------------------------------+ | package |popcon|size | description | |-------------------+------+-----+------------------------------| | |V:860,| |event-based init(8) daemon for| |systemd |I:966 |11168|concurrency (alternative to | | | | |sysvinit) | |-------------------+------+-----+------------------------------| |cloud-init |V:3, |2870 |initialization system for | | |I:5 | |infrastructure cloud instances| |-------------------+------+-----+------------------------------| | |V:832,| |the manual pages and links | |systemd-sysv |I:964 |80 |needed for systemd to replace | | | | |sysvinit | |-------------------+------+-----+------------------------------| |init-system-helpers|V:699,|130 |helper tools for switching | | |I:974 | |between sysvinit and systemd | |-------------------+------+-----+------------------------------| |initscripts |V:33, |198 |scripts for initializing and | | |I:133 | |shutting down the system | |-------------------+------+-----+------------------------------| |sysvinit-core |V:4, |361 |System-V-like init(8) | | |I:5 | |utilities | |-------------------+------+-----+------------------------------|     |sysv-rc |V:66, |88 |System-V-like runlevel change | | |I:145 | |mechanism | |-------------------+------+-----+------------------------------| |sysvinit-utils |V:897,|102 |System-V-like utilities | | |I:999 | |(startpar(8), bootlogd(8), …) | |-------------------+------+-----+------------------------------| |lsb-base |V:634,|12 |Linux Standard Base 3.2 init | | |I:675 | |script functionality | |-------------------+------+-----+------------------------------| | |V:88, | |tool to organize boot sequence| |insserv |I:144 |132 |using LSB init.d script | | | | |dependencies | |-------------------+------+-----+------------------------------| |kexec-tools |V:1, |316 |kexec tool for kexec(8) | | |I:6 | |reboots (warm reboot) | |-------------------+------+-----+------------------------------| |systemd-bootchart |V:0, |131 |boot process performance | | |I:0 | |analyser | |-------------------+------+-----+------------------------------| |mingetty |V:0, |36 |console-only getty(8) | | |I:2 | | | |-------------------+------+-----+------------------------------| |mgetty |V:0, |315 |smart modem getty(8) | | |I:0 | |replacement | +---------------------------------------------------------------+ Tip     See Debian wiki: BootProcessSpeedup for the latest tips to speed up the boot process. 3.2. Systemd 3.2.1. Systemd init When the Debian system starts, /usr/sbin/init symlinked to /usr/     lib/systemd is started as the init system process (PID=1) owned by root (UID=0). See systemd(1). The systemd init process spawns processes in parallel based on     the unit configuration files (see systemd.unit(5)) which are written in declarative style instead of SysV-like procedural style. The spawned processes are placed in individual Linux control     groups named after the unit which they belong to in the private systemd hierarchy (see cgroups and Section 4.7.5, “Linux security features”). Units for the system mode are loaded from the "System Unit Search     Path" described in systemd.unit(5). The main ones are as follows in the order of priority: * "/etc/systemd/system/*": System units created by the administrator     * "/run/systemd/system/*": Runtime units * "/lib/systemd/system/*": System units installed by the distribution package manager Their inter-dependencies are specified by the directives "Wants=     ", "Requires=", "Before=", "After=", … (see "MAPPING OF UNIT PROPERTIES TO THEIR INVERSES" in systemd.unit(5)). The resource controls are also defined (see systemd.resource-control(5)).     The suffix of the unit configuration file encodes their types as: * *.service describes the process controlled and supervised by systemd. See systemd.service(5). * *.device describes the device exposed in the sysfs(5) as udev (7) device tree. See systemd.device(5). * *.mount describes the file system mount point controlled and supervised by systemd. See systemd.mount(5). * *.automount describes the file system auto mount point controlled and supervised by systemd. See systemd.automount (5). * *.swap describes the swap device or file controlled and supervised by systemd. See systemd.swap(5).     * *.path describes the path monitored by systemd for path-based activation. See systemd.path(5). * *.socket describes the socket controlled and supervised by systemd for socket-based activation. See systemd.socket(5). * *.timer describes the timer controlled and supervised by systemd for timer-based activation. See systemd.timer(5). * *.slice manages resources with the cgroups(7). See systemd.slice(5). * *.scope is created programmatically using the bus interfaces of systemd to manages a set of system processes. See systemd.scope(5). * *.target groups other unit configuration files to create the synchronization point during start-up. See systemd.target(5). Upon system start up (i.e., init), the systemd process tries to start the "/lib/systemd/system/default.target (normally symlinked to "graphical.target"). First, some special target units (see     systemd.special(7)) such as "local-fs.target", "swap.target" and "cryptsetup.target" are pulled in to mount the filesystems. Then, other target units are also pulled in by the target unit dependencies. For details, read bootup(7). systemd offers backward compatibility features. SysV-style boot     scripts in "/etc/init.d/rc[0123456S].d/[KS]name" are still parsed and telinit(8) is translated into systemd unit activation requests. Caution     Emulated runlevel 2 to 4 are all symlinked to the same "multi-user.target". 3.2.2. Systemd login When a user logins to the Debian system via gdm3(8), sshd(8),     etc., /lib/systemd/system --user is started as the user service manager process owned by the corresponding user. See systemd(1). The systemd user service manager process spawns processes in     parallel based on the declarative unit configuration files (see systemd.unit(5) and user@.service(5)). Units for the user mode are loaded from the "User Unit Search     Path" described in systemd.unit(5). The main ones are as follows in the order of priority: * "~/.config/systemd/user/*": User configuration units * "/etc/systemd/user/*": User units created by the administrator     * "/run/systemd/user/*": Runtime units * "/lib/systemd/user/*": User units installed by the distribution package manager     These are managed in the same way as Section 3.2.1, “Systemd init”. 3.3. The kernel message     The kernel error message displayed to the console can be configured by setting its threshold level.     # dmesg -n3 Table 3.4. List of kernel error levels +---------------------------------------------------------------+ | error level | error level | meaning | | value | name | | |----------------+---------------+------------------------------| |0 |KERN_EMERG |system is unusable | |----------------+---------------+------------------------------| |1 |KERN_ALERT |action must be taken | | | |immediately | |----------------+---------------+------------------------------| |2 |KERN_CRIT |critical conditions |     |----------------+---------------+------------------------------| |3 |KERN_ERR |error conditions | |----------------+---------------+------------------------------| |4 |KERN_WARNING |warning conditions | |----------------+---------------+------------------------------| |5 |KERN_NOTICE |normal but significant | | | |condition | |----------------+---------------+------------------------------| |6 |KERN_INFO |informational | |----------------+---------------+------------------------------| |7 |KERN_DEBUG |debug-level messages | +---------------------------------------------------------------+ 3.4. The system message Under systemd, both kernel and system messages are logged by the journal service systemd-journald.service (a.k.a journald) either     into a persistent binary data below "/var/log/journal" or into a volatile binary data below "/run/log/journal/". These binary log data are accessed by the journalctl(1) command. For example, you can display log from the last boot as:     $ journalctl -b Table 3.5. List of typical journalctl command snippets +---------------------------------------------------------------+ | Operation | Command snippets | |-------------------------------------------+-------------------| |View log for system services and kernel |"journalctl -b | |from the last boot |--system" | |-------------------------------------------+-------------------|     |View log for services of the current user |"journalctl -b | |from the last boot |--user" | |-------------------------------------------+-------------------| |View job log of "$unit" from the last boot |"journalctl -b -u | | |$unit" | |-------------------------------------------+-------------------| |View job log of "$unit" ("tail -f" style) |"journalctl -b -u | |from the last boot |$unit -f" | +---------------------------------------------------------------+ Under systemd, the system logging utility rsyslogd(8) may be uninstalled. If it is installed, it changes its behavior to read the volatile binary log data (instead of pre-systemd default "/     dev/log") and to create traditional permanent ASCII system log data. This can be customized by "/etc/default/rsyslog" and "/etc/ rsyslog.conf" for both the log file and on-screen display. See rsyslogd(8) and rsyslog.conf(5). See also Section 9.3.2, “Log analyzer”. 3.5. System management     The systemd offers not only init system but also generic system management operations with the systemctl(1) command. Table 3.6. List of typical systemctl command snippets +---------------------------------------------------------------+ | Operation | Command snippets | |---------------------------------+-----------------------------| |List all available unit types |"systemctl list-units --type=| | |help" | |---------------------------------+-----------------------------| |List all target units in memory |"systemctl list-units --type=| | |target" | |---------------------------------+-----------------------------| |List all service units in memory |"systemctl list-units --type=| | |service" | |---------------------------------+-----------------------------| |List all device units in memory |"systemctl list-units --type=| | |device" | |---------------------------------+-----------------------------| |List all mount units in memory |"systemctl list-units --type=| | |mount" | |---------------------------------+-----------------------------| |List all socket units in memory |"systemctl list-sockets" | |---------------------------------+-----------------------------| |List all timer units in memory |"systemctl list-timers" | |---------------------------------+-----------------------------| |Start "$unit" |"systemctl start $unit" | |---------------------------------+-----------------------------| |Stop "$unit" |"systemctl stop $unit" | |---------------------------------+-----------------------------| |Reload service-specific |"systemctl reload $unit" | |configuration | | |---------------------------------+-----------------------------| |Stop and start all "$unit" |"systemctl restart $unit" | |---------------------------------+-----------------------------| |Start "$unit" and stop all others|"systemctl isolate $unit" | |---------------------------------+-----------------------------| |Switch to "graphical" (GUI |"systemctl isolate graphical"| |system) | | |---------------------------------+-----------------------------| |Switch to "multi-user" (CLI |"systemctl isolate | |system) |multi-user" | |---------------------------------+-----------------------------| |Switch to "rescue" (single user |"systemctl isolate rescue" | |CLI system) | | |---------------------------------+-----------------------------| |Send kill signal to "$unit" |"systemctl kill $unit" | |---------------------------------+-----------------------------| |Check if "$unit" service is |"systemctl is-active $unit" | |active | | |---------------------------------+-----------------------------| |Check if "$unit" service is |"systemctl is-failed $unit" | |failed | |     |---------------------------------+-----------------------------| |Check status of "$unit|$PID| |"systemctl status $unit|$PID|| |device" |$device" | |---------------------------------+-----------------------------| |Show properties of "$unit|$job" |"systemctl show $unit|$job" | |---------------------------------+-----------------------------| |Reset failed "$unit" |"systemctl reset-failed | | |$unit" | |---------------------------------+-----------------------------| |List dependency of all unit |"systemctl list-dependencies | |services |--all" | |---------------------------------+-----------------------------| |List unit files installed on the |"systemctl list-unit-files" | |system | | |---------------------------------+-----------------------------| |Enable "$unit" (add symlink) |"systemctl enable $unit" | |---------------------------------+-----------------------------| |Disable "$unit" (remove symlink) |"systemctl disable $unit" | |---------------------------------+-----------------------------| |Unmask "$unit" (remove symlink to|"systemctl unmask $unit" | |"/dev/null") | | |---------------------------------+-----------------------------| |Mask "$unit" (add symlink to "/ |"systemctl mask $unit" | |dev/null") | | |---------------------------------+-----------------------------| |Get default-target setting |"systemctl get-default" | |---------------------------------+-----------------------------| |Set default-target to "graphical"|"systemctl set-default | |(GUI system) |graphical" | |---------------------------------+-----------------------------| |Set default-target to |"systemctl set-default | |"multi-user" (CLI system) |multi-user" | |---------------------------------+-----------------------------| |Show job environment |"systemctl show-environment" | |---------------------------------+-----------------------------| |Set job environment "variable" to|"systemctl set-environment | |"value" |variable=value" | |---------------------------------+-----------------------------| |Unset job environment "variable" |"systemctl unset-environment | | |variable" | |---------------------------------+-----------------------------| |Reload all unit files and daemons|"systemctl daemon-reload" | |---------------------------------+-----------------------------| |Shut down the system |"systemctl poweroff" | |---------------------------------+-----------------------------| |Shut down and reboot the system |"systemctl reboot" | |---------------------------------+-----------------------------| |Suspend the system |"systemctl suspend" | |---------------------------------+-----------------------------| |Hibernate the system |"systemctl hibernate" | +---------------------------------------------------------------+ Here, "$unit" in the above examples may be a single unit name (suffix such as .service and .target are optional) or, in many     cases, multiple unit specifications (shell-style globs "*", "?", "[]" using fnmatch(3) which will be matched against the primary names of all units currently in memory). System state changing commands in the above examples are     typically preceded by the "sudo" to attain the required administrative privilege.     The output of the "systemctl status $unit|$PID|$device" uses color of the dot ("●") to summarize the unit state at a glance. * White "●" indicates an "inactive" or "deactivating" state.     * Red "●" indicates a "failed" or "error" state. * Green "●" indicates an "active", "reloading" or "activating" state. 3.6. Other system monitors     Here are a list of other monitoring command snippets under systemd. Please read the pertinent manpages including cgroups(7). Table 3.7. List of other monitoring command snippets under systemd +---------------------------------------------------------------+ | Operation | Command snippets | |-----------------------------------------+---------------------| |Show time spent for each initialization |"systemd-analyze | |steps |time" | |-----------------------------------------+---------------------| |List of all units by the time to |"systemd-analyze | |initialize |blame" | |-----------------------------------------+---------------------| |Load and detect errors in "$unit" file |"systemd-analyze | | |verify $unit" |     |-----------------------------------------+---------------------| |Show terse runtime status information of |"loginctl | |the user of the caller's session |user-status" | |-----------------------------------------+---------------------| |Show terse runtime status information of |"loginctl | |the caller's session |session-status" | |-----------------------------------------+---------------------| |Track boot process by the cgroups |"systemd-cgls" | |-----------------------------------------+---------------------| |Track boot process by the cgroups |"ps xawf -eo | | |pid,user,cgroup,args"| |-----------------------------------------+---------------------| |Track boot process by the cgroups |Read sysfs under "/ | | |sys/fs/cgroup/" | +---------------------------------------------------------------+ 3.7. System configuration 3.7.1. The hostname The kernel maintains the system hostname. The system unit started by systemd-hostnamed.service sets the system hostname at boot     time to the name stored in "/etc/hostname". This file should contain only the system hostname, not a fully qualified domain name.     To print out the current hostname run hostname(1) without an argument. 3.7.2. The filesystem The mount options of normal disk and network filesystems are set     in "/etc/fstab". See fstab(5) and Section 9.6.7, “Optimization of filesystem by mount options”.     The configuration of the encrypted filesystem is set in "/etc/ crypttab". See crypttab(5)     The configuration of software RAID with mdadm(8) is set in "/etc/ mdadm/mdadm.conf". See mdadm.conf(5). Warning     After mounting all the filesystems, temporary files in "/tmp", "/ var/lock", and "/var/run" are cleaned for each boot up. 3.7.3. Network interface initialization Network interfaces are typically initialized in     "networking.service" for the lo interface and "NetworkManager.service" for other interfaces on modern Debian desktop system under systemd.     See Chapter 5, Network setup for how to configure them. 3.7.4. Cloud system initialization The cloud system instance may be launched as a clone of "Debian Official Cloud Images" or similar images. For such system instance, personalities such as hostname, filesystem, networking, locale, SSH keys, users and groups may be configured using     functionalities provided by cloud-init and netplan.io packages with multiple data sources such as files placed in the original system image and external data provided during its launch. These packages enable the declarative system configuration using YAML data. See more at "Cloud Computing with Debian and its descendants",     "Cloud-init documentation" and Section 5.4, “The modern network configuration for cloud”. 3.7.5. Customization example to tweak sshd service With default installation, many network services (see Chapter 6, Network applications) are started as daemon processes after     network.target at boot time by systemd. The "sshd" is no exception. Let's change this to on-demand start of "sshd" as a customization example.     First, disable system installed service unit.     $ sudo systemctl stop sshd.service $ sudo systemctl mask sshd.service The on-demand socket activation system of the classic Unix     services was through the inetd (or xinetd) superserver. Under systemd, the equivalent can be enabled by adding *.socket and *.service unit configuration files.     sshd.socket for specifying a socket to listen on [Unit] Description=SSH Socket for Per-Connection Servers [Socket]     ListenStream=22 Accept=yes [Install] WantedBy=sockets.target     sshd@.service as the matching service file of sshd.socket [Unit] Description=SSH Per-Connection Server     [Service] ExecStart=-/usr/sbin/sshd -i StandardInput=socket     Then reload.     $ sudo systemctl daemon-reload 3.8. The udev system The udev system provides mechanism for the automatic hardware discovery and initialization (see udev(7)) since Linux kernel 2.6. Upon discovery of each device by the kernel, the udev system     starts a user process which uses information from the sysfs filesystem (see Section 1.2.12, “procfs and sysfs”), loads required kernel modules supporting it using the modprobe(8) program (see Section 3.9, “The kernel module initialization”), and creates corresponding device nodes. Tip If "/lib/modules/kernel-version/modules.dep" was not generated properly by depmod(8) for some reason, modules may not be loaded     as expected by the udev system. Execute "depmod -a" to fix it. For mounting rules in "/etc/fstab", device nodes do not need to be static ones. You can use UUID to mount devices instead of device names such as "/dev/sda". See Section 9.6.3, “Accessing partition using UUID”. Since the udev system is somewhat a moving target, I leave     details to other documentations and describe the minimum information here. Warning     Don't try to run long running programs such as backup script with RUN in udev rules as mentioned in udev(7). Please create a proper systemd.service(5) file and activate it, instead. See Section 10.2.3.2, “Mount event triggered backup”. 3.9. The kernel module initialization The modprobe(8) program enables us to configure running Linux     kernel from user process by adding and removing kernel modules. The udev system (see Section 3.8, “The udev system”) automates its invocation to help the kernel module initialization. There are non-hardware modules and special hardware driver     modules as the following which need to be pre-loaded by listing them in the "/etc/modules" file (see modules(5)). * TUN/TAP modules providing virtual Point-to-Point network device (TUN) and virtual Ethernet network device (TAP),     * netfilter modules providing netfilter firewall capabilities (iptables(8), Section 5.7, “Netfilter infrastructure”), and * watchdog timer driver modules. The configuration files for the modprobe(8) program are located under the "/etc/modprobes.d/" directory as explained in     modprobe.conf(5). (If you want to avoid some kernel modules to be auto-loaded, consider to blacklist them in the "/etc/modprobes.d/ blacklist" file.) The "/lib/modules/version/modules.dep" file generated by the     depmod(8) program describes module dependencies used by the modprobe(8) program. Note     If you experience module loading issues with boot time module loading or with modprobe(8), "depmod -a" may resolve these issues by reconstructing "modules.dep".     The modinfo(8) program shows information about a Linux kernel module.     The lsmod(8) program nicely formats the contents of the "/proc/ modules", showing what kernel modules are currently loaded. Tip You can identify exact hardware on your system. See Section 9.5.3, “Hardware identification”.     You may configure hardware at boot time to activate expected hardware features. See Section 9.5.4, “Hardware configuration”. You can probably add support for your special device by recompiling the kernel. See Section 9.10, “The kernel”. Chapter 4. Authentication and access controls     When a person (or a program) requests access to the system, authentication confirms the identity to be a trusted one. Warning     Configuration errors of PAM may lock you out of your own system. You must have a rescue CD handy or setup an alternative boot partition. To recover, boot the system with them and correct things from there. 4.1. Normal Unix authentication Normal Unix authentication is provided by the pam_unix(8) module     under the PAM (Pluggable Authentication Modules). Its 3 important configuration files, with ":" separated entries, are the following. Table 4.1. 3 important configuration files for pam_unix(8) +---------------------------------------------------------------+ | file |permission|user|group | description | |---------+----------+----+------+------------------------------| |/etc/ |-rw-r--r--|root|root |(sanitized) user account | |passwd | | | |information |     |---------+----------+----+------+------------------------------| |/etc/ |-rw-r-----|root|shadow|secure user account | |shadow | | | |information | |---------+----------+----+------+------------------------------| |/etc/ |-rw-r--r--|root|root |group information | |group | | | | | +---------------------------------------------------------------+     "/etc/passwd" contains the following. ...     user1:x:1000:1000:User1 Name,,,:/home/user1:/bin/bash user2:x:1001:1001:User2 Name,,,:/home/user2:/bin/bash ...     As explained in passwd(5), each ":" separated entry of this file means the following. * Login name * Password specification entry * Numerical user ID     * Numerical group ID * User name or comment field * User home directory * Optional user command interpreter The second entry of "/etc/passwd" was used for the encrypted     password entry. After the introduction of "/etc/shadow", this entry is used for the password specification entry. Table 4.2. The second entry content of "/etc/passwd" +--------------------------------------------------+ |content| meaning |     |-------+------------------------------------------| |(empty)|passwordless account | |-------+------------------------------------------| |x |the encrypted password is in "/etc/shadow"| +--------------------------------------------------+     "/etc/shadow" contains the following. ...     user1:$1$Xop0FYH9$IfxyQwBe9b8tiyIkt2P4F/:13262:0:99999:7::: user2:$1$vXGZLVbS$ElyErNf/agUDsm1DehJMS/:13261:0:99999:7::: ...     As explained in shadow(5), each ":" separated entry of this file means the following. * Login name * Encrypted password (The initial "$1$" indicates use of the MD5 encryption. The "*" indicates no login.) * Date of the last password change, expressed as the number of days since Jan 1, 1970 * Number of days the user will have to wait before she will be allowed to change her password again     * Number of days after which the user will have to change her password * Number of days before a password is going to expire during which the user should be warned * Number of days after a password has expired during which the password should still be accepted * Date of expiration of the account, expressed as the number of days since Jan 1, 1970 * …     "/etc/group" contains the following.     group1:x:20:user1,user2     As explained in group(5), each ":" separated entry of this file means the following. * Group name * Encrypted password (not really used)     * Numerical group ID * "," separated list of user names Note     "/etc/gshadow" provides the similar function as "/etc/shadow" for "/etc/group" but is not really used. Note     The actual group membership of a user may be dynamically added if "auth optional pam_group.so" line is added to "/etc/pam.d/ common-auth" and set it in "/etc/security/group.conf". See pam_group(8). Note     The base-passwd package contains an authoritative list of the user and the group: "/usr/share/doc/base-passwd/ users-and-groups.html". 4.2. Managing account and password information     Here are few notable commands to manage account information. Table 4.3. List of commands to manage account information +---------------------------------------------------------------+ | command | function | |--------------------+------------------------------------------| |getent passwd |browse account information of "user_name" | |user_name | | |--------------------+------------------------------------------| |getent shadow |browse shadowed account information of " | |user_name |user_name" |     |--------------------+------------------------------------------| |getent group |browse group information of "group_name" | |group_name | | |--------------------+------------------------------------------| |passwd |manage password for the account | |--------------------+------------------------------------------| |passwd -e |set one-time password for the account | | |activation | |--------------------+------------------------------------------| |chage |manage password aging information | +---------------------------------------------------------------+     You may need to have the root privilege for some functions to work. See crypt(3) for the password and data encryption. Note     On the system set up with PAM and NSS as the Debian salsa machine, the content of local "/etc/passwd", "/etc/group" and "/ etc/shadow" may not be actively used by the system. Above commands are valid even under such environment. 4.3. Good password When creating an account during your system installation or with the passwd(1) command, you should choose a good password which     consists of at least 6 to 8 characters including one or more characters from each of the following sets according to passwd (1). * Lower case alphabetics     * Digits 0 through 9 * Punctuation marks Warning Do not choose guessable words for the password. Account name,     social security number, phone number, address, birthday, name of your family members or pets, dictionary words, simple sequence of characters such as "12345" or "qwerty", … are all bad choice for the password. 4.4. Creating encrypted password     There are independent tools to generate encrypted passwords with salt. Table 4.4. List of tools to generate password +---------------------------------------------------------------+ |package| popcon |size| command | function | |-------+--------+----+---------+-------------------------------|     |whois |V:25, |387 |mkpasswd |over-featured front end to the | | |I:251 | | |crypt(3) library | |-------+--------+----+---------+-------------------------------| |openssl|V:841, |2111|openssl |compute password hashes | | |I:995 | |passwd |(OpenSSL). passwd(1ssl) | +---------------------------------------------------------------+ 4.5. PAM and NSS Modern Unix-like systems such as the Debian system provide PAM     (Pluggable Authentication Modules) and NSS (Name Service Switch) mechanism to the local system administrator to configure his system. The role of these can be summarizes as the following. * PAM offers a flexible authentication mechanism used by the application software thus involves password data exchange.     * NSS offers a flexible name service mechanism which is frequently used by the C standard library to obtain the user and group name for programs such as ls(1) and id(1).     These PAM and NSS systems need to be configured consistently.     The notable packages of PAM and NSS systems are the following. Table 4.5. List of notable PAM and NSS systems +---------------------------------------------------------------+ | package |popcon|size | description | |-------------------+------+-----+------------------------------| |libpam-modules |V:889,|984 |Pluggable Authentication | | |I:999 | |Modules (basic service) | |-------------------+------+-----+------------------------------| | |V:0, | |Pluggable Authentication | |libpam-ldap |I:6 |249 |Module allowing LDAP | | | | |interfaces | |-------------------+------+-----+------------------------------| | |V:0, | |Pluggable Authentication | |libpam-cracklib |I:8 |117 |Module to enable cracklib | | | | |support | |-------------------+------+-----+------------------------------| | |V:571,| |Pluggable Authentication | |libpam-systemd |I:936 |627 |Module to register user | | | | |sessions for logind | |-------------------+------+-----+------------------------------| | | | |Pluggable Authentication | |libpam-doc |I:0 |152 |Modules (documentation in html|     | | | |and text) | |-------------------+------+-----+------------------------------| | |V:917,| |GNU C Library: Shared | |libc6 |I:999 |12988|libraries which also provides | | | | |"Name Service Switch" service | |-------------------+------+-----+------------------------------| |glibc-doc |I:8 |3503 |GNU C Library: Manpages | |-------------------+------+-----+------------------------------| | | | |GNU C Library: Reference | |glibc-doc-reference|I:4 |13841|manual in info, pdf and html | | | | |format (non-free) | |-------------------+------+-----+------------------------------| |libnss-mdns |I:510 |141 |NSS module for Multicast DNS | | | | |name resolution | |-------------------+------+-----+------------------------------| |libnss-ldap |I:5 |265 |NSS module for using LDAP as a| | | | |naming service | |-------------------+------+-----+------------------------------| | | | |NSS module for using LDAP as a| |libnss-ldapd |I:15 |129 |naming service (new fork of | | | | |libnss-ldap) | +---------------------------------------------------------------+ * "The Linux-PAM System Administrators' Guide" in libpam-doc is essential for learning PAM configuration.     * "System Databases and Name Service Switch" section in glibc-doc-reference is essential for learning NSS configuration. Note     You can see more extensive and current list by "aptitude search 'libpam-|libnss-'" command. The acronym NSS may also mean "Network Security Service" which is different from "Name Service Switch". Note     PAM is the most basic way to initialize environment variables for each program with the system wide default value. Under systemd, libpam-systemd package is installed to manage user     logins by registering user sessions in the systemd control group hierarchy for logind. See systemd-logind(8), logind.conf(5), and pam_systemd(8). 4.5.1. Configuration files accessed by PAM and NSS     Here are a few notable configuration files accessed by PAM and NSS. Table 4.6. List of configuration files accessed by PAM and NSS +---------------------------------------------------------------+ |configuration| function | | file | | |-------------+-------------------------------------------------| |/etc/pam.d/ |set up PAM configuration for the "program_name" | |program_name |program; see pam(7) and pam.d(5) | |-------------+-------------------------------------------------| |/etc/ |set up NSS configuration with the entry for each | |nsswitch.conf|service. See nsswitch.conf(5) | |-------------+-------------------------------------------------| |/etc/nologin |limit the user login by the pam_nologin(8) module| |-------------+-------------------------------------------------| |/etc/ |limit the tty for the root access by the | |securetty |pam_securetty(8) module | |-------------+-------------------------------------------------| |/etc/security|set access limit by the pam_access(8) module | |/access.conf | | |-------------+-------------------------------------------------|     |/etc/security|set group based restraint by the pam_group(8) | |/group.conf |module | |-------------+-------------------------------------------------| |/etc/security|set environment variables by the pam_env(8) | |/pam_env.conf|module | |-------------+-------------------------------------------------| |/etc/ |set additional environment variables by the | |environment |pam_env(8) module with the "readenv=1" argument | |-------------+-------------------------------------------------| |/etc/default/|set locale by pam_env(8) module with the "readenv| |locale |=1 envfile=/etc/default/locale" argument (Debian)| |-------------+-------------------------------------------------| |/etc/security|set resource restraint (ulimit, core, …) by the | |/limits.conf |pam_linits(8) module | |-------------+-------------------------------------------------| |/etc/security|set time restraint by the pam_time(8) module | |/time.conf | | |-------------+-------------------------------------------------| |/etc/systemd/|set systemd login manager configuration (see | |logind.conf |logind.conf(5) and systemd-logind.service(8)) | +---------------------------------------------------------------+ The limitation of the password selection is implemented by the     PAM modules, pam_unix(8) and pam_cracklib(8). They can be configured by their arguments. Tip     PAM modules use suffix ".so" for their filenames. 4.5.2. The modern centralized system management The modern centralized system management can be deployed using the centralized Lightweight Directory Access Protocol (LDAP)     server to administer many Unix-like and non-Unix-like systems on the network. The open source implementation of the Lightweight Directory Access Protocol is OpenLDAP Software. The LDAP server provides the account information through the use of PAM and NSS with libpam-ldap and libnss-ldap packages for the     Debian system. Several actions are required to enable this (I have not used this setup and the following is purely secondary information. Please read this in this context.). * You set up a centralized LDAP server by running a program such as the stand-alone LDAP daemon, slapd(8). * You change the PAM configuration files in the "/etc/pam.d/" directory to use "pam_ldap.so" instead of the default "pam_unix.so". + Debian uses "/etc/pam_ldap.conf" as the configuration file for libpam-ldap and "/etc/pam_ldap.secret" as the file to store the password of the root. * You change the NSS configuration in the "/etc/nsswitch.conf" file to use "ldap" instead of the default ("compat" or     "file"). + Debian uses "/etc/libnss-ldap.conf" as the configuration file for libnss-ldap. * You must make libpam-ldap to use SSL (or TLS) connection for the security of password. * You may make libnss-ldap to use SSL (or TLS) connection to ensure integrity of data at the cost of the LDAP network overhead. * You should run nscd(8) locally to cache any LDAP search results in order to reduce the LDAP network traffic. See documentations in pam_ldap.conf(5) and "/usr/share/doc/     libpam-doc/html/" offered by the libpam-doc package and "info libc 'Name Service Switch'" offered by the glibc-doc package.     Similarly, you can set up alternative centralized systems with other methods. * Integration of user and group with the Windows system. + Access Windows domain services by the winbind and libpam_winbind packages. + See winbindd(8) and Integrating MS Windows Networks with Samba.     * Integration of user and group with the legacy Unix-like system. + Access NIS (originally called YP) or NIS+ by the nis package. + See The Linux NIS(YP)/NYS/NIS+ HOWTO. 4.5.3. "Why GNU su does not support the wheel group" This is the famous phrase at the bottom of the old "info su" page by Richard M. Stallman. Not to worry: the current su command in     Debian uses PAM, so that one can restrict the ability to use su to the root group by enabling the line with "pam_wheel.so" in "/ etc/pam.d/su". 4.5.4. Stricter password rule     Installing the libpam-cracklib package enables you to force stricter password rule.     On a typical GNOME system which automatically installs libpam-gnome-keyring, "/etc/pam.d/common-password" looks like: # here are the per-package modules (the "Primary" block) password requisite pam_cracklib.so retry=3 minlen=8 difok=3 password [success=1 default=ignore] pam_unix.so obscure use_authtok try_first_pass yescrypt # here's the fallback if no module succeeds password requisite pam_deny.so     # prime the stack with a positive return value if there isn't one already; # this avoids us returning an error just because nothing sets a success code # since the modules above will each just jump around password required pam_permit.so # and here are more per-package modules (the "Additional" block) password optional pam_gnome_keyring.so # end of pam-auth-update config 4.6. Security of authentication Note     The information here may not be sufficient for your security needs but it should be a good start. 4.6.1. Secure password on the Internet Many popular transportation layer services communicate messages including password authentication in the plain text. It is very bad idea to transmit password in the plain text over the wild     Internet where it can be intercepted. You can run these services over "Transport Layer Security" (TLS) or its predecessor, "Secure Sockets Layer" (SSL) to secure entire communication including password by the encryption. Table 4.7. List of insecure and secure services and ports +---------------------------------------------------+ |insecure service name|port|secure service name|port| |---------------------+----+-------------------+----| |www (http) |80 |https |443 | |---------------------+----+-------------------+----| |smtp (mail) |25 |ssmtp (smtps) |465 | |---------------------+----+-------------------+----| |ftp-data |20 |ftps-data |989 |     |---------------------+----+-------------------+----| |ftp |21 |ftps |990 | |---------------------+----+-------------------+----| |telnet |23 |telnets |992 | |---------------------+----+-------------------+----| |imap2 |143 |imaps |993 | |---------------------+----+-------------------+----| |pop3 |110 |pop3s |995 | |---------------------+----+-------------------+----| |ldap |389 |ldaps |636 | +---------------------------------------------------+ The encryption costs CPU time. As a CPU friendly alternative, you can keep communication in plain text while securing just the password with the secure authentication protocol such as "Authenticated Post Office Protocol" (APOP) for POP and     "Challenge-Response Authentication Mechanism MD5" (CRAM-MD5) for SMTP and IMAP. (For sending mail messages over the Internet to your mail server from your mail client, it is recently popular to use new message submission port 587 instead of traditional SMTP port 25 to avoid port 25 blocking by the network provider while authenticating yourself with CRAM-MD5.) 4.6.2. Secure Shell The Secure Shell (SSH) program provides secure encrypted communications between two untrusted hosts over an insecure network with the secure authentication. It consists of the     OpenSSH client, ssh(1), and the OpenSSH daemon, sshd(8). This SSH can be used to tunnel an insecure protocol communication such as POP and X securely over the Internet with the port forwarding feature. The client tries to authenticate itself using host-based authentication, public key authentication, challenge-response     authentication, or password authentication. The use of public key authentication enables the remote password-less login. See Section 6.3, “The remote access server and utilities (SSH)”. 4.6.3. Extra security measures for the Internet Even when you run secure services such as Secure Shell (SSH) and Point-to-point tunneling protocol (PPTP) servers, there are still     chances for the break-ins using brute force password guessing attack etc. from the Internet. Use of the firewall policy (see Section 5.7, “Netfilter infrastructure”) together with the following security tools may improve the security situation. Table 4.8. List of tools to provide extra security measures +---------------------------------------------------------------+ | package |popcon |size| description | |-------------+-------+----+------------------------------------| |knockd |V:0, |110 |small port-knock daemon knockd(1) | | |I:2 | |and client knock(1) |     |-------------+-------+----+------------------------------------| |fail2ban |V:98, |2126|ban IPs that cause multiple | | |I:111 | |authentication errors | |-------------+-------+----+------------------------------------| |libpam-shield|V:0, |115 |lock out remote attackers trying | | |I:0 | |password guessing | +---------------------------------------------------------------+ 4.6.4. Securing the root password     To prevent people to access your machine with root privilege, you need to make following actions. * Prevent physical access to the hard disk * Lock UEFI/BIOS and prevent booting from the removable media     * Set password for GRUB interactive session * Lock GRUB menu from editing     With physical access to hard disk, resetting the password is relatively easy with following steps. 1. Move the hard disk to a PC with CD bootable UEFI/BIOS. 2. Boot system with a rescue media (Debian boot disk, Knoppix CD, GRUB CD, …).     3. Mount root partition with read/write access. 4. Edit "/etc/passwd" in the root partition and make the second entry for the root account empty. If you have edit access to the GRUB menu entry (see     Section 3.1.2, “Stage 2: the boot loader”) for grub-rescue-pc at boot time, it is even easier with following steps. 1. Boot system with the kernel parameter changed to something like "root=/dev/hda6 rw init=/bin/sh".     2. Edit "/etc/passwd" and make the second entry for the root account empty. 3. Reboot system.     The root shell of the system is now accessible without password. Note Once one has root shell access, he can access everything on the     system and reset any passwords on the system. Further more, he may compromise password for all user accounts using brute force password cracking tools such as john and crack packages (see Section 9.5.11, “System security and integrity check”). This cracked password may lead to compromise other systems. The only reasonable software solution to avoid all these concerns     is to use software encrypted root partition (or "/etc" partition) using dm-crypt and initramfs (see Section 9.9, “Data encryption tips”). You always need password to boot the system, though. 4.7. Other access controls     There are access controls to the system other than the password based authentication and file permissions. Note     See Section 9.4.16, “Alt-SysRq key” for restricting the kernel secure attention key (SAK) feature. 4.7.1. Access control lists (ACLs)     ACLs are a superset of the regular permissions as explained in Section 1.2.3, “Filesystem permissions”. You encounter ACLs in action on modern desktop environment. When     a formatted USB storage device is auto mounted as, e.g., "/media/ penguin/USBSTICK", a normal user penguin can execute: $ cd /media/penguin $ ls -la     total 16 drwxr-x---+ 1 root root 16 Jan 17 22:55 . drwxr-xr-x 1 root root 28 Sep 17 19:03 .. drwxr-xr-x 1 penguin penguin 18 Jan 6 07:05 USBSTICK "+" in the 11th column indicates ACLs are in action. Without     ACLs, a normal user penguin shouldn't be able to list like this since penguin isn't in root group. You can see ACLs as: $ getfacl . # file: . # owner: root # group: root     user::rwx user:penguin:r-x group::--- mask::r-x other::---     Here: * "user::rwx", "group::---", and "other::---" correspond to the regular owner, group, and other permissions.     * The ACL "user:penguin:r-x" allows a normal user penguin to have "r-x" permissions. This enabled "ls -la" to list directory content. * The ACL "mask::r-x" sets the upper bound of permissions.     See "POSIX Access Control Lists on Linux", acl(5), getfacl(1), and setfacl(1) for more. 4.7.2. sudo sudo(8) is a program designed to allow a sysadmin to give limited root privileges to users and log root activity. sudo requires     only an ordinary user's password. Install sudo package and activate it by setting options in "/etc/sudoers". See configuration example at "/usr/share/doc/sudo/examples/sudoers" and Section 1.1.12, “sudo configuration”. My usage of sudo for the single user system (see Section 1.1.12, “sudo configuration”) is aimed to protect myself from my own     stupidity. Personally, I consider using sudo a better alternative than using the system from the root account all the time. For example, the following changes the owner of "some_file" to " my_name".     $ sudo chown my_name some_file Of course if you know the root password (as self-installed Debian     users do), any command can be run under root from any user's account using "su -c". 4.7.3. PolicyKit     PolicyKit is an operating system component for controlling system-wide privileges in Unix-like operating systems. Newer GUI applications are not designed to run as privileged     processes. They talk to privileged processes via PolicyKit to perform administrative operations.     PolicyKit limits such operations to user accounts belonging to the sudo group on the Debian system.     See polkit(8). 4.7.4. Restricting access to some server services For system security, it is a good idea to disable as much server     programs as possible. This becomes critical for network servers. Having unused servers, activated either directly as daemon or via super-server program, are considered security risks.     Many programs, such as sshd(8), use PAM based access control. There are many ways to restrict access to some server services. * configuration files: "/etc/default/program_name" * Systemd service unit configuration for daemon * PAM (Pluggable Authentication Modules) * "/etc/inetd.conf" for super-server     * "/etc/hosts.deny" and "/etc/hosts.allow" for TCP wrapper, tcpd(8) * "/etc/rpc.conf" for Sun RPC * "/etc/at.allow" and "/etc/at.deny" for atd(8) * "/etc/cron.allow" and "/etc/cron.deny" for crontab(1) * Network firewall of netfilter infrastructure See Section 3.5, “System management”, Section 4.5.1,     “Configuration files accessed by PAM and NSS”, and Section 5.7, “Netfilter infrastructure”. Tip     Sun RPC services need to be active for NFS and other RPC based programs. Tip     If you have problems with remote access in a recent Debian system, comment out offending configuration such as "ALL: PARANOID" in "/etc/hosts.deny" if it exists. (But you must be careful on security risks involved with this kind of action.) 4.7.5. Linux security features     Linux kernel has evolved and supports security features not found in traditional UNIX implementations.     Linux supports extended attributes which extend the traditional UNIX attributes (see xattr(7)). Linux divides the privileges traditionally associated with     superuser into distinct units, known as capabilities(7), which can be independently enabled and disabled. Capabilities are a per-thread attribute since kernel version 2.2. The Linux Security Module (LSM) framework provides a mechanism     for various security checks to be hooked by new kernel extensions. For example: * AppArmor * Security-Enhanced Linux (SELinux)     * Smack (Simplified Mandatory Access Control Kernel) * Tomoyo Linux Since these extensions may tighten privilege model tighter than     the ordinary Unix-like security model policies, even the root power may be restricted. You are advised to read the Linux Security Module (LSM) framework document at kernel.org. Linux namespaces wrap a global system resource in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the global resource.     Changes to the global resource are visible to other processes that are members of the namespace, but are invisible to other processes. Since kernel version 5.6, there are 8 kinds of namespaces (see namespaces(7), unshare(1), nsenter(1)).     As of Debian 11 Bullseye (2021), Debian uses unified cgroup hierarchy (a.k.a. cgroups-v2).     Usage examples of namespaces with cgroups to isolate their processes and to allow resource control are: * Systemd. See Section 3.2.1, “Systemd init”.     * Sandbox environment. See Section 7.7, “Sandbox”. * Linux containers such as Docker, LXC. See Section 9.11, “Virtualized system”. These functionalities can't be realized by Section 4.1, “Normal     Unix authentication”. These advanced topics are mostly out-of-scope for this introductory document. Chapter 5. Network setup Tip     For modern Debian specific guide to the networking, read The Debian Administrator's Handbook — Configuring the Network. Tip     Under systemd, networkd may be used to manage networks. See systemd-networkd(8). 5.1. The basic network infrastructure     Let's review the basic network infrastructure on the modern Debian system. Table 5.1. List of network configuration tools +-----------------------------------------------------------------------+ | packages |popcon|size | type | description | |---------------------+------+-----+-------------------+----------------| | | | | |NetworkManager | |network-manager |V:392,|15542|config::NM |(daemon): manage| | |I:459 | | |the network | | | | | |automatically | |---------------------+------+-----+-------------------+----------------| |network-manager-gnome|V:121,|5583 |config::NM |NetworkManager | | |I:369 | | |(GNOME frontend)| |---------------------+------+-----+-------------------+----------------| | | | | |Netplan | | | | | |(generator): | | | | | |Unified, | | |V:1, | | |declarative | |netplan.io |I:5 |319 |config::NM+networkd|interface to | | | | | |NetworkManager | | | | | |and | | | | | |systemd-networkd| | | | | |backends | |---------------------+------+-----+-------------------+----------------| | | | | |standardized | | |V:608,| | |tool to bring up| |ifupdown |I:979 |199 |config::ifupdown |and down the | | | | | |network (Debian | | | | | |specific) | |---------------------+------+-----+-------------------+----------------| |isc-dhcp-client |V:217,|2875 |config::low-level |DHCP client | | |I:981 | | | | |---------------------+------+-----+-------------------+----------------| | |V:0, | | |configuration | |pppoeconf |I:5 |186 |config::helper |helper for PPPoE| | | | | |connection | |---------------------+------+-----+-------------------+----------------| | |V:353,| | |client support | |wpasupplicant |I:513 |3862 |config::helper |for WPA and WPA2| | | | | |(IEEE 802.11i) | |---------------------+------+-----+-------------------+----------------| | |V:0, | | |Qt GUI client | |wpagui |I:1 |774 |config::helper |for | | | | | |wpa_supplicant | |---------------------+------+-----+-------------------+----------------| | | | | |tools for | |wireless-tools |V:179,|292 |config::helper |manipulating | | |I:244 | | |Linux Wireless | | | | | |Extensions | |---------------------+------+-----+-------------------+----------------| | | | | |tool for | |iw |V:34, |302 |config::helper |configuring | | |I:475 | | |Linux wireless | | | | | |devices | |---------------------+------+-----+-------------------+----------------| | | | | |iproute2, IPv6 | | | | | |and other | |iproute2 |V:736,|3606 |config::iproute2 |advanced network| | |I:972 | | |configuration: | | | | | |ip(8), tc(8), | | | | | |etc | |---------------------+------+-----+-------------------+----------------| | | | | |administration | |iptables |V:319,|2414 |config::Netfilter |tools for packet| | |I:718 | | |filtering and | | | | | |NAT (Netfilter) | |---------------------+------+-----+-------------------+----------------| | | | | |administration | | | | | |tools for packet| | |V:106,| | |filtering and | |nftables |I:701 |182 |config::Netfilter |NAT (Netfilter) | | | | | |(successor to | | | | | |{ip,ip6,arp,eb} | | | | | |tables) | |---------------------+------+-----+-------------------+----------------| | | | | |test network | | | | | |reachability of | |iputils-ping |V:194,|122 |test |a remote host by| | |I:997 | | |hostname or IP | | | | | |address |     | | | | |(iproute2) | |---------------------+------+-----+-------------------+----------------| | | | | |test network | | |V:3, | | |reachability of | |iputils-arping |I:36 |50 |test |a remote host | | | | | |specified by the| | | | | |ARP address | |---------------------+------+-----+-------------------+----------------| | |V:2, | | |trace the | |iputils-tracepath |I:30 |47 |test |network path to | | | | | |a remote host | |---------------------+------+-----+-------------------+----------------| | |V:95, | | |display or | |ethtool |I:267 |739 |test |change Ethernet | | | | | |device settings | |---------------------+------+-----+-------------------+----------------| | | | | |trace the | |mtr-tiny |V:5, |156 |test::low-level |network path to | | |I:46 | | |a remote host | | | | | |(curses) | |---------------------+------+-----+-------------------+----------------| | | | | |trace the | |mtr |V:4, |209 |test::low-level |network path to | | |I:41 | | |a remote host | | | | | |(curses and GTK)| |---------------------+------+-----+-------------------+----------------| | | | | |tools for common| | |V:0, | | |network | |gnome-nettool |I:17 |2492 |test::low-level |information | | | | | |operations | | | | | |(GNOME) | |---------------------+------+-----+-------------------+----------------| | |V:25, | | |network mapper /| |nmap |I:199 |4498 |test::low-level |port scanner | | | | | |(Nmap, console) | |---------------------+------+-----+-------------------+----------------| | | | | |network traffic | |tcpdump |V:17, |1340 |test::low-level |analyzer | | |I:175 | | |(Tcpdump, | | | | | |console) | |---------------------+------+-----+-------------------+----------------| | | | | |network traffic | |wireshark |I:45 |10417|test::low-level |analyzer | | | | | |(Wireshark, GTK)| |---------------------+------+-----+-------------------+----------------| | |V:2, | | |network traffic | |tshark |I:25 |400 |test::low-level |analyzer | | | | | |(console) | |---------------------+------+-----+-------------------+----------------| | | | | |produce a | | |V:0, | | |summarization of| |tcptrace |I:2 |401 |test::low-level |the connections | | | | | |from tcpdump | | | | | |output | |---------------------+------+-----+-------------------+----------------| | | | | |flexible network| |snort |V:0, |2203 |test::low-level |intrusion | | |I:0 | | |detection system| | | | | |(Snort) | |---------------------+------+-----+-------------------+----------------| | |V:0, | | |display network | |ntopng |I:1 |15904|test::low-level |usage in web | | | | | |browser | |---------------------+------+-----+-------------------+----------------| | | | | |network clients | | |V:16, | | |provided with | |dnsutils |I:280 |276 |test::low-level |BIND: nslookup | | | | | |(8), nsupdate | | | | | |(8), dig(8) | |---------------------+------+-----+-------------------+----------------| | | | | |check DNS zone | |dlint |V:0, |53 |test::low-level |information | | |I:3 | | |using nameserver| | | | | |lookups | |---------------------+------+-----+-------------------+----------------| | |V:0, | | |trace a chain of| |dnstracer |I:1 |59 |test::low-level |DNS servers to | | | | | |the source | +-----------------------------------------------------------------------+ 5.1.1. The hostname resolution The hostname resolution is currently supported by the NSS (Name     Service Switch) mechanism too. The flow of this resolution is the following. 1. The "/etc/nsswitch.conf" file with stanza like "hosts: files dns" dictates the hostname resolution order. (This replaces the old functionality of the "order" stanza in "/etc/ host.conf".) 2. The files method is invoked first. If the hostname is found     in the "/etc/hosts" file, it returns all valid addresses for it and exits. (The "/etc/host.conf" file contains "multi on".) 3. The dns method is invoked. If the hostname is found by the query to the Internet Domain Name System (DNS) identified by the "/etc/resolv.conf" file, it returns all valid addresses for it and exits. A typical workstation may be installed with its host name set to,     e.g., "host_name" and its optional domain name set to an empty string. Then, "/etc/hosts" looks like the following. 127.0.0.1 localhost 127.0.1.1 host_name     # The following lines are desirable for IPv6 capable hosts ::1 localhost ip6-localhost ip6-loopback ff02::1 ip6-allnodes ff02::2 ip6-allrouters     Each line starts with a IP address and it is followed by the associated hostname. The IP address 127.0.1.1 in the second line of this example may not be found on some other Unix-like systems. The Debian     Installer creates this entry for a system without a permanent IP address as a workaround for some software (e.g., GNOME) as documented in the bug #719621.     The host_name matches the hostname defined in the "/etc/hostname" (see Section 3.7.1, “The hostname”).     For a system with a permanent IP address, that permanent IP address should be used here instead of 127.0.1.1. For a system with a permanent IP address and a fully qualified     domain name (FQDN) provided by the Domain Name System (DNS), that canonical host_name.domain_name should be used instead of just host_name. The "/etc/resolv.conf" is a static file if the resolvconf package is not installed. If installed, it is a symbolic link. Either     way, it contains information that initialize the resolver routines. If the DNS is found at IP="192.168.11.1", it contains the following.     nameserver 192.168.11.1 The resolvconf package makes this "/etc/resolv.conf" into a     symbolic link and manages its contents by the hook scripts automatically. For the PC workstation on the typical adhoc LAN environment, the     hostname can be resolved via Multicast DNS (mDNS) in addition to the basic files and dns methods. * Avahi provides a framework for Multicast DNS Service Discovery on Debian. * It is equivalent of Apple Bonjour / Apple Rendezvous. * The libnss-mdns plugin package provides host name resolution via mDNS for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc).     * The "/etc/nsswitch.conf" file should have stanza like "hosts: files mdns4_minimal [NOTFOUND=return] dns" (see /usr/share/ doc/libnss-mdns/README.Debian for other configurations). * A host name suffixed with the ".local" pseudo-top-level domain is resolved by sending a mDNS query message in a multicast UDP packet using IPv4 address "224.0.0.251" or IPv6 address "FF02::FB". Note     The expansion of generic Top-Level Domains (gTLD) in the Domain Name System is underway. Watch out for the name collision when chosing a domain name used only within LAN. Note Use of packages such as libnss-resolve together with systemd-resolved, or libnss-myhostname, or libnss-mymachine, with     coressponding listings on the "hosts" line in the "/etc/ nsswitch.conf" file may override the traditional network configuration discussed in the above. See nss-resolve(8), systemd-resolved(8), nss-myhostname(8), and nss-mymachines(8) for more. 5.1.2. The network interface name     The systemd uses "Predictable Network Interface Names" such as "enp0s25". 5.1.3. The network address range for the LAN Let us be reminded of the IPv4 32 bit address ranges in each     class reserved for use on the local area networks (LANs) by rfc1918. These addresses are guaranteed not to conflict with any addresses on the Internet proper. Note     IP address written with colon are IPv6 address, e.g., "::1" for localhost. Table 5.2. List of network address ranges +---------------------------------------------------------------+ |Class| network addresses | net mask |net mask /| number of | | | | | bits | subnets | |-----+-------------------+-------------+----------+------------| |A |10.x.x.x |255.0.0.0 |/8 |1 |     |-----+-------------------+-------------+----------+------------| |B |172.16.x.x — |255.255.0.0 |/16 |16 | | |172.31.x.x | | | | |-----+-------------------+-------------+----------+------------| |C |192.168.0.x — |255.255.255.0|/24 |256 | | |192.168.255.x | | | | +---------------------------------------------------------------+ Note If one of these addresses is assigned to a host, then that host     must not access the Internet directly but must access it through a gateway that acts as a proxy for individual services or else does Network Address Translation (NAT). The broadband router usually performs NAT for the consumer LAN environment. 5.1.4. The network device support Although most hardware devices are supported by the Debian     system, there are some network devices which require DFSG non-free firmware to support them. Please see Section 9.10.5, “Hardware drivers and firmware”. 5.2. The modern network configuration for desktop Network interfaces are typically initialized in     "networking.service" for the lo interface and "NetworkManager.service" for other interfaces on modern Debian desktop system under systemd. Debian can manage the network connection via management daemon     software such as NetworkManager (NM) (network-manager and associated packages). * They come with their own GUI and command-line programs as their user interfaces. * They come with their own daemon as their backend system.     * They allow easy connection of your system to the Internet. * They allow easy management of wired and wireless network configuration. * They allow us to configure network independent of the legacy ifupdown package. Note     Do not use these automatic network configuration tools for servers. These are aimed primarily for mobile desktop users on laptops. These modern network configuration tools need to be configured     properly to avoid conflicting with the legacy ifupdown package and its configuration file "/etc/network/interfaces". 5.2.1. GUI network configuration tools     Official documentations for NM on Debian are provided in "/usr/ share/doc/network-manager/README.Debian".     Essentially, the network configuration for desktop is done as follows. 1. Make desktop user, e.g. foo, belong to group "netdev" by the following (Alternatively, do it automatically via D-bus under modern desktop environments such as GNOME and KDE). $ sudo usermod -a -G foo netdev 2. Keep configuration of "/etc/network/interfaces" as simple as in the following.     auto lo iface lo inet loopback 3. Restart NM by the following. $ sudo systemctl restart network-manager 4. Configure your network via GUI. Note     Only interfaces which are not listed in "/etc/network/interfaces" are managed by NM to avoid conflict with ifupdown. Tip If you wish to extend network configuration capabilities of NM,     please seek appropriate plug-in modules and supplemental packages such as network-manager-openconnect, network-manager-openvpn-gnome, network-manager-pptp-gnome, mobile-broadband-provider-info, gnome-bluetooth, etc. 5.3. The modern network configuration without GUI Under systemd, the network may be configured in /etc/systemd/     network/ instead. See systemd-resolved(8), resolved.conf(5), and systemd-networkd(8).     This allows the modern network configuration without GUI.     A DHCP client configuration can be set up by creating "/etc/ systemd/network/dhcp.network". E.g.: [Match] Name=en*     [Network] DHCP=yes     A static network configuration can be set up by creating "/etc/ systemd/network/static.network". E.g.: [Match] Name=en*     [Network] Address=192.168.0.15/24 Gateway=192.168.0.1 5.4. The modern network configuration for cloud The modern network configuration for cloud may use cloud-init and     netplan.io packages (see Section 3.7.4, “Cloud system initialization”). The netplan.io package supports systemd-networkd and     NetworkManager as its network configuration backends, and enables the declarative network configuration using YAML data. When you change YAML: * Run "netplan generate" command to generate all the necessary backend configuration from YAML.     * Run "netplan apply" command to apply the generated configuration to the backends.     See "Netplan documentation", netplan(5), netplan-generate(8), and netplan-apply(8). See also "Cloud-init documentation" (especially around     "Configuration sources" and "Netplan Passthrough") for how cloud-init can integrate netplan.io configuration with alternative data sources. 5.4.1. The modern network configuration for cloud with DHCP     A DHCP client configuration can be set up by creating a data source file "/etc/netplan/50-dhcp.yaml": network: version: 2 ethernets:     all-en: match: name: "en*" dhcp4: true dhcp6: true 5.4.2. The modern network configuration for cloud with static IP     A static network configuration can be set up by creating a data source file "/etc/netplan/50-static.yaml": network: version: 2 ethernets: eth0:     addresses: - 192.168.0.15/24 routes: - to: default via: 192.168.0.1 5.4.3. The modern network configuration for cloud with Network Manger The network client configuration using Network Manger     infrastructure can be set up by creating a data source file "/etc /netplan/00-network-manager.yaml": network:     version: 2 renderer: NetworkManager 5.5. The low level network configuration     For the low level network configuration on Linux, use the iproute2 programs (ip(8), …) . 5.5.1. Iproute2 commands Iproute2 commands offer complete low-level network configuration     capabilities. Here is a translation table from obsolete net-tools commands to new iproute2 etc. commands. Table 5.3. Translation table from obsolete net-tools commands to new iproute2 commands +---------------------------------------------------------------+ | obsolete |new iproute2 | manipulation | | net-tools | etc. | | |-------------+-------------+-----------------------------------| |ifconfig(8) |ip addr |protocol (IP or IPv6) address on a | | | |device | |-------------+-------------+-----------------------------------| |route(8) |ip route |routing table entry |     |-------------+-------------+-----------------------------------| |arp(8) |ip neigh |ARP or NDISC cache entry | |-------------+-------------+-----------------------------------| |ipmaddr |ip maddr |multicast address | |-------------+-------------+-----------------------------------| |iptunnel |ip tunnel |tunnel over IP | |-------------+-------------+-----------------------------------| |nameif(8) |ifrename(8) |name network interfaces based on | | | |MAC addresses | |-------------+-------------+-----------------------------------| |mii-tool(8) |ethtool(8) |Ethernet device settings | +---------------------------------------------------------------+     See ip(8) and Linux Advanced Routing & Traffic Control. 5.5.2. Safe low level network operations     You may use low level network commands as follows safely since they do not change network configuration. Table 5.4. List of low level network commands +---------------------------------------------------------------+ | command | description | |----------------------+----------------------------------------| |ip addr show |display the link and address status of | | |active interfaces | |----------------------+----------------------------------------| |route -n |display all the routing table in | | |numerical addresses | |----------------------+----------------------------------------| |ip route show |display all the routing table in | | |numerical addresses | |----------------------+----------------------------------------| |arp |display the current content of the ARP | | |cache tables | |----------------------+----------------------------------------| |ip neigh |display the current content of the ARP | | |cache tables | |----------------------+----------------------------------------| |plog |display ppp daemon log | |----------------------+----------------------------------------| |ping yahoo.com |check the Internet connection to | | |"yahoo.com" | |----------------------+----------------------------------------| |whois yahoo.com |check who registered "yahoo.com" in the |     | |domains database | |----------------------+----------------------------------------| |traceroute yahoo.com |trace the Internet connection to | | |"yahoo.com" | |----------------------+----------------------------------------| |tracepath yahoo.com |trace the Internet connection to | | |"yahoo.com" | |----------------------+----------------------------------------| |mtr yahoo.com |trace the Internet connection to | | |"yahoo.com" (repeatedly) | |----------------------+----------------------------------------| |dig [@dns-server.com] |check DNS records of "example.com" by | |example.com [{a|mx| |"dns-server.com" for a "a", "mx", or | |any}] |"any" record | |----------------------+----------------------------------------| |iptables -L -n |check packet filter | |----------------------+----------------------------------------| |netstat -a |find all open ports | |----------------------+----------------------------------------| |netstat -l --inet |find listening ports | |----------------------+----------------------------------------| |netstat -ln --tcp |find listening TCP ports (numeric) | |----------------------+----------------------------------------| |dlint example.com |check DNS zone information of | | |"example.com" | +---------------------------------------------------------------+ Tip     Some of these low level network configuration tools reside in "/ usr/sbin/". You may need to issue full command path such as "/usr /sbin/ifconfig" or add "/usr/sbin" to the "$PATH" list in your "~ /.bashrc". 5.6. Network optimization Generic network optimization is beyond the scope of this     documentation. I touch only subjects pertinent to the consumer grade connection. Table 5.5. List of network optimization tools +---------------------------------------------------------------+ |packages |popcon |size| description | |---------+-------+----+----------------------------------------| |iftop |V:7, |93 |display bandwidth usage information on | | |I:100 | |an network interface | |---------+-------+----+----------------------------------------| |iperf |V:3, |360 |Internet Protocol bandwidth measuring | | |I:43 | |tool | |---------+-------+----+----------------------------------------| |ifstat |V:0, |60 |InterFace STATistics Monitoring | | |I:7 | | | |---------+-------+----+----------------------------------------| |bmon |V:1, |144 |portable bandwidth monitor and rate | | |I:18 | |estimator |     |---------+-------+----+----------------------------------------| |ethstatus|V:0, |40 |script that quickly measures network | | |I:3 | |device throughput | |---------+-------+----+----------------------------------------| |bing |V:0, |80 |empirical stochastic bandwidth tester | | |I:0 | | | |---------+-------+----+----------------------------------------| |bwm-ng |V:1, |95 |small and simple console-based bandwidth| | |I:13 | |monitor | |---------+-------+----+----------------------------------------| |ethstats |V:0, |23 |console-based Ethernet statistics | | |I:0 | |monitor | |---------+-------+----+----------------------------------------| |ipfm |V:0, |82 |bandwidth analysis tool | | |I:0 | | | +---------------------------------------------------------------+ 5.6.1. Finding optimal MTU     NM normally sets optimal Maximum Transmission Unit (MTU) automatically. In some occasion, you may wish to set MTU manually after experiments with ping(8) with "-M do" option to send a ICMP packet with various data packet size. MTU is the maximum     succeeding data packet size without IP fragmentation plus 28 bytes for the IPv4 and plus 48 bytes for the IPv6. For example the following finds MTU for IPv4 connection to be 1460 and MTU for IPv6 connection to be 1500. $ ping -4 -c 1 -s $((1500-28)) -M do www.debian.org PING (149.20.4.15) 1472(1500) bytes of data. ping: local error: message too long, mtu=1460 --- ping statistics --- 1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms $ ping -4 -c 1 -s $((1460-28)) -M do www.debian.org PING (130.89.148.77) 1432(1460) bytes of data. 1440 bytes from klecker-misc.debian.org (130.89.148.77): icmp_seq=1 ttl=50 time=325 ms     --- ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 325.318/325.318/325.318/0.000 ms $ ping -6 -c 1 -s $((1500-48)) -M do www.debian.org PING www.debian.org(mirror-csail.debian.org (2603:400a:ffff:bb8::801f:3e)) 1452 data bytes 1460 bytes from mirror-csail.debian.org (2603:400a:ffff:bb8::801f:3e): icmp_seq=1 ttl=47 time=191 ms --- www.debian.org ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 191.332/191.332/191.332/0.000 ms     This process is Path MTU (PMTU) discovery (RFC1191) and the tracepath(8) command can automate this. Table 5.6. Basic guide lines of the optimal MTU value +-----------------------------------------------------------+ | network environment |MTU | rationale |     |---------------------------------+----+--------------------| |Dial-up link (IP: PPP) |576 |standard | |---------------------------------+----+--------------------| |Ethernet link (IP: DHCP or fixed)|1500|standard and default| +-----------------------------------------------------------+     In addition to these basic guide lines, you should know the following. * Any use of tunneling methods (VPN etc.) may reduce optimal MTU further by their overheads.     * The MTU value should not exceed the experimentally determined PMTU value. * The bigger MTU value is generally better when other limitations are met. The maximum segment size (MSS) is used as an alternative measure     of packet size. The relationship between MSS and MTU are the following. * MSS = MTU - 40 for IPv4     * MSS = MTU - 60 for IPv6 Note     The iptables(8) (see Section 5.7, “Netfilter infrastructure”) based optimization can clamp packet size by the MSS and is useful for the router. See "TCPMSS" in iptables(8). 5.6.2. WAN TCP optimization The TCP throughput can be maximized by adjusting TCP buffer size parameters as in "TCP tuning" for the modern high-bandwidth and     high-latency WAN. So far, the current Debian default settings serve well even for my LAN connected by the fast 1G bps FTTP service. 5.7. Netfilter infrastructure Netfilter provides infrastructure for stateful firewall and     network address translation (NAT) with Linux kernel modules (see Section 3.9, “The kernel module initialization”). Table 5.7. List of firewall tools +---------------------------------------------------------------+ | packages |popcon|size| description | |---------------+------+----+-----------------------------------| | | | |administration tools for packet | |nftables |V:106,|182 |filtering and NAT (Netfilter) | | |I:701 | |(successor to {ip,ip6,arp,eb} | | | | |tables) | |---------------+------+----+-----------------------------------| | |V:319,| |administration tools for netfilter | |iptables |I:718 |2414|(iptables(8) for IPv4, ip6tables(8)| | | | |for IPv6) | |---------------+------+----+-----------------------------------| |arptables |V:0, |100 |administration tools for netfilter | | |I:1 | |(arptables(8) for ARP) | |---------------+------+----+-----------------------------------| |ebtables |V:14, |276 |administration tools for netfilter | | |I:29 | |(ebtables(8) for Ethernet bridging)| |---------------+------+----+-----------------------------------| |iptstate |V:0, |119 |continuously monitor netfilter | | |I:2 | |state (similar to top(1)) | |---------------+------+----+-----------------------------------| | |V:55, | |Uncomplicated Firewall (UFW) is a | |ufw |I:77 |859 |program for managing a netfilter | | | | |firewall | |---------------+------+----+-----------------------------------|     |gufw |V:5, |3660|graphical user interface for | | |I:10 | |Uncomplicated Firewall (UFW) | |---------------+------+----+-----------------------------------| | |V:11, | |firewalld is a dynamically managed | |firewalld |I:16 |2613|firewall program with support for | | | | |network zones | |---------------+------+----+-----------------------------------| |firewall-config|V:0, |1163|graphical user interface for | | |I:3 | |firewalld | |---------------+------+----+-----------------------------------| |shorewall-init |V:0, |88 |Shoreline Firewall initialization | | |I:0 | | | |---------------+------+----+-----------------------------------| |shorewall |V:3, |3090|Shoreline Firewall, netfilter | | |I:8 | |configuration file generator | |---------------+------+----+-----------------------------------| | |V:0, | |Shoreline Firewall, netfilter | |shorewall-lite |I:0 |71 |configuration file generator (light| | | | |version) | |---------------+------+----+-----------------------------------| | |V:0, | |Shoreline Firewall, netfilter | |shorewall6 |I:1 |1334|configuration file generator (IPv6 | | | | |version) | |---------------+------+----+-----------------------------------| | |V:0, | |Shoreline Firewall, netfilter | |shorewall6-lite|I:0 |71 |configuration file generator (IPv6,| | | | |light version) | +---------------------------------------------------------------+ Main user space program of netfilter is iptables(8). You can     manually configure netfilter interactively from shell, save its state with iptables-save(8), and restore it via init script with iptables-restore(8) upon system reboot.     Configuration helper scripts such as shorewall ease this process.     See documentations at Netfilter Documentation (or in "/usr/share/ doc/iptables/html/"). * Linux Networking-concepts HOWTO     * Linux 2.4 Packet Filtering HOWTO * Linux 2.4 NAT HOWTO Tip     Although these were written for Linux 2.4, both iptables(8) command and netfilter kernel function apply for Linux 2.6 and 3.x kernel series. Chapter 6. Network applications     After establishing network connectivity (see Chapter 5, Network setup), you can run various network applications. Tip     For modern Debian specific guide to the network infrastructure, read The Debian Administrator's Handbook — Network Infrastructure. Tip     If you enabled "2-Step Verification" with some ISP, you need to obtain an application password to access POP and SMTP services from your program. You may need to approve your host IP in advance. 6.1. Web browsers     There are many web browser packages to access remote contents with Hypertext Transfer Protocol (HTTP). Table 6.1. List of web browsers +---------------------------------------------------------------+ | package |popcon| size | type | description of web | | | | | | browser | |----------------+------+------+--------+-----------------------| |chromium |V:35, |234084|X |Chromium, (open-source | | |I:108 | | |browser from Google) | |----------------+------+------+--------+-----------------------| | | | | |Firefox, (open-source | |firefox |V:10, |239492|, , |browser from Mozilla, | | |I:15 | | |only available in | | | | | |Debian Unstable) | |----------------+------+------+--------+-----------------------| | |V:198,| | |Firefox ESR, (Firefox | |firefox-esr |I:435 |228981|, , |Extended Support | | | | | |Release) | |----------------+------+------+--------+-----------------------| |epiphany-browser|V:3, |2154 |, , |GNOME, HIG compliant, | | |I:15 | | |Epiphany | |----------------+------+------+--------+-----------------------|     |konqueror |V:24, |25905 |, , |KDE, Konqueror | | |I:106 | | | | |----------------+------+------+--------+-----------------------| |dillo |V:0, |1565 |, , |Dillo, (light weight | | |I:5 | | |browser, FLTK based) | |----------------+------+------+--------+-----------------------| |w3m |V:15, |2837 |text |w3m | | |I:187 | | | | |----------------+------+------+--------+-----------------------| |lynx |V:25, |1948 |, , |Lynx | | |I:344 | | | | |----------------+------+------+--------+-----------------------| |elinks |V:3, |1654 |, , |ELinks | | |I:20 | | | | |----------------+------+------+--------+-----------------------| |links |V:3, |2314 |, , |Links (text only) | | |I:28 | | | | |----------------+------+------+--------+-----------------------| |links2 |V:1, |5492 |graphics|Links (console graphics| | |I:12 | | |without X) | +---------------------------------------------------------------+ 6.1.1. Spoofing the User-Agent string In order to access some overly restrictive web sites, you may     need to spoof the User-Agent string returned by the web browser program. See: * MDN Web Docs: userAgent * Chrome Developers: Override the user agent string * How to change your user agent     * How to Change User-Agent in Chrome, Firefox, Safari, and more * How to Change Your Browser’s User Agent Without Installing Any Extensions * How to change the User Agent in Gnome Web (epiphany) Caution     Spoofed user-agent string may cause bad side effects with Java. 6.1.2. Browser extension     All modern GUI browsers support source code based browser extension and it is becoming standardized as web extensions. 6.2. The mail system     This section focuses on typical mobile workstations on consumer grade Internet connections. Caution     If you are to set up the mail server to exchange mail directly with the Internet, you should be better than reading this elementary document. 6.2.1. Email basics     An email message consists of three components, the message envelope, the message header, and the message body. * The "To" and "From" information in the message envelope is used by the SMTP to deliver the email. (The "From" information in the message envelope is also called bounce address, From_, etc.). * The "To" and "From" information in the message header is displayed by the email client. (While it is most common for     these to be the same as ones in the message envelope, such is not always the case.) * The email message format covering header and body data is extended by Multipurpose Internet Mail Extensions (MIME) from the plain ASCII text to other character encodings, as well as attachments of audio, video, images, and application programs.     Full featured GUI based email clients offer all the following functions using the GUI based intuitive configuration. * It creates and interprets the message header and body data using Multipurpose Internet Mail Extensions (MIME) to deal the content data type and encoding. * It authenticates itself to the ISP's SMTP and IMAP servers using the legacy basic access authentication or modern OAuth 2.0. (For OAuth 2.0, set it via Desktop environment settings. E.g., "Settings" -> "Online Accounts".)     * It sends the message to the ISP's smarthost SMTP server listening to the message submission port (587). * It receives the stored message on the ISP's server from the TLS/IMAP4 port (993). * It can filter mails by their attributes. * It may offer additional functionalities: Contacts, Calendar, Tasks, Memos. Table 6.2. List of mail user agent (MUA) +---------------------------------------------------------------+ | package | popcon | size | type | |-----------+--------+------+-----------------------------------| |evolution |V:30, |486 |X GUI program (GNOME3, groupware | | |I:239 | |suite) | |-----------+--------+------+-----------------------------------| |thunderbird|V:48, |224760|X GUI program (GTK, Mozilla | | |I:119 | |Thunderbird) |     |-----------+--------+------+-----------------------------------| |kmail |V:38, |23871 |X GUI program (KDE) | | |I:97 | | | |-----------+--------+------+-----------------------------------| |mutt |V:16, |7104 |character terminal program probably| | |I:149 | |used with vim | |-----------+--------+------+-----------------------------------| |mew |V:0, I:0|2319 |character terminal program under | | | | |(x)emacs | +---------------------------------------------------------------+ 6.2.2. Modern mail service limitation Modern mail service are under some limitations in order to     minimize exposure to the spam (unwanted and unsolicited email) problems. * It is not realistic to run SMTP server on the consumer grade network to send mail directly to the remote host reliably. * A mail may be rejected by any host en route to the     destination quietly unless it appears as authentic as possible. * It is not realistic to expect a single smarthost to send mails of unrelated source mail addresses to the remote host reliably.     This is because: * The SMTP port (25) connections from hosts serviced by the consumer grade network to the Internet are blocked. * The SMTP port (25) connections to hosts serviced by the consumer grade network from the Internet are blocked. * The outgoing messages from hosts serviced by the consumer grade network to the Internet can only be sent via the message submission port (587).     * Anti-spam techniques such as DomainKeys Identified Mail (DKIM), Sender_Policy_Framework (SPF), and Domain-based Message Authentication, Reporting and Conformance (DMARC) are widely used for the email filtering. * The DomainKeys Identified Mail service may be provided for your mail sent through the smarthost. * The smarthost may rewrite the source mail address in the message header to your mail account on the smarthost to prevent email address spoofing. 6.2.3. Historic mail service expectation Some programs on Debian expect to access the /usr/sbin/sendmail     command to send emails as their default or customized setting since the mail service on a UNIX system functioned historically as: * An email is created as a text file. * The email is handed to the /usr/sbin/sendmail command. * For the destination address on the same host, the /usr/sbin/ sendmail command makes local delivery of the email by appending it to the /var/mail/$username file.     + Commands expecting this feature: apt-listchanges, cron, at, ... * For the destination address on the remote host, the /usr/sbin /sendmail command makes remote transfer of the email to the destination host found by the DNS MX record using SMTP. + Commands expecting this feature: popcon, reportbug, bts, ... 6.2.4. Mail transport agent (MTA) Debian mobile workstations can be configured just with full     featured GUI based email clients without mail transfer agent (MTA) program after Debian 12 Bookworm. Debian traditionally installed some MTA program to support programs expecting the /usr/sbin/sendmail command. Such MTA on     mobile workstations must cope with Section 6.2.2, “Modern mail service limitation” and Section 6.2.3, “Historic mail service expectation”. For mobile workstations, the typical choice of MTA is either exim4-daemon-light or postfix with its installation option such     as "Mail sent by smarthost; received via SMTP or fetchmail" selected. These are light weight MTAs that respect "/etc/ aliases". Tip Configuring exim4 to send the Internet mail via multiple     corresponding smarthosts for multiple source email addresses is non-trivial. If you need such capability for some programs, set them up to use msmtp which is easy to set up for multiple source email addresses. Then leave main MTA only for a single email address. Table 6.3. List of basic mail transport agent related packages +---------------------------------------------------------------+ | package |popcon|size| description | |------------------+------+----+--------------------------------| |exim4-daemon-light|V:217,|1575|Exim4 mail transport agent (MTA:| | |I:227 | |Debian default) | |------------------+------+----+--------------------------------| |exim4-daemon-heavy|V:6, |1743|Exim4 mail transport agent (MTA:| | |I:6 | |flexible alternative) | |------------------+------+----+--------------------------------| |exim4-base |V:224,|1699|Exim4 documentation (text) and | | |I:234 | |common files | |------------------+------+----+--------------------------------| |exim4-doc-html |I:1 |3746|Exim4 documentation (html) | |------------------+------+----+--------------------------------| |exim4-doc-info |I:0 |637 |Exim4 documentation (info) | |------------------+------+----+--------------------------------| |postfix |V:124,|4039|Postfix mail transport agent | | |I:133 | |(MTA: secure alternative) | |------------------+------+----+--------------------------------| |postfix-doc |I:6 |4646|Postfix documentation | | | | |(html+text) | |------------------+------+----+--------------------------------| | |V:5, | |Cyrus SASL API implementation | |sasl2-bin |I:13 |371 |(supplement postfix for SMTP | | | | |AUTH) | |------------------+------+----+--------------------------------| |cyrus-sasl2-doc |I:0 |2154|Cyrus SASL - documentation |     |------------------+------+----+--------------------------------| |msmtp |V:6, |667 |Light weight MTA | | |I:11 | | | |------------------+------+----+--------------------------------| | |V:4, | |Light weight MTA (sendmail | |msmtp-mta |I:6 |124 |compatibility extension to | | | | |msmtp) | |------------------+------+----+--------------------------------| |esmtp |V:0, |129 |Light weight MTA | | |I:0 | | | |------------------+------+----+--------------------------------| | |V:0, | |Light weight MTA (sendmail | |esmtp-run |I:0 |32 |compatibility extension to | | | | |esmtp) | |------------------+------+----+--------------------------------| |nullmailer |V:8, |474 |Strip down MTA, no local mail | | |I:9 | | | |------------------+------+----+--------------------------------| |ssmtp |V:5, |2 |Strip down MTA, no local mail | | |I:8 | | | |------------------+------+----+--------------------------------| |sendmail-bin |V:13, |1901|Full featured MTA (only if you | | |I:13 | |are already familiar) | |------------------+------+----+--------------------------------| |courier-mta |V:0, |2407|Full featured MTA (web interface| | |I:0 | |etc.) | |------------------+------+----+--------------------------------| |git-email |V:0, |1087|git-send-email(1) program for | | |I:10 | |sending series of patch emails | +---------------------------------------------------------------+ 6.2.4.1. The configuration of exim4     For the Internet mail via smarthost, you (re)configure exim4-* packages as the following.     $ sudo systemctl stop exim4 $ sudo dpkg-reconfigure exim4-config     Select "mail sent by smarthost; received via SMTP or fetchmail" for "General type of mail configuration".     Set "System mail name:" to its default as the FQDN (see Section 5.1.1, “The hostname resolution”).     Set "IP-addresses to listen on for incoming SMTP connections:" to its default as "127.0.0.1 ; ::1".     Unset contents of "Other destinations for which mail is accepted:".     Unset contents of "Machines to relay mail for:".     Set "IP address or host name of the outgoing smarthost:" to "smtp.hostname.dom:587". Select "No" for "Hide local mail name in outgoing mail?". (Use "/     etc/email-addresses" as in Section 6.2.4.3, “The mail address configuration”, instead.)     Reply to "Keep number of DNS-queries minimal (Dial-on-Demand)?" as one of the following. * "No" if the system is connected to the Internet while booting.     * "Yes" if the system is not connected to the Internet while booting.     Set "Delivery method for local mail:" to "mbox format in /var/ mail/".     Select "Yes" for "Split configuration into small files?:".     Create password entries for the smarthost by editing "/etc/exim4/ passwd.client". $ sudo vim /etc/exim4/passwd.client     ... $ cat /etc/exim4/passwd.client ^smtp.*\.hostname\.dom:username@hostname.dom:password Configure exim4(8) with "QUEUERUNNER='queueonly'", "QUEUERUNNER=     'nodaemon'", etc. in "/etc/default/exim4" to minimize system resource usages. (optional)     Start exim4 by the following.     $ sudo systemctl start exim4     The host name in "/etc/exim4/passwd.client" should not be the alias. You check the real host name with the following. $ host smtp.hostname.dom     smtp.hostname.dom is an alias for smtp99.hostname.dom. smtp99.hostname.dom has address 123.234.123.89 I use regex in "/etc/exim4/passwd.client" to work around the     alias issue. SMTP AUTH probably works even if the ISP moves host pointed by the alias.     You can manually update exim4 configuration by the following: * Update exim4 configuration files in "/etc/exim4/". + creating "/etc/exim4/exim4.conf.localmacros" to set MACROs and editing "/etc/exim4/exim4.conf.template". (non-split configuration)     + creating new files or editing existing files in the "/etc /exim4/exim4.conf.d" subdirectories. (split configuration) * Run "systemctl reload exim4". Caution     Starting exim4 takes long time if "No" (default value) was chosen for the debconf query of "Keep number of DNS-queries minimal (Dial-on-Demand)?" and the system is not connected to the Internet while booting.         Please read the official guide at: "/usr/share/doc/exim4-base/ README.Debian.gz" and update-exim4.conf(8). Warning     For all practical consideration, use SMTP with STARTTLS on port 587 or SMTPS (SMTP over SSL) on port 465, instead of plain SMTP on port 25. 6.2.4.2. The configuration of postfix with SASL     For the Internet mail via smarthost, you should first read postfix documentation and key manual pages. Table 6.4. List of important postfix manual pages +-----------------------------------------------+ | command | function | |------------+----------------------------------| |postfix(1) |Postfix control program | |------------+----------------------------------|     |postconf(1) |Postfix configuration utility | |------------+----------------------------------| |postconf(5) |Postfix configuration parameters | |------------+----------------------------------| |postmap(1) |Postfix lookup table maintenance | |------------+----------------------------------| |postalias(1)|Postfix alias database maintenance| +-----------------------------------------------+     You (re)configure postfix and sasl2-bin packages as follows.     $ sudo systemctl stop postfix $ sudo dpkg-reconfigure postfix     Chose "Internet with smarthost".     Set "SMTP relay host (blank for none):" to " [smtp.hostname.dom]:587" and configure it by the following. $ sudo postconf -e 'smtp_sender_dependent_authentication = yes' $ sudo postconf -e 'smtp_sasl_auth_enable = yes'     $ sudo postconf -e 'smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd' $ sudo postconf -e 'smtp_sasl_type = cyrus' $ sudo vim /etc/postfix/sasl_passwd     Create password entries for the smarthost. $ cat /etc/postfix/sasl_passwd     [smtp.hostname.dom]:587 username:password $ sudo postmap hush:/etc/postfix/sasl_passwd     Start the postfix by the following.     $ sudo systemctl start postfix Here the use of "[" and "]" in the dpkg-reconfigure dialog and "/ etc/postfix/sasl_passwd" ensures not to check MX record but     directly use exact hostname specified. See "Enabling SASL authentication in the Postfix SMTP client" in "/usr/share/doc/ postfix/html/SASL_README.html". 6.2.4.3. The mail address configuration     There are a few mail address configuration files for mail transport, delivery and user agents. Table 6.5. List of mail address related configuration files +---------------------------------------------------------------+ | file | function | application | |---------------+-----------------+-----------------------------| | |default host name| | |/etc/mailname |for (outgoing) |Debian specific, mailname(5) | | |mail | | |---------------+-----------------+-----------------------------| |/etc/ |host name |exim(8) specific, |     |email-addresses|spoofing for |exim4-config_files(5) | | |outgoing mail | | |---------------+-----------------+-----------------------------| |/etc/postfix/ |host name |postfix(1) specific, | |generic |spoofing for |activated after postmap(1) | | |outgoing mail |command execution. | |---------------+-----------------+-----------------------------| | |account name |general, activated after | |/etc/aliases |alias for |newaliases(1) command | | |incoming mail |execution. | +---------------------------------------------------------------+ The mailname in the "/etc/mailname" file is usually a fully qualified domain name (FQDN) that resolves to one of the host's     IP addresses. For the mobile workstation which does not have a hostname with resolvable IP address, set this mailname to the value of "hostname -f". (This is safe choice and works for both exim4-* and postfix.) Tip The contents of "/etc/mailname" is used by many non-MTA programs     for their default behavior. For mutt, set "hostname" and "from" variables in ~/muttrc file to override the mailname value. For programs in the devscripts package, such as bts(1) and dch(1), export environment variables "$DEBFULLNAME" and "$DEBEMAIL" to override it. Tip The popularity-contest package normally send mail from root account with FQDN. You need to set MAILFROM in /etc/     popularity-contest.conf as described in the /usr/share/ popularity-contest/default.conf file. Otherwise, your mail will be rejected by the smarthost SMTP server. Although this is tedious, this approach is safer than rewriting the source address for all mails from root by MTA and should be used for other daemons and cron scripts.     When setting the mailname to "hostname -f", the spoofing of the source mail address via MTA can be realized by the following. * "/etc/email-addresses" file for exim4(8) as explained in the exim4-config_files(5)     * "/etc/postfix/generic" file for postfix(1) as explained in the generic(5)     For postfix, the following extra steps are needed. # postmap hash:/etc/postfix/generic     # postconf -e 'smtp_generic_maps = hash:/etc/postfix/generic' # postfix reload     You can test mail address configuration using the following. * exim(8) with -brw, -bf, -bF, -bV, … options     * postmap(1) with -q option. Tip     Exim comes with several utility programs such as exiqgrep(8) and exipick(8). See "dpkg -L exim4-base|grep man8/" for available commands. 6.2.4.4. Basic MTA operations     There are several basic MTA operations. Some may be performed via sendmail(1) compatibility interface. Table 6.6. List of basic MTA operation +---------------------------------------------------------------+ | exim | postfix command | description | | command | | | |----------+--------------------+-------------------------------| |sendmail |sendmail |read mails from standard input | | | |and arrange for delivery (-bm) | |----------+--------------------+-------------------------------| |mailq |mailq |list the mail queue with status| | | |and queue ID (-bp) | |----------+--------------------+-------------------------------| |newaliases|newaliases |initialize alias database (-I) | |----------+--------------------+-------------------------------| |exim4 -q |postqueue -f |flush waiting mails (-q) |     |----------+--------------------+-------------------------------| | |postsuper -r ALL | | |exim4 -qf |deferred; postqueue |flush all mails | | |-f | | |----------+--------------------+-------------------------------| |exim4 -qff|postsuper -r ALL; |flush even frozen mails | | |postqueue -f | | |----------+--------------------+-------------------------------| |exim4 -Mg |postsuper -h |freeze one message by its queue| |queue_id |queue_id |ID | |----------+--------------------+-------------------------------| |exim4 -Mrm|postsuper -d |remove one message by its queue| |queue_id |queue_id |ID | |----------+--------------------+-------------------------------| |N/A |postsuper -d ALL |remove all messages | +---------------------------------------------------------------+ Tip     It may be a good idea to flush all mails by a script in "/etc/ppp /ip-up.d/*". 6.3. The remote access server and utilities (SSH) The Secure SHell (SSH) is the secure way to connect over the     Internet. A free version of SSH called OpenSSH is available as openssh-client and openssh-server packages in Debian. For the user, ssh(1) functions as a smarter and more secure     telnet(1). Unlike telnet command, ssh command does not stop on the telnet escape character (initial default CTRL-]). Table 6.7. List of remote access server and utilities +---------------------------------------------------------------------+ | package |popcon|size| tool |description| |----------------------+------+----+----------------------+-----------| | |V:866,| | |Secure | |openssh-client |I:996 |4959|ssh(1) |shell | | | | | |client | |----------------------+------+----+----------------------+-----------| | |V:730,| | |Secure | |openssh-server |I:814 |1804|sshd(8) |shell | | | | | |server | |----------------------+------+----+----------------------+-----------| | | | | |asks user | | | | | |for a pass | |ssh-askpass |I:23 |102 |ssh-askpass(1) |phrase for | | | | | |ssh-add | | | | | |(plain X) | |----------------------+------+----+----------------------+-----------| | | | | |asks user |     | |V:0, | | |for a pass | |ssh-askpass-gnome |I:3 |200 |ssh-askpass-gnome(1) |phrase for | | | | | |ssh-add | | | | | |(GNOME) | |----------------------+------+----+----------------------+-----------| | | | | |asks user | | | | | |for a pass | | |V:0, | |ssh-askpass-fullscreen|phrase for | |ssh-askpass-fullscreen|I:0 |48 |(1) |ssh-add | | | | | |(GNOME) | | | | | |with extra | | | | | |eye candy | |----------------------+------+----+----------------------+-----------| | | | | |web server | | | | | |for browser| |shellinabox |V:0, |507 |shellinaboxd(1) |accessible | | |I:1 | | |VT100 | | | | | |terminal | | | | | |emulator | +---------------------------------------------------------------------+     Although shellinabox is not a SSH program, it is listed here as an interesting alternative for the remote terminal access.     See also Section 7.9, “X server connection” for connecting to remote X client programs. Caution     See Section 4.6.3, “Extra security measures for the Internet” if your SSH is accessible from the Internet. Tip     Please use the screen(1) program to enable remote shell process to survive the interrupted connection (see Section 9.1.2, “The screen program”). 6.3.1. Basics of SSH     The OpenSSH SSH daemon supports SSH protocol 2 only. Please read "/usr/share/doc/openssh-client/README.Debian.gz", ssh     (1), sshd(8), ssh-agent(1), and ssh-keygen(1), ssh-add(1) and ssh-agent(1). Warning "/etc/ssh/sshd_not_to_be_run" must not be present if one wishes     to run the OpenSSH server. Don't enable rhost based authentication (HostbasedAuthentication in /etc/ssh/sshd_config). Table 6.8. List of SSH configuration files +---------------------------------------------------------------+ | configuration | description of configuration file | | file | | |----------------+----------------------------------------------| |/etc/ssh/ |SSH client defaults, see ssh_config(5) | |ssh_config | | |----------------+----------------------------------------------| |/etc/ssh/ |SSH server defaults, see sshd_config(5) |     |sshd_config | | |----------------+----------------------------------------------| |~/.ssh/ |default public SSH keys that clients use to | |authorized_keys |connect to this account on this SSH server | |----------------+----------------------------------------------| |~/.ssh/id_rsa |secret SSH-2 RSA key of the user | |----------------+----------------------------------------------| |~/.ssh/id_ |secret SSH-2 key-type-name key such as ecdsa, | |key-type-name |ed25519, ... of the user | +---------------------------------------------------------------+     The following starts an ssh(1) connection from a client. Table 6.9. List of SSH client startup examples +---------------------------------------------------------------+ | command | description | |-----------------------------------+---------------------------| |ssh username@hostname.domain.ext |connect with default mode | |-----------------------------------+---------------------------| |ssh -v username@hostname.domain.ext|connect with default mode | | |with debugging messages |     |-----------------------------------+---------------------------| |ssh -o PreferredAuthentications= |force to use password with | |password |SSH version 2 | |username@hostname.domain.ext | | |-----------------------------------+---------------------------| |ssh -t username@hostname.domain.ext|run passwd program to | |passwd |update password on a remote| | |host | +---------------------------------------------------------------+ 6.3.2. User name on the remote host     If you use the same user name on the local and the remote host, you can eliminate typing "username@". Even if you use different user name on the local and the remote     host, you can eliminate it using "~/.ssh/config". For Debian Salsa service with account name "foo-guest", you set "~/.ssh/ config" to contain the following.     Host salsa.debian.org people.debian.org User foo-guest 6.3.3. Connecting without remote passwords     One can avoid having to remember passwords for remote systems by using "PubkeyAuthentication" (SSH-2 protocol).     On the remote system, set the respective entries, "PubkeyAuthentication yes", in "/etc/ssh/sshd_config".     Generate authentication keys locally and install the public key on the remote system by the following.     $ ssh-keygen -t rsa $ cat .ssh/id_rsa.pub | ssh user1@remote "cat - >>.ssh/authorized_keys" You can add options to the entries in "~/.ssh/authorized_keys" to     limit hosts and to run specific commands. See sshd(8) "AUTHORIZED_KEYS FILE FORMAT". 6.3.4. Dealing with alien SSH clients     There are some free SSH clients available for other platforms. Table 6.10. List of free SSH clients for other platforms +---------------------------------------------------------------+ |environment | free SSH program | |------------+--------------------------------------------------| |Windows |puTTY (PuTTY: a free SSH and Telnet client) (GPL) |     |------------+--------------------------------------------------| |Windows |SSH in cygwin (Cygwin: Get that Linux feeling - on| |(cygwin) |Windows) (GPL) | |------------+--------------------------------------------------| |Mac OS X |OpenSSH; use ssh in the Terminal application (GPL)| +---------------------------------------------------------------+ 6.3.5. Setting up ssh-agent It is safer to protect your SSH authentication secret keys with a     pass phrase. If a pass phrase was not set, use "ssh-keygen -p" to set it. Place your public SSH key (e.g. "~/.ssh/id_rsa.pub") into "~/.ssh     /authorized_keys" on a remote host using a password-based connection to the remote host as described above. $ ssh-agent bash     $ ssh-add ~/.ssh/id_rsa Enter passphrase for /home/username/.ssh/id_rsa: Identity added: /home/username/.ssh/id_rsa (/home/username/.ssh/id_rsa)     No remote password needed from here on for the next command.     $ scp foo username@remote.host:foo     Press ^D to terminating ssh-agent session. For the X server, the normal Debian startup script executes     ssh-agent as the parent process. So you only need to execute ssh-add once. For more, read ssh-agent(1) and ssh-add(1). 6.3.6. Sending a mail from a remote host If you have an SSH shell account on a server with proper DNS     settings, you can send a mail generated on your workstation as an email genuinely sent from the remote server.     $ ssh username@example.org /usr/sbin/sendmail -bm -ti -f "username@example.org" < mail_data.txt 6.3.7. Port forwarding for SMTP/POP3 tunneling To establish a pipe to connect to port 25 of remote-server from     port 4025 of localhost, and to port 110 of remote-server from port 4110 of localhost through ssh, execute on the local host as the following.     # ssh -q -L 4025:remote-server:25 4110:remote-server:110 username@remote-server This is a secure way to make connections to SMTP/POP3 servers     over the Internet. Set the "AllowTcpForwarding" entry to "yes" in "/etc/ssh/sshd_config" of the remote host. 6.3.8. How to shutdown the remote system on SSH You need to protect the process doing "shutdown -h now" (see     Section 1.1.8, “How to shutdown the system”) from the termination of SSH using the at(1) command (see Section 9.4.13, “Scheduling tasks once”) by the following.     # echo "shutdown -h now" | at now     Running "shutdown -h now" in screen(1) (see Section 9.1.2, “The screen program”) session is another way to do the same. 6.3.9. Troubleshooting SSH     If you have problems, check the permissions of configuration files and run ssh with the "-v" option.     Use the "-p" option if you are root and have trouble with a firewall; this avoids the use of server ports 1 — 1023. If ssh connections to a remote site suddenly stop working, it may be the result of tinkering by the sysadmin, most likely a change     in "host_key" during system maintenance. After making sure this is the case and nobody is trying to fake the remote host by some clever hack, one can regain a connection by removing the "host_key" entry from "~/.ssh/known_hosts" on the local host. 6.4. The print server and utilities In the old Unix-like system, the BSD Line printer daemon (lpd) was the standard and the standard print out format of the classic     free software was PostScript (PS). Some filter system was used along with Ghostscript to enable printing to the non-PostScript printer. See Section 11.4.1, “Ghostscript”. In the modern Debian system, the Common UNIX Printing System     (CUPS) is the de facto standard and the standard print out format of the modern free software is Portable Document Format (PDF). The CUPS uses Internet Printing Protocol (IPP). The IPP is now     supported by other OSs such as Windows XP and Mac OS X and has became new cross-platform de facto standard for remote printing with bi-directional communication capability. Thanks to the file format dependent auto-conversion feature of     the CUPS system, simply feeding any data to the lpr command should generate the expected print output. (In CUPS, lpr can be enabled by installing the cups-bsd package.)     The Debian system has some notable packages for the print servers and utilities. Table 6.11. List of print servers and utilities +---------------------------------------------------------------+ | package |popcon|size| port | description | |-------------------------+------+----+----------+--------------| | |V:2, | |printer |BSD lpr/lpd | |lpr |I:3 |367 |(515) |(Line printer | | | | | |daemon) | |-------------------------+------+----+----------+--------------| |lprng |V:0, |3051|, , |, , (Enhanced)| | |I:0 | | | | |-------------------------+------+----+----------+--------------| | |V:97, | | |Internet | |cups |I:441 |1061|IPP (631) |Printing CUPS | | | | | |server | |-------------------------+------+----+----------+--------------| | | | | |System V | | | | | |printer |     | | | | |commands for | | | | | |CUPS: lp(1), | |cups-client |V:119,|426 |, , |lpstat(1), | | |I:461 | | |lpoptions(1), | | | | | |cancel(1), | | | | | |lpmove(8), | | | | | |lpinfo(8), | | | | | |lpadmin(8), … | |-------------------------+------+----+----------+--------------| | | | | |BSD printer | | |V:32, | | |commands for | |cups-bsd |I:219 |131 |, , |CUPS: lpr(1), | | | | | |lpq(1), lprm | | | | | |(1), lpc(8) | |-------------------------+------+----+----------+--------------| | |V:20, | |Not |printer | |printer-driver-gutenprint|I:114 |1219|applicable|drivers for | | | | | |CUPS | +---------------------------------------------------------------+ Tip     You can configure CUPS system by pointing your web browser to "http://localhost:631/" . 6.5. Other network application servers     Here are other network application servers. Table 6.12. List of other network application servers +---------------------------------------------------------------+ | package |popcon|size|protocol| description | |-----------------+------+----+--------+------------------------| |telnetd |V:0, |54 |TELNET |TELNET server | | |I:2 | | | | |-----------------+------+----+--------+------------------------| |telnetd-ssl |V:0, |159 |, , |, , (SSL support) | | |I:0 | | | | |-----------------+------+----+--------+------------------------| |nfs-kernel-server|V:49, |769 |NFS |Unix file sharing | | |I:63 | | | | |-----------------+------+----+--------+------------------------| |samba |V:108,|3995|SMB |Windows file and printer| | |I:131 | | |sharing | |-----------------+------+----+--------+------------------------| | |V:1, | | |Apple/Mac file and |     |netatalk |I:1 |2003|ATP |printer sharing | | | | | |(AppleTalk) | |-----------------+------+----+--------+------------------------| |proftpd-basic |V:8, |452 |FTP |General file download | | |I:16 | | | | |-----------------+------+----+--------+------------------------| |apache2 |V:214,|561 |HTTP |General web server | | |I:263 | | | | |-----------------+------+----+--------+------------------------| |squid |V:11, |9265|, , |General web proxy server| | |I:12 | | | | |-----------------+------+----+--------+------------------------| |bind9 |V:43, |1124|DNS |IP address for other | | |I:49 | | |hosts | |-----------------+------+----+--------+------------------------| |isc-dhcp-server |V:18, |6082|DHCP |IP address of client | | |I:36 | | |itself | +---------------------------------------------------------------+ Common Internet File System Protocol (CIFS) is the same protocol     as Server Message Block (SMB) and is used widely by Microsoft Windows. Tip     See Section 4.5.2, “The modern centralized system management” for integration of server systems. Tip     The hostname resolution is usually provided by the DNS server. For the host IP address dynamically assigned by DHCP, Dynamic DNS can be set up for the hostname resolution using bind9 and isc-dhcp-server as described in the DDNS page on the Debian wiki. Tip     Use of proxy server such as squid is much more efficient for saving bandwidth than use of local mirror server with the full Debian archive contents. 6.6. Other network application clients     Here are other network application clients. Table 6.13. List of network application clients +---------------------------------------------------------------+ | package |popcon|size|protocol| description | |---------------+------+----+--------+--------------------------| |netcat |I:27 |16 |TCP/IP |TCP/IP swiss army knife | |---------------+------+----+--------+--------------------------| | |V:841,| | |Secure Socket Layer (SSL) | |openssl |I:995 |2111|SSL |binary and related | | | | | |cryptographic tools | |---------------+------+----+--------+--------------------------| |stunnel4 |V:7, |548 |, , |universal SSL Wrapper | | |I:12 | | | | |---------------+------+----+--------+--------------------------| |telnet |V:29, |54 |TELNET |TELNET client | | |I:511 | | | | |---------------+------+----+--------+--------------------------| |telnet-ssl |V:0, |196 |, , |, , (SSL support) | | |I:2 | | | | |---------------+------+----+--------+--------------------------| |nfs-common |V:152,|1124|NFS |Unix file sharing | | |I:234 | | | | |---------------+------+----+--------+--------------------------| |smbclient |V:24, |2071|SMB |MS Windows file and | | |I:204 | | |printer sharing client | |---------------+------+----+--------+--------------------------| |cifs-utils |V:29, |317 |, , |mount and umount commands | | |I:121 | | |for remote MS Windows file| |---------------+------+----+--------+--------------------------| |ftp |V:7, |53 |FTP |FTP client | | |I:114 | | | |     |---------------+------+----+--------+--------------------------| |lftp |V:4, |2361|, , |, , | | |I:30 | | | | |---------------+------+----+--------+--------------------------| |ncftp |V:1, |1389|, , |full screen FTP client | | |I:14 | | | | |---------------+------+----+--------+--------------------------| |wget |V:208,|3681|HTTP and|web downloader | | |I:981 | |FTP | | |---------------+------+----+--------+--------------------------| |curl |V:185,|517 |, , |, , | | |I:620 | | | | |---------------+------+----+--------+--------------------------| |axel |V:0, |224 |, , |accelerated downloader | | |I:3 | | | | |---------------+------+----+--------+--------------------------| | |V:3, | | |accelerated downloader | |aria2 |I:20 |1981|, , |with BitTorrent and | | | | | |Metalink supports | |---------------+------+----+--------+--------------------------| |bind9-host |V:115,|393 |DNS |host(1) from bind9, | | |I:939 | | |"Priority: standard" | |---------------+------+----+--------+--------------------------| |dnsutils |V:16, |276 |, , |dig(1) from bind, | | |I:280 | | |"Priority: standard" | |---------------+------+----+--------+--------------------------| |isc-dhcp-client|V:217,|2875|DHCP |obtain IP address | | |I:981 | | | | |---------------+------+----+--------+--------------------------| |ldap-utils |V:12, |767 |LDAP |obtain data from LDAP | | |I:63 | | |server | +---------------------------------------------------------------+ 6.7. The diagnosis of the system daemons     The telnet program enables manual connection to the system daemons and its diagnosis.     For testing plain POP3 service, try the following     $ telnet mail.ispname.net pop3 For testing the TLS/SSL enabled POP3 service by some ISPs, you     need TLS/SSL enabled telnet client by the telnet-ssl or openssl packages.     $ telnet -z ssl pop.gmail.com 995     $ openssl s_client -connect pop.gmail.com:995     The following RFCs provide required knowledge to each system daemon. Table 6.14. List of popular RFCs +---------------------------------------------------------------+ | RFC | description | |-------------------+-------------------------------------------| |rfc1939 and rfc2449|POP3 service | |-------------------+-------------------------------------------| |rfc3501 |IMAP4 service | |-------------------+-------------------------------------------| |rfc2821 (rfc821) |SMTP service | |-------------------+-------------------------------------------|     |rfc2822 (rfc822) |Mail file format | |-------------------+-------------------------------------------| |rfc2045 |Multipurpose Internet Mail Extensions | | |(MIME) | |-------------------+-------------------------------------------| |rfc819 |DNS service | |-------------------+-------------------------------------------| |rfc2616 |HTTP service | |-------------------+-------------------------------------------| |rfc2396 |URI definition | +---------------------------------------------------------------+     The port usage is described in "/etc/services". Chapter 7. GUI System 7.1. GUI desktop environment     There are several choices for the full featured GUI desktop environment on the Debian system. Table 7.1. List of desktop environment +---------------------------------------------------------------+ | task package |popcon|size| description | |----------------------------+------+----+----------------------| |task-gnome-desktop |I:198 |9 |GNOME desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-xfce-desktop |I:96 |9 |Xfce desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-kde-desktop |I:81 |6 |KDE Plasma desktop | | | | |environment | |----------------------------+------+----+----------------------|     |task-mate-desktop |I:43 |9 |MATE desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-cinnamon-desktop |I:41 |9 |Cinnamon desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-lxde-desktop |I:29 |9 |LXDE desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-lxqt-desktop |I:18 |9 |LXQt desktop | | | | |environment | |----------------------------+------+----+----------------------| |task-gnome-flashback-desktop|I:13 |6 |GNOME Flashback | | | | |desktop environment | +---------------------------------------------------------------+ Tip Dependency packages selected by a task metapackage may be out of sync with the latest package transition state under the Debian unstable/testing environment. For task-gnome-desktop, you may need to adjust package selections as follows: * Start aptitude(8) as sudo aptitude -u. * Move cursor to "Tasks" and press "Enter". * Move cursor to "End-user" press "Enter".     * Move cursor to "GNOME" press "Enter". * Move cursor to task-gnome-desktop and press "Enter". * Move cursor to "Depends" and press "m" (manually selected). * Move cursor to "Recommends" and press "m" (manually selected). * Move cursor to "task-gnome-desktop and press "-". (drop) * Adjust selected packages while dropping problematic ones causing package conflicts. * Press "g" to start install.     This chapter will focus mostly on the default desktop environment of Debian: task-gnome-desktop offering GNOME on wayland. 7.2. GUI communication protocol     GUI communication protocol used on the GNOME desktop can be: * Wayland (display server protocol) (native)     * X Window System core protocol (via xwayland)     Please check freedesktop.org site for how Wayland architecture is different from X Window architecture.     From user's perspective, differences can be colloquially summarized as: * Wayland is a same-host GUI communication protocol: new, simpler, faster, no setuid root binary     * X Window is a network-capable GUI communication protocol: traditional, complex, slower, setuid root binary For applications using Wayland protocol, the access to their     display contents from a remote host is supported by the VNC or RDP. See Section 7.8, “Remote desktop” Modern X servers have the MIT Shared Memory Extension and communicate with their local X clients using the local shared     memory. This bypasses the network transparent Xlib interprocess communication channel and gains performance. This situation was the background of creating Wayland as a local-only GUI communication protocol.     Using the xeyes program started from the GNOME terminal, you can check GUI communication protocol used by each GUI application.     $ xeyes * If the mouse cursor is on an application such as "GNOME terminal" which uses Wayland display server protocol, eyes don't move with the mouse cursor.     * If the mouse cursor is on an application such as "xterm" which uses X Window System core protocol, eyes move with the mouse cursor exposing not-so-isolated nature of X Window architecture. As of April 2021, many popular GUI applications such as GNOME and LibreOffice (LO) applications have been migrated to the Wayland     display server protocol. I see xterm, gitk, chromium, firefox, gimp, dia, and KDE applications still use X Window System core protocol. Note For both the xwayland on Wayland or the native X Window System,     the old X server configuration file "/etc/X11/xorg.conf" shouldn't exist on the system. The graphics and input devices are now configured by the kernel with DRM, KMS, and udev. The native X server has been rewritten to use them. See "modedb default video mode support" in the Linux kernel documentation. 7.3. GUI infrastructure     Here are notable GUI infrastructure packages for the GNOME on Wayland environment. Table 7.2. List of notable GUI infrastructure packages +---------------------------------------------------------------+ | package |popcon|package| description | | | | size | | |---------------------------+------+-------+--------------------| | |V:1, | |GNOME's mutter | |mutter |I:60 |187 |window manager | | | | |[auto] | |---------------------------+------+-------+--------------------| | |V:240,| |An X server running | |xwayland |I:319 |2388 |on top of wayland | | | | |[auto] |     |---------------------------+------+-------+--------------------| | | | |Remote desktop | |gnome-remote-desktop |V:35, |1068 |daemon for GNOME | | |I:221 | |using PipeWire | | | | |[auto] | |---------------------------+------+-------+--------------------| | |V:21, | |Advanced | |gnome-tweaks |I:229 |1170 |configuration | | | | |settings for GNOME | |---------------------------+------+-------+--------------------| | |V:13, | |Tool to enable / | |gnome-shell-extension-prefs|I:213 |60 |disable GNOME Shell | | | | |extensions | +---------------------------------------------------------------+     Here, "[auto]" means that these packages are automatically installed when task-gnome-desktop is installed. Tip gnome-tweaks is the indispensable configuration utility. For example:     * You can force "Over-Amplification" of sound volume from "General". * You can force "Caps" to become "Esc" from "Keyboard & Mouse" -> "Keyboard" -> "Additional Layout Option". Tip     Detail features of GNOME desktop environment can be configured with utilities started by typing "settings", "tweaks", or "extensions" after pressing Super-key. 7.4. GUI applications Many useful GUI applications are available on Debian now. Installing software packages such as scribus (KDE) on GNOME     desktop environment are quite acceptable since corresponding functionality is not available under GNOME desktop environment. But installing too many packages with duplicated functionalities may clutter your system.     Here is a list of GUI applications which caught my eyes. Table 7.3. List of notable GUI applications +---------------------------------------------------------------+ | package |popcon|package| type | description | | | | size | | | |-------------------+------+-------+------+---------------------| | |V:30, | | |Personal information | |evolution |I:239 |486 |GNOME |Management (groupware| | | | | |and email) | |-------------------+------+-------+------+---------------------| |thunderbird |V:48, |224760 |GTK |Email client (Mozilla| | |I:119 | | |Thunderbird) | |-------------------+------+-------+------+---------------------| | |V:1, | | |Personal information | |kontact |I:12 |2208 |KDE |Management (groupware| | | | | |and email) | |-------------------+------+-------+------+---------------------| |libreoffice-writer |V:117,|31474 |LO |word processor | | |I:435 | | | | |-------------------+------+-------+------+---------------------| |abiword |V:1, |3542 |GNOME |word processor | | |I:8 | | | | |-------------------+------+-------+------+---------------------| |calligrawords |V:0, |6097 |KDE |word processor | | |I:7 | | | | |-------------------+------+-------+------+---------------------| | |V:1, | | |desktop publishing | |scribus |I:16 |31345 |KDE |editor to edit PDF | | | | | |files | |-------------------+------+-------+------+---------------------| |glabels |V:0, |1338 |GNOME |label editor | | |I:3 | | | | |-------------------+------+-------+------+---------------------| |libreoffice-calc |V:111,|26009 |LO |spreadsheet | | |I:432 | | | | |-------------------+------+-------+------+---------------------| |gnumeric |V:4, |9910 |GNOME |spreadsheet | | |I:14 | | | | |-------------------+------+-------+------+---------------------| |calligrasheets |V:0, |11396 |KDE |spreadsheet | | |I:5 | | | | |-------------------+------+-------+------+---------------------| |libreoffice-impress|V:69, |2646 |LO |presentation | | |I:429 | | | | |-------------------+------+-------+------+---------------------| |calligrastage |V:0, |5339 |KDE |presentation | | |I:5 | | | | |-------------------+------+-------+------+---------------------| |libreoffice-base |V:26, |5003 |LO |database management | | |I:121 | | | | |-------------------+------+-------+------+---------------------| |kexi |V:0, |7118 |KDE |database management | | |I:1 | | | | |-------------------+------+-------+------+---------------------| |libreoffice-draw |V:72, |10312 |LO |vector graphics | | |I:430 | | |editor (draw) | |-------------------+------+-------+------+---------------------| |inkscape |V:15, |99800 |GNOME |vector graphics | | |I:112 | | |editor (draw) | |-------------------+------+-------+------+---------------------| |karbon |V:0, |3610 |KDE |vector graphics | | |I:6 | | |editor (draw) | |-------------------+------+-------+------+---------------------| |dia |V:2, |3741 |GTK |flowchart and diagram| | |I:22 | | |editor |     |-------------------+------+-------+------+---------------------| |gimp |V:50, |19304 |GTK |bitmap graphics | | |I:252 | | |editor (paint) | |-------------------+------+-------+------+---------------------| |shotwell |V:17, |6263 |GTK |digital photo | | |I:255 | | |organizer | |-------------------+------+-------+------+---------------------| |digikam |V:1, |293 |KDE |digital photo | | |I:9 | | |organizer | |-------------------+------+-------+------+---------------------| | |V:4, | | |lighttable and | |darktable |I:13 |30554 |GTK |darkroom for | | | | | |photographers | |-------------------+------+-------+------+---------------------| |planner |V:0, |1394 |GNOME |project management | | |I:4 | | | | |-------------------+------+-------+------+---------------------| |calligraplan |V:0, |19013 |KDE |project management | | |I:2 | | | | |-------------------+------+-------+------+---------------------| |gnucash |V:2, |28928 |GNOME |personal accounting | | |I:8 | | | | |-------------------+------+-------+------+---------------------| |homebank |V:0, |1218 |GTK |personal accounting | | |I:2 | | | | |-------------------+------+-------+------+---------------------| |lilypond |V:0, |16092 |- |music typesetter | | |I:7 | | | | |-------------------+------+-------+------+---------------------| |kmymoney |V:0, |13937 |KDE |personal accounting | | |I:2 | | | | |-------------------+------+-------+------+---------------------| |librecad |V:1, |8963 |Qt-app|computer-aided design| | |I:15 | | |(CAD) system (2D) | |-------------------+------+-------+------+---------------------| |freecad |I:18 |36 |Qt-app|computer-aided design| | | | | |(CAD) system (3D) | |-------------------+------+-------+------+---------------------| | |V:3, | | |electronic schematic | |kicad |I:14 |236461 |GTK |and PCB design | | | | | |software | |-------------------+------+-------+------+---------------------| |xsane |V:12, |2339 |GTK |scanner frontend | | |I:144 | | | | |-------------------+------+-------+------+---------------------| |libreoffice-math |V:51, |1898 |LO |mathematical equation| | |I:432 | | |/formula editor | |-------------------+------+-------+------+---------------------| |calibre |V:6, |63385 |KDE |e-book converter and | | |I:28 | | |library management | |-------------------+------+-------+------+---------------------| |fbreader |V:1, |3783 |GTK |e-book reader | | |I:9 | | | | |-------------------+------+-------+------+---------------------| |evince |V:92, |941 |GNOME |document(pdf) viewer | | |I:314 | | | | |-------------------+------+-------+------+---------------------| |okular |V:40, |17728 |KDE |document(pdf) viewer | | |I:123 | | | | |-------------------+------+-------+------+---------------------| |x11-apps |V:31, |2460 |pure |xeyes(1), etc. | | |I:463 | |X-app | | |-------------------+------+-------+------+---------------------| |x11-utils |V:192,|651 |pure |xev(1), xwininfo(1), | | |I:566 | |X-app |etc. | +---------------------------------------------------------------+ 7.5. User directories Default names for user directories such as "~/Desktop", "~/     Documents", ..., used by the Desktop environment depend on the locale used for the system installation. You can reset them to the English ones by:     $ LANGUAGE=C xdg-user-dirs-update --force     Then you manually move all the data to the newer directories. See xdg-user-dirs-update(1).     You can also set them to any names by editing "~/.config/ user-dirs.dirs". See user-dirs.dirs(5). 7.6. Fonts Many useful scalable fonts are available for users on Debian.     User's concern is how to avoid redundancy and how to configure parts of installed fonts to be disabled. Otherwise, useless font choices may clutter your GUI application menus.     Debian system uses FreeType 2.0 library to rasterise many scalable font formats for screen and print: * Type 1 (PostScript) fonts which use cubic Bézier curves (almost obsolete format)     * TrueType fonts which use quadratic Bézier curves (good choice format) * OpenType fonts which use cubic Bézier curves (best choice format) 7.6.1. Basic fonts The following table is compiled in the hope to help users to chose appropriate scalable fonts with clear understanding of the     metric compatibility and the glyph coverage. Most fonts cover all Latin, Greek, and Cyril characters. The final choice of activated fonts can also be affected by your aesthetics. These fonts can be used for the screen display or for the paper printing. Table 7.4. List of notable TrueType and OpenType fonts +--------------------------------------------------------------------+ | package |popcon|size |sans|serif|mono|note on font | |-------------------------+------+-----+----+-----+----+-------------| | |V:216,| | | | |Cantarell | |fonts-cantarell |I:306 |572 |59 |- |- |(GNOME 3, | | | | | | | |display) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Noto fonts | |fonts-noto |I:153 |31 |61 |63 |40 |(Google, | | | | | | | |multi-lingual| | | | | | | |with CJK) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |DejaVu (GNOME| | | | | | | |2, | |fonts-dejavu |I:421 |35 |58 |68 |40 |MCM:Verdana, | | | | | | | |extended | | | | | | | |Bitstream | | | | | | | |Vera) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Liberation | | |V:130,| | | | |fonts for | |fonts-liberation2 |I:427 |15 |56 |60 |40 |LibreOffice | | | | | | | |(Red Hat, | | | | | | | |MCMATC) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Chrome OS: | | |V:20, | | | | |Arimo, Tinos | |fonts-croscore |I:40 |5274 |56 |60 |40 |and Cousine | | | | | | | |(Google, | | | | | | | |MCMATC) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Chrome OS: | |fonts-crosextra-carlito |V:21, |2696 |57 |- |- |Carlito | | |I:131 | | | | |(Google, | | | | | | | |MCM:Calibri )| |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Chrome OS: | | | | | | | |Caladea | |fonts-crosextra-caladea |I:128 |347 |- |55 |- |(Google, |     | | | | | | |MCM:Cambria )| | | | | | | |(Latin only )| |-------------------------+------+-----+----+-----+----+-------------| | |V:76, | | | | |GNU FreeFont | |fonts-freefont-ttf |I:218 |14460|57 |59 |40 |(extended URW| | | | | | | |Nimbus) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Debian | | |V:123,| | | | |task-desktop,| |fonts-quicksand |I:438 |392 |56 |- |- |Quicksand | | | | | | | |(display, | | | | | | | |Latin only) | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |A typeface | | |V:24, | | | | |designed for | |fonts-hack |I:120 |2508 |- |- |40 P|source code | | | | | | | |Hack | | | | | | | |(Facebook) | |-------------------------+------+-----+----+-----+----+-------------| |fonts-sil-gentiumplus |I:32 |14345|- |54 |- |Gentium SIL | |-------------------------+------+-----+----+-----+----+-------------| |fonts-sil-charis |I:27 |6704 |- |59 |- |Charis SIL | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |URW Nimbus | | |V:167,| | | | |(Nimbus Sans,| |fonts-urw-base35 |I:472 |15560|56 |60 |40 |Roman No. 9 | | | | | | | |L, Mono L, | | | | | | | |MCAHTC) | |-------------------------+------+-----+----+-----+----+-------------| |fonts-ubuntu |V:2, |4339 |58 |- |33 P|Ubuntu fonts | | |I:5 | | | | |(display) | |-------------------------+------+-----+----+-----+----+-------------| | |V:0, | | | | |Cool retro | |fonts-terminus |I:3 |452 |- |- |33 |terminal | | | | | | | |fonts | |-------------------------+------+-----+----+-----+----+-------------| | | | | | | |Downloader of| | |V:1, | | | | |Microsoft | |ttf-mscorefonts-installer|I:49 |85 |56? |60 |40 |non-free | | | | | | | |fonts (see | | | | | | | |below) | +--------------------------------------------------------------------+     Here: * "MCM" stands for "metric compatible with fonts provided by Microsoft" * "MCMATC" stands for "metric compatible with fonts provided by Microsoft: Arial, Times New Roman, Courier New" * "MCAHTC" stands for "metric compatible with fonts provided by Adobe: Helvetica, Times, Courier"     * Numbers in font type columns stands for the rough relative "M" width for the same point size font. * "P" in mono font type columns stands for its usability for programming having clearly distinguishable "0"/"O" and "1"/ "I"/"l". * The ttf-mscorefonts-installer package downloads Microsoft's "Core fonts for the Web" and installs Arial, Times New Roman, Courier New, Verdana, ... . These installed font data are non-free data.     Many free Latin fonts have their lineage traced to URW Nimbus family or Bitstream Vera. Tip If your locale needs fonts not covered well by the above fonts,     please use aptitude to check under task packages listed under "Tasks" -> "Localization". The font packages listed as "Depends:" or "Recommends:" in the localization task packages are the primary candidates. 7.6.2. Font rasterization Debian uses FreeType to rasterize fonts. Its font choice     infrastructure is provided by the Fontconfig font configuration library. Table 7.5. List of notable font environment and related packages +---------------------------------------------------------------+ | package | popcon |size| description | |---------------------+--------+----+---------------------------| |libfreetype6 |V:560, |938 |FreeType font rasterization| | |I:997 | |library | |---------------------+--------+----+---------------------------| |libfontconfig1 |V:554, |587 |Fontconfig font | | |I:849 | |configuration library |     |---------------------+--------+----+---------------------------| |fontconfig |V:437, |680 |fc-*: CLI commands for | | |I:721 | |Fontconfig | |---------------------+--------+----+---------------------------| |font-manager |V:2, I:8|1022|Font Manager: GUI command | | | | |for Fontconfig | |---------------------+--------+----+---------------------------| |nautilus-font-manager|V:0, I:0|37 |Nautilus extension for Font| | | | |Manager | +---------------------------------------------------------------+ Tip Some font packages such as fonts-noto* install too many fonts. You may also want to keep some font packages installed but disabled under the normal use situation. The multiple glyphs are     expected for some Unicode code points due to Han unification and unwanted gliphs may be chosen by the unconfigured Fontconfig library. One of the most annoying case is "U+3001 IDEOGRAPHIC COMMA" and "U+3002 IDEOGRAPHIC FULL STOP" among CJK countries. You can avoid this problematic situation easily by configuring font availability using Font Manager GUI (font-manager).     You can list font configuration state from the command line, too. * "fc-match(1)" for fontconfig font default     * "fc-list(1)" for available fontconfig fonts     You can configure font configuration state from the text editor but this is non-trivial. See fonts.conf(5). 7.7. Sandbox     Many mostly GUI applications on Linux are available in binary formats from non-Debian sources. * AppImage -- Linux apps that run anywhere     * FLATHUB -- Apps for Linux, right here * snapcraft -- The app store for Linux Warning     Binaries from these sites may include proprietary non-free software packages. There is some raison d'être for these binary format distributions for Free Software aficionados using Debian since these can     accommodate clean set of libraries used for each application by the respective upstream developer independent of the ones provided by Debian. The inherent risk of running external binaries can be reduced by     using the sandbox environment which leverages modern Linux security features (see Section 4.7.5, “Linux security features”). * For binaries from AppImage and some upstream sites, run them in firejail with manual configuration.     * For binaries from FLATHUB, run them in Flatpak . (No manual configuration required.) * For binaries from snapcraft, run them in Snap . (No manual configuration required. Compatible with daemon programs.) The xdg-desktop-portal package provides a standardized API to     common desktop features. See xdg-desktop-portal (flatpak) and xdg-desktop-portal (snap) . Table 7.6. List of notable sandbox environment and related packages +---------------------------------------------------------------+ | package |popcon|size | description | |-----------------------------+------+-----+--------------------| | |V:65, | |Flatpak application | |flatpak |I:70 |7499 |deployment framework| | | | |for desktop apps | |-----------------------------+------+-----+--------------------| |gnome-software-plugin-flatpak|V:20, |254 |Flatpak support for | | |I:29 | |GNOME Software | |-----------------------------+------+-----+--------------------| | |V:67, | |Daemon and tooling | |snapd |I:70 |60022|that enable snap | | | | |packages | |-----------------------------+------+-----+--------------------| |gnome-software-plugin-snap |V:1, |121 |Snap support for | | |I:2 | |GNOME Software | |-----------------------------+------+-----+--------------------|     | |V:303,| |desktop integration | |xdg-desktop-portal |I:394 |1936 |portal for Flatpak | | | | |and Snap | |-----------------------------+------+-----+--------------------| | |V:274,| |xdg-desktop-portal | |xdg-desktop-portal-gtk |I:392 |715 |backend for gtk | | | | |(GNOME) | |-----------------------------+------+-----+--------------------| |xdg-desktop-portal-kde |V:53, |1438 |xdg-desktop-portal | | |I:72 | |backend for Qt (KDE)| |-----------------------------+------+-----+--------------------| | |V:0, | |xdg-desktop-portal | |xdg-desktop-portal-wlr |I:4 |135 |backend for wlroots | | | | |(Wayland) | |-----------------------------+------+-----+--------------------| | | | |a SUID security | |firejail |V:1, |1771 |sandbox program | | |I:4 | |firejail for use | | | | |with AppImage | +---------------------------------------------------------------+ This sandbox environment technology is very much like apps on     smart phone OS where apps are executed under controlled resource accesses. Some large GUI applications such as web browsers on Debian also     use sandbox environment technology internally to make them more secure. 7.8. Remote desktop Table 7.7. List of notable remote access server Access to the desktop and applications which use Wayland protocol and run on the remote host is supported by the GNOME Remote Desktop on the remote host through VNC or RDP to the local client. Access to the desktop capabilities of all QEMU virtual machines is supported by the SPICE (the Simple Protocol for Independent Computing Environments) protocol. +---------------------------------------------------------------+ | package |popcon|size|protocols| description | |--------------------------+------+----+---------+--------------| |gnome-remote-desktop |V:35, |1068|RDP |GNOME Remote | | |I:221 | | |Desktop server| |--------------------------+------+----+---------+--------------| | | | | |xrdp, Remote | |xrdp |V:22, |3202|RDP |Desktop | | |I:24 | | |Protocol (RDP)| | | | | |server | |--------------------------+------+----+---------+--------------| | | | | |x11vnc, Remote| |x11vnc |V:6, |2107|RFB (VNC)|Framebuffer | | |I:23 | | |Protocol (VNC)| | | | | |server | |--------------------------+------+----+---------+--------------| | | | | |TigerVNC, | | |V:4, | | |Remote | |tigervnc-standalone-server|I:15 |2768|RFB (VNC)|Framebuffer | | | | | |Protocol (VNC)|     | | | | |server | |--------------------------+------+----+---------+--------------| |gnome-connections |V:0, |1356|RDP, RFB |GNOME remote | | |I:1 | |(VNC) |desktop client| |--------------------------+------+----+---------+--------------| | | | |RDP, RFB |Vinagre: GNOME| |vinagre |V:2, |4249|(VNC), |remote desktop| | |I:68 | |SPICE, |client | | | | |SSH | | |--------------------------+------+----+---------+--------------| | | | |RDP, RFB |Remmina: GTK | |remmina |V:15, |953 |(VNC), |remote desktop| | |I:72 | |SPICE, |client | | | | |SSH, ... | | |--------------------------+------+----+---------+--------------| | |V:1, | |RDP, RFB |KRDC: KDE | |krdc |I:17 |3873|(VNC) |remote desktop| | | | | |client | |--------------------------+------+----+---------+--------------| | | | | |Apache | | | | |RDP, RFB |Guacamole: | |guacd |V:0, |83 |(VNC), |clientless | | |I:0 | |SSH / |remote desktop| | | | |HTML5 |gateway | | | | | |(HTML5) | |--------------------------+------+----+---------+--------------| | | | | |Virtual | | |V:5, | |RFB |Machine | |virt-viewer |I:52 |1284|(VNC), |Manager's GUI | | | | |SPICE |display client| | | | | |of guest OS | +---------------------------------------------------------------+ 7.9. X server connection     There are several ways to connect from an application on a remote host to the X server including xwayland on the local host. Table 7.8. List of connection methods to the X server +---------------------------------------------------------------+ | package |popcon|size| command | description | |-----------------+------+----+-----------------+---------------| |openssh-server |V:730,|1804|sshd with option |SSH server | | |I:814 | |X11-forwarding |(secure) | |-----------------+------+----+-----------------+---------------|     |openssh-client |V:866,|4959|ssh -X |SSH client | | |I:996 | | |(secure) | |-----------------+------+----+-----------------+---------------| |xauth |V:165,|81 |xauth |X authority | | |I:961 | | |file utility | |-----------------+------+----+-----------------+---------------| |x11-xserver-utils|V:302,|568 |xhost |server access | | |I:528 | | |control for X | +---------------------------------------------------------------+ 7.9.1. X server local connection Access to the local X server by the local applications which use X core protocol can be locally connected through a local UNIX domain socket. This can be authorized by the authority file     holding access cookie. The authority file location is identified by the "$XAUTHORITY" environment variable and X display is identified by the "$DISPLAY" environment variable. Since these are normally set automatically, no special action is needed, e.g. "gitk" as the following.     username $ gitk Note     For xwayland, XAUTHORITY holds value like "/run/user/1000 /.mutter-Xwaylandauth.YVSU30". 7.9.2. X server remote connection Access to the local X server display from the remote applications     which use X core protocol is supported by using the X11 forwarding feature. * Open an gnome-terminal on the local host. * Run ssh(1) with -X option to establish a connection with the remote site as the following.     localname @ localhost $ ssh -q -X loginname@remotehost.domain Password: * Run an X application command, e.g. "gitk", on the remote site as the following. loginname @ remotehost $ gitk     This method can display the output from a remote X client as if it were locally connected through a local UNIX domain socket.     See Section 6.3, “The remote access server and utilities (SSH)” for SSH/SSHD. Warning     A remote TCP/IP connection to the X server is disabled by default on the Debian system for security reasons. Don't enable them by simply setting "xhost +" nor by enabling XDMCP connection, if you can avoid it. 7.9.3. X server chroot connection Access to the X server by the applications which use X core protocol and run on the same host but in an environment such as     chroot where the authority file is not accessible, can be authorized securely with xhost by using the User-based access, e.g. "gitk" as the following. username $ xhost + si:localuser:root ; sudo chroot /path/to # cd /src     # gitk # exit username $ xhost - 7.10. Clipboard     For clipping text to clipboard, see Section 1.4.4, “Mouse operations”.     For clipping graphics to clipboard, see Section 11.6, “Graphic data tools”.     Some CLI commands can manipulate character clipboard (PRIMARY and CLIPBOARD), too. Table 7.9. List of programs related to manipulating character clipboard +---------------------------------------------------------------+ | package |popcon|package|target | description | | | | size | | | |------------+------+-------+-------+---------------------------| |xsel |V:9, |55 |X |command line interface to X| | |I:42 | | |selections (clipboard) | |------------+------+-------+-------+---------------------------|     |xclip |V:12, |62 |X |command line interface to X| | |I:63 | | |selections (clipboard) | |------------+------+-------+-------+---------------------------| | |V:3, | | |wl-copy wl-paste: command | |wl-clipboard|I:14 |162 |Wayland|line interface to Wayland | | | | | |clipboard | |------------+------+-------+-------+---------------------------| | |V:10, | |Linux |a daemon that captures | |gpm |I:12 |521 |console|mouse events on Linux | | | | | |console | +---------------------------------------------------------------+ Chapter 8. I18N and L10N     Multilingualization (M17N) or Native Language Support for an application software is done in 2 steps. * Internationalization (I18N): To make a software potentially handle multiple locales.     * Localization (L10N): To make a software handle an specific locale. Tip     There are 17, 18, or 10 letters between "m" and "n", "i" and "n", or "l" and "n" in multilingualization, internationalization, and localization which correspond to M17N, I18N, and L10N. See Internationalization and localization for details. 8.1. The locale The behavior of programs supporting internationalization are configured by the environment variable "$LANG" to support     localization. Actual support of locale dependent features by the libc library requires to install locales or locales-all packages. The locales package requires to be initialized properly. If neither locales or locales-all package are installed, support     of locale features are lost and system uses US English messages and handles data as ASCII. This behavior is the same way as "$LANG" is set by "LANG=", "LANG=C", or "LANG=POSIX". The modern software such as GNOME and KDE are multilingualized. They are internationalized by making them handle UTF-8 data and     localized by providing their translated messages through the gettext(1) infrastructure. Translated messages may be provided as separate localization packages. The current Debian desktop GUI system normally sets the locale under GUI environment as "LANG=xx_YY.UTF-8". Here, "xx" is ISO     639 language codes and "YY" is ISO 3166 country codes. These values are set by the desktop configuration GUI dialogue and change the program behavior. See Section 1.5.2, “The "$LANG" variable” 8.1.1. Rationale for UTF-8 locale The simplest representation of the text data is ASCII which is     sufficient for English and uses less than 127 characters (representable with 7 bits). Even plain English text may contain non-ASCII characters, e.g.     slightly curly left and right quotation marks are not available in ASCII.     “double quoted text” is not "double quoted ASCII" ‘single quoted text’ is not 'single quoted ASCII' In order to support more characters, many character sets and     encoding systems have been used to support many languages (see Table 11.2, “List of encoding values and their usage”). Unicode character set can represent practically all characters     known to human with 21 bit code point range (i.e., 0 to 10FFFF in hexadecimal notation). Text encoding system UTF-8 fits Unicode code points into a sensible 8 bit data stream mostly compatible with the ASCII data processing system. This makes UTF-8 the modern preferred choice.     UTF stands for Unicode Transformation Format. When ASCII plain text data is converted to UTF-8 one, it has exactly the same content and size as the original ASCII one. So you loose nothing by deploying UTF-8 locale. Under UTF-8 locale with the compatible application program, you can display and edit any foreign language text data as long as     required fonts and input methods are installed and enabled. For example under "LANG=fr_FR.UTF-8" locale, gedit(1) (text editor for the GNOME desktop) can display and edit Chinese character text data while presenting menus in French. Tip Both the new standard "en_US.UTF-8" locale and the old standard     "C"/"POSIX" locale use the standard US English message, they have subtle differences in sorting order etc. If you want to handle not only ASCII characters but also handle all UTF-8 encoded characters gracefully while maintaining the old "C" local behavior, use the non-standard "C.UTF-8" locale on Debian. Note Some programs consume more memory after supporting I18N. This is     because they are coded to use UTF-32(UCS4) internally to support Unicode for speed optimization and consume 4 bytes per each ASCII character data independent of locale selected. Again, you loose nothing by deploying UTF-8 locale. 8.1.2. The reconfiguration of the locale     In order for the system to access a particular locale, the locale data must be compiled from the locale database.     The locales package does not come with pre-compiled locale data. You need to configure it as:     # dpkg-reconfigure locales     This process involves 2 steps. 1. Select all required locale data to be compiled into the binary form. (Please make sure to include at least one UTF-8 locale)     2. Set the system wide default locale value by creating "/etc/ default/locale" for use by PAM (see Section 4.5, “PAM and NSS”).     The system wide default locale value set in "/etc/default/locale" may be overridden by the GUI configuration for GUI applications. Note     Actual traditional encoding system can be identified by "/usr/ share/i18n/SUPPORTED". Thus, the "LANG=en_US" is "LANG= en_US.ISO-8859-1". The locales-all package comes with pre-compiled locale data for     all locale data. Since it doesn't create "/etc/default/locale", you may still need to install the locales package, too. Tip     The locales package of some Debian derivative distributions come with pre-compiled locale data for all locale data. You need to install both locales and locales-all packages on Debian to emulate such system environment. 8.1.3. Filename encoding For cross platform data exchanges (see Section 10.1.7, “Removable storage device”), you may need to mount some filesystem with     particular encodings. For example, mount(8) for vfat filesystem assumes CP437 if used without option. You need to provide explicit mount option to use UTF-8 or CP932 for filenames. Note When auto-mounting a hot-pluggable USB memory stick under modern     desktop environment such as GNOME, you may provide such mount option by right clicking the icon on the desktop, click "Drive" tab, click to expand "Setting", and entering "utf8" to "Mount options:". The next time this memory stick is mounted, mount with UTF-8 is enabled. Note If you are upgrading system or moving disk drives from older     non-UTF-8 system, file names with non-ASCII characters may be encoded in the historic and deprecated encodings such as ISO-8859-1 or eucJP. Please seek help of text conversion tools to convert them to UTF-8. See Section 11.1, “Text data conversion tools”. Samba uses Unicode for newer clients (Windows NT, 200x, XP) but     uses CP850 for older clients (DOS and Windows 9x/Me) as default. This default for older clients can be changed using "dos charset" in the "/etc/samba/smb.conf" file, e.g., to CP932 for Japanese. 8.1.4. Localized messages and translated documentation Translations exist for many of the text messages and documents that are displayed in the Debian system, such as error messages,     standard program output, menus, and manual pages. GNU gettext(1) command tool chain is used as the backend tool for most translation activities. Under "Tasks" → "Localization" aptitude(8) provides an extensive     list of useful binary packages which add localized messages to applications and provide translated documentation. For example, you can obtain the localized message for manpage by     installing the manpages-LANG package. To read the Italian-language manpage for programname from "/usr/share/man/it/ ", execute as the following.     LANG=it_IT.UTF-8 man programname     GNU gettext can accommodate priority list of translation languages with $LANGUAGE environment variable. For example:     $ export LANGUAGE="pt:pt_BR:es:it:fr"     For more, see info gettext and read the section "The LANGUAGE variable". 8.1.5. Effects of the locale The sort order of characters with sort(1) and ls(1) are affected     by the locale. Exporting LANG=en_US.UTF-8 sorts in the dictionary A->a->B->b...->Z->z order, while exporting LANG=C.UTF-8 sorts in ASCII binary A->B->...->Z->a->b... order.     The date format of ls(1) is affected by the locale (see Section 9.3.4, “Customized display of time and date”).     The date format of date(1) is affected by the locale. For example: $ unset LC_ALL $ LANG=en_US.UTF-8 date Thu Dec 24 08:30:00 PM JST 2023 $ LANG=en_GB.UTF-8 date     Thu 24 Dec 20:30:10 JST 2023 $ LANG=es_ES.UTF-8 date jue 24 dic 2023 20:30:20 JST $ LC_TIME=en_DK.UTF-8 date 2023-12-24T20:30:30 JST Number punctuation are different for locales. For example, in     English locale, one thousand point one is displayed as "1,000.1" while in German locale, it is displayed as "1.000,1". You may see this difference in spreadsheet program. Each detail feature of "$LANG" environment variable may be overridden by setting "$LC_*" variables. These environment variables can be overridden again by setting "$LC_ALL" variable.     See locale(7) manpage for the details. Unless you have strong reason to create complicated configuration, please stay away from them and use only "$LANG" variable set to one of the UTF-8 locales. 8.2. The keyboard input 8.2.1. The keyboard input for Linux console and X Window The Debian system can be configured to work with many     international keyboard arrangements using the keyboard-configuration and console-setup packages.     # dpkg-reconfigure keyboard-configuration # dpkg-reconfigure console-setup For the Linux console and the X Window system, this updates configuration parameters in "/etc/default/keyboard" and "/etc/     default/console-setup". This also configures the Linux console font. Many non-ASCII characters including accented characters used by many European languages can be made available with dead key, AltGr key, and compose key. 8.2.2. The keyboard input for Wayland For GNOME on Wayland desktop system, Section 8.2.1, “The keyboard input for Linux console and X Window” can't support non-English European languages. IBus was made to support not only Asian languages but also European languages. The package dependency of     GNOME desktop Environment recommends "ibus" via "gnome-shell". The code of "ibus" has been updated to integrate setxkbmap and XKB option functionalities. You need to configure ibus from "GNOME Settings" or "GNOME Tweaks" for the multilingualized keyboard input. Note If ibus is active, your classic X keyboard configuration by the     setxkbmap may be overridden by ibus even under classic X-based desktop environment. You can disable installed ibus using im-config to set input method to "None". For more, see Debian Wiki on keyboard. 8.2.3. The input method support with IBus     Since GNOME desktop Environment recommends "ibus" via "gnome-shell", "ibus" is the good choice for input method.     Multilingual input to the application is processed as: Keyboard Application | ^     | | +-> Linux kernel -> Input method (ibus) -> Gtk, Qt, X, Wayland +-- Engine--+     The list of IBus and its engine packages are the following. Table 8.1. List of IBus and its engine packages +---------------------------------------------------------------+ | package |popcon|size | supported locale | |---------------------+------+-----+----------------------------| |ibus |V:198,|1723 |input method framework using| | |I:245 | |dbus | |---------------------+------+-----+----------------------------| |ibus-mozc |V:1, |944 |Japanese | | |I:3 | | | |---------------------+------+-----+----------------------------| |ibus-anthy |V:0, |8856 |, , | | |I:1 | | | |---------------------+------+-----+----------------------------| |ibus-skk |V:0, |242 |, , | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-kkc |V:0, |210 |, , | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-libpinyin |V:1, |2761 |Chinese (for zh_CN) | | |I:3 | | | |---------------------+------+-----+----------------------------| |ibus-chewing |V:0, |247 |, , (for zh_TW) | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-libzhuyin |V:0, |40995|, , (for zh_TW) | | |I:0 | | | |---------------------+------+-----+----------------------------|     |ibus-rime |V:0, |73 |, , (for zh_CN/zh_TW) | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-cangjie |V:0, |119 |, , (for zh_HK) | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-hangul |V:0, |264 |Korean | | |I:2 | | | |---------------------+------+-----+----------------------------| |ibus-libthai |I:0 |90 |Thai | |---------------------+------+-----+----------------------------| |ibus-table-thai |I:0 |58 |Thai | |---------------------+------+-----+----------------------------| |ibus-unikey |V:0, |318 |Vietnamese | | |I:0 | | | |---------------------+------+-----+----------------------------| |ibus-keyman |V:0, |161 |Multilingual: Keyman engine | | |I:0 | |for over 2000 languages | |---------------------+------+-----+----------------------------| |ibus-table |V:0, |2176 |table engine for IBus | | |I:1 | | | |---------------------+------+-----+----------------------------| |ibus-m17n |V:0, |395 |Multilingual: Indic, Arabic | | |I:1 | |and others | |---------------------+------+-----+----------------------------| | |V:50, | |additional widgets for | |plasma-widgets-addons|I:100 |1992 |Plasma 5 containing Keyboard| | | | |Indicator | +---------------------------------------------------------------+ Note For Chinese, "fcitx5" may be an alternative input method     framework. For Emacs aficionados, "uim" may be an alternative. Either cases, you may need to do extra manual configuration with im-config. Some old classic input methods such as "kinput2" may still exist in Debian repository but are not recommended for the modern environment. 8.2.4. An example for Japanese I find the Japanese input method started under English     environment ("en_US.UTF-8") very useful. Here is how I did this with IBus for GNOME on Wayland: 1. Install the Japanese input tool package ibus-mozc (or ibus-anthy) with its recommended packages such as im-config. 2. Select "Settings" → "Keyboard" → "Input Sources" → click "+" in "Input Sources" → "Japanese" → "Japanese mozc (or anthy)" and click "Add" if it hasn't been activated.     3. You may chose as many input sources. 4. Relogin to user's account. 5. Setup each input source by right clicking the GUI toolbar icon. 6. Switch among installed input sources by SUPER-SPACE. (SUPER is normally the Windows key.) Tip If you wish to have access to alphabet only keyboard environment     with the physical Japanese keyboard on which shift-2 has " (double quotation mark) engraved, you select "Japanese" in the above procedure. You can enter Japanese using "Japanese mozc (or anthy)" with physical "US" keyboard on which shift-2 has @ (at mark) engraved. * The GUI menu entry for im-config(8) is "Input method". * Alternatively, execute "im-config" from user's shell.     * im-config(8) behaves differently if command is executed from root or not. * im-config(8) enables the best input method on the system as default without any user actions. 8.3. The display output Linux console can only display limited characters. (You need to     use special terminal program such as jfbterm(1) to display non-European languages on the non-GUI console.) GUI environment (Chapter 7, GUI System) can display any     characters in the UTF-8 as long as required fonts are installed and enabled. (The encoding of the original font data is taken care and transparent to the user.) 8.4. East Asian Ambiguous Character Width Characters Under the East Asian locale, the box drawing, Greek, and Cyrillic     characters may be displayed wider than your desired width to cause the unaligned terminal output (see Unicode Standard Annex # 11).     You can work around this problem: * gnome-terminal: Preferences → Profiles → Profile name →     Compatibility → Ambiguous-wide characters → Narrow * ncurses: Set environment export NCURSES_NO_UTF8_ACS=0. Chapter 9. System tips     Here, I describe basic tips to configure and manage systems, mostly from the console. 9.1. The console tips     There are some utility programs to help your console activities. Table 9.1. List of programs to support console activities +---------------------------------------------------------------+ |package |popcon |size| description | |--------+-------+----+-----------------------------------------| |mc |V:50, |1542|See Section 1.3, “Midnight Commander (MC)| | |I:209 | |” | |--------+-------+----+-----------------------------------------| |bsdutils|V:519, |356 |script(1) command to make a record of | | |I:999 | |terminal session | |--------+-------+----+-----------------------------------------| |screen |V:71, |1003|terminal multiplexer with VT100/ANSI | | |I:230 | |terminal emulation | |--------+-------+----+-----------------------------------------| |tmux |V:43, |1180|terminal multiplexer alternative (Use | | |I:146 | |"Control-B" instead) | |--------+-------+----+-----------------------------------------|     |fzf |V:4, |3648|fuzzy text finder | | |I:16 | | | |--------+-------+----+-----------------------------------------| |fzy |V:0, |54 |fuzzy text finder | | |I:0 | | | |--------+-------+----+-----------------------------------------| |rlwrap |V:1, |330 |readline feature command line wrapper | | |I:15 | | | |--------+-------+----+-----------------------------------------| |ledit |V:0, |331 |readline feature command line wrapper | | |I:11 | | | |--------+-------+----+-----------------------------------------| |rlfe |V:0, |45 |readline feature command line wrapper | | |I:0 | | | |--------+-------+----+-----------------------------------------| |ripgrep |V:5, |5152|fast recursive string search in the | | |I:19 | |source code tree with automatic filtering| +---------------------------------------------------------------+ 9.1.1. Recording the shell activities cleanly The simple use of script(1) (see Section 1.4.9, “Recording the     shell activities”) to record shell activity produces a file with control characters. This can be avoided by using col(1) as the following.     $ script Script started, file is typescript     Do whatever … and press Ctrl-D to exit script.     $ col -bx < typescript > cleanedfile $ vim cleanedfile     There are alternative methods to record the shell activities: * Use tee (usable during the boot process in the initramfs): $ sh -i 2>&1 | tee typescript * Use gnome-terminal with the extend line buffer for scrollback. * Use screen with "^A H" (see Section 9.1.2, “The screen     program”) to perform recording of console. * Use vim with ":terminal" to enter the terminal mode. Use "Ctrl-W N" to exit from terminal mode to normal mode. Use ":w typescript" to write the buffer to a file. * Use emacs with "M-x shell", "M-x eshell", or "M-x term" to enter recording console. Use "C-x C-w" to write the buffer to a file. 9.1.2. The screen program screen(1) not only allows one terminal window to work with     multiple processes, but also allows remote shell process to survive interrupted connections. Here is a typical use scenario of screen(1). 1. You login to a remote machine. 2. You start screen on a single console. 3. You execute multiple programs in screen windows created with ^A c ("Control-A" followed by "c"). 4. You switch among the multiple screen windows by ^A n ("Control-A" followed by "n"). 5. Suddenly you need to leave your terminal, but you don't want to lose your active work by keeping the connection. 6. You may detach the screen session by any methods.     + Brutally unplug your network connection + Type ^A d ("Control-A" followed by "d") and manually logging out from the remote connection + Type ^A DD ("Control-A" followed by "DD") to have screen detach and log you out 7. You log in again to the same remote machine (even from a different terminal). 8. You start screen as "screen -r". 9. screen magically reattaches all previous screen windows with all actively running programs. Tip     You can save connection fees with screen for metered network connections such as dial-up and packet ones, because you can leave a process active while disconnected, and then re-attach it later when you connect again. In a screen session, all keyboard inputs are sent to your current     window except for the command keystroke. All screen command keystrokes are entered by typing ^A ("Control-A") plus a single key [plus any parameters]. Here are important ones to remember. Table 9.2. List of key bindings for screen +-------------------------------------------------------------+ |key binding| meaning | |-----------+-------------------------------------------------| |^A ? |show a help screen (display key bindings) | |-----------+-------------------------------------------------| |^A c |create a new window and switch to it | |-----------+-------------------------------------------------| |^A n |go to next window | |-----------+-------------------------------------------------| |^A p |go to previous window | |-----------+-------------------------------------------------| |^A 0 |go to window number 0 | |-----------+-------------------------------------------------|     |^A 1 |go to window number 1 | |-----------+-------------------------------------------------| |^A w |show a list of windows | |-----------+-------------------------------------------------| |^A a |send a Ctrl-A to current window as keyboard input| |-----------+-------------------------------------------------| |^A h |write a hardcopy of current window to file | |-----------+-------------------------------------------------| |^A H |begin/end logging current window to file | |-----------+-------------------------------------------------| |^A ^X |lock the terminal (password protected) | |-----------+-------------------------------------------------| |^A d |detach screen session from the terminal | |-----------+-------------------------------------------------| |^A DD |detach screen session and log out | +-------------------------------------------------------------+     See screen(1) for details.     See tmux(1) for functionalities of the alternative command. 9.1.3. Navigating around directories     In Section 1.4.2, “Customizing bash”, 2 tips to allow quick navigation around directories are described: $CDPATH and mc.     If you use fuzzy text filter program, you can do without typing the exact path. For fzf, include following in ~/.bashrc. FZF_KEYBINDINGS_PATH=/usr/share/doc/fzf/examples/key-bindings.bash     if [ -f $FZF_KEYBINDINGS_PATH ]; then . $FZF_KEYBINDINGS_PATH fi     For example: * You can jump to a very deep subdirectory with minimal efforts. You first type "cd **" and press Tab. Then you will be prompted with candidate paths. Typing in partial path strings, e.g., s/d/b foo, will narrow down candidate paths. You select the path to be used by cd with cursor and return keys.     * You can select a command from the command history more efficiently with minimal efforts. You press Ctrl-R at the command prompt. Then you will be prompted with candidate commands. Typing in partial command strings, e.g., vim d, will narrow down candidates. You select the one to be used with cursor and return keys. 9.1.4. Readline wrapper Some commands such as /usr/bin/dash which lacks command line     history editing capability can add such functionality transparently by running under rlwrap or its equivalents.     $ rlwrap dash -i     This provides convenient platform to test subtle points for dash with friendly bash-like environment. 9.1.5. Scanning the source code tree The rg(1) command in the ripgrep package offers a faster alternative to the grep(1) command for scanning the source code     tree for typical situation. It takes advantage of modern multi-core CPUs and automatically applies reasonable filters to skip some files. 9.2. Customizing vim After you learn basics of vim(1) through Section 1.4.8, “Using     vim”, please read Bram Moolenaar's "Seven habits of effective text editing (2000)" to understand how vim should be used. 9.2.1. Customizing vim with internal features The behavior of vim can be changed significantly by enabling its     internal features through the Ex-mode commands such as "set ..." to set vim options. These Ex-mode commands can be included in user's vimrc file,     traditional "~/.vimrc" or git-friendly "~/.vim/vimrc". Here is a very simple example ^[2]: """ Generic baseline Vim and Neovim configuration (~/.vimrc) """ - For NeoVim, use "nvim -u ~/.vimrc [filename]" set nocompatible " :h 'cp -- sensible (n)vim mode syntax on " :h :syn-on filetype plugin indent on " :h :filetype-overview set encoding=utf-8 " :h 'enc (default: latin1) -- sensible encoding """ current vim option value can be verified by :set encoding? set backspace=indent,eol,start " :h 'bs (default: nobs) -- sensible BS set statusline=%<%f%m%r%h%w%=%y[U+%04B]%2l/%2L=%P,%2c%V set listchars=eol:¶,tab:⇄\ ,extends:↦,precedes:↤,nbsp:␣ set viminfo=!,'100,<5000,s100,h " :h 'vi -- bigger copy buffer etc. """ Pick "colorscheme" from blue darkblue default delek desert elflord evening """ habamax industry koehler lunaperche morning murphy pablo peachpuff quiet ron """ shine slate torte zellner colorscheme industry     "colorscheme default set scrolloff=5 " :h 'scr -- show 5 lines around cursor set laststatus=2 " :h 'ls (default 1) k """ boolian options can be unset by prefixing "no" "set list " :h 'list (default nolist) set smartcase " :h 'scs -- Override the 'ignorecase' option set autoindent " :h 'ai set smartindent " :h 'si set nowrap " :h 'wrap set nolist " :h 'list -- (enable when needed) set noerrorbells " :h 'eb set novisualbell " :h 'vb set t_vb= " :h 't_vb -- termcap visual bell set spell " :h 'spell set spelllang=en_us,cjk " :h 'spl -- english spell, ignore CJK set clipboard=unnamedplus " :h 'clipboard -- cut/copy/paste with other app     The keymap of vim can be changed in user's vimrc file. E.g.: Caution     Don't try to change the default key bindings without very good reasons. """ *** popular mappings *** """ Window moves without using CTRL-W which is dangerous in INSERT mode nnoremap h nnoremap j nnoremap k silent! nnoremap l """ nohlsearch with ( is mapped as above) inoremap noh nnoremap noh """ execute macro recorded with qq.....q with Q nnoremap Q @q """ center after jump next nnoremap n nzz nnoremap N Nzz """ fast "jk" to get out of INSERT mode ()     inoremap jk """ double to get out of TERM mode (CTRL-\ CTRL-N) tnoremap """ fast "jk" to get out of TERM mode (CTRL-\ CTRL-N) tnoremap jk """ """ *** nvim default mappings (no need for nvim) *** """ copy to EOL (no delete) like D for d noremap Y y$ """ disable prefix repeats for erases inoremap u inoremap u """ search visual selected string for visual mode xnoremap * y/\V" xnoremap # y?\V" """ repeat last substitute and *KEEP* flags nnoremap & :&& In order for the above keybindings to function properly, the     terminal program needs to be configured to generate "ASCII DEL" for Backspace-key and "Escape sequence" for Delete-key.     Other miscellaneous configuration can be changed in user's vimrc file. E.g.: """ Force to use underline for spell check results augroup SpellUnderline autocmd! autocmd ColorScheme * highlight SpellBad term=Underline gui=Undercurl autocmd ColorScheme * highlight SpellCap term=Underline gui=Undercurl autocmd ColorScheme * highlight SpellLocal term=Underline gui=Undercurl autocmd ColorScheme * highlight SpellRare term=Underline gui=Undercurl augroup END " """ highlight tailing spaces except when typing as red (set after colorscheme) highlight TailingWhitespaces ctermbg=red guibg=red     """ \s\+ 1 or more whitespace character: and """ \%#\@ 0 && line ("'\"") <= line("$") | \ exe "normal! g'\"" | \ endif augroup END 9.2.2. Customizing vim with external packages     Interesting external plugin packages can be found: * Vim - the ubiquitous text editor -- The official upstream site of Vim and vim scripts     * VimAwsome -- The listing of Vim plugins * vim-scripts -- Debian package: a collection of vim scripts     Plugin packages in the vim-scripts package can be enabled using user's vimrc file. E.g.: packadd! secure-modelines packadd! winmanager let mapleader = ' '     " Toggle paste mode with p for Vim (no need for Nvim) set pastetoggle=p " IDE-like UI for files and buffers with w nnoremap w :WMToggle The new native Vim package system works nicely with "git" and     "git submodule". One such example configuration can be found at my git repository: dot-vim. This does essentially: * By using "git" and "git submodule", latest external packages, such as "name", are placed into ~/.vim/pack/*/opt/name and similar. * By adding :packadd! name line to user's vimrc file, these     packages are placed on runtimepath. * Vim loads these packages on runtimepath during its initialization. * At the end of its initialization, tags for the installed documents are updated with "helptags ALL".     For more, please start vim with "vim --startuptime vimstart.log" to check actual execution sequence and time spent for each step. It is quite confusing to see too many ways^[3] to manage and load     these external packages to vim. Checking the original information is the best cure. Table 9.3. Information on the initialization of vim +---------------------------------------------------------------+ |key strokes | information | |------------+--------------------------------------------------| |:help |explanation on the vim package mechanism | |package | | |------------+--------------------------------------------------| |:help |explanation on the runtimepath mechanism | |runtimepath | | |------------+--------------------------------------------------| |:version |internal states including candidates for the vimrc|     | |file | |------------+--------------------------------------------------| |:echo $VIM |the environment variable "$VIM" used to locate the| | |vimrc file | |------------+--------------------------------------------------| |:set |list of directories which will be searched for all| |runtimepath?|runtime support files | |------------+--------------------------------------------------| |:echo |the environment variable "$VIMRUNTIME" used to | |$VIMRUNTIME |locate various system provided runtime support | | |files | +---------------------------------------------------------------+ 9.3. Data recording and presentation 9.3.1. The log daemon     Many traditional programs record their activities in the text file format under the "/var/log/" directory.     logrotate(8) is used to simplify the administration of log files on a system which generates a lot of log files. Many new programs record their activities in the binary file     format using systemd-journald(8) Journal service under the "/var/ log/journal" directory.     You can log data to the systemd-journald(8) Journal from a shell script by using the systemd-cat(1) command.     See Section 3.4, “The system message” and Section 3.3, “The kernel message”. 9.3.2. Log analyzer     Here are notable log analyzers ("~Gsecurity::log-analyzer" in aptitude(8)). Table 9.4. List of system log analyzers +---------------------------------------------------------------+ | package |popcon |size| description | |----------+-------+----+---------------------------------------| |logwatch |V:11, |2328|log analyzer with nice output written | | |I:13 | |in Perl | |----------+-------+----+---------------------------------------| |fail2ban |V:98, |2126|ban IPs that cause multiple | | |I:111 | |authentication errors | |----------+-------+----+---------------------------------------| |analog |V:3, |3739|web server log analyzer | | |I:96 | | | |----------+-------+----+---------------------------------------| |awstats |V:6, |6928|powerful and featureful web server log | | |I:10 | |analyzer | |----------+-------+----+---------------------------------------| |sarg |V:1, |845 |squid analysis report generator | | |I:1 | | |     |----------+-------+----+---------------------------------------| |pflogsumm |V:1, |109 |Postfix log entry summarizer | | |I:4 | | | |----------+-------+----+---------------------------------------| |fwlogwatch|V:0, |481 |firewall log analyzer | | |I:0 | | | |----------+-------+----+---------------------------------------| |squidview |V:0, |189 |monitor and analyze squid access.log | | |I:0 | |files | |----------+-------+----+---------------------------------------| |swatch |V:0, |99 |log file viewer with regexp matching, | | |I:0 | |highlighting, and hooks | |----------+-------+----+---------------------------------------| |crm114 |V:0, |1119|Controllable Regex Mutilator and Spam | | |I:0 | |Filter (CRM114) | |----------+-------+----+---------------------------------------| |icmpinfo |V:0, |44 |interpret ICMP messages | | |I:0 | | | +---------------------------------------------------------------+ Note     CRM114 provides language infrastructure to write fuzzy filters with the TRE regex library. Its popular use is spam mail filter but it can be used as log analyzer. 9.3.3. Customized display of text data Although pager tools such as more(1) and less(1) (see Section 1.4.5, “The pager”) and custom tools for highlighting and     formatting (see Section 11.1.8, “Highlighting and formatting plain text data”) can display text data nicely, general purpose editors (see Section 1.4.6, “The text editor”) are most versatile and customizable. Tip     For vim(1) and its pager mode alias view(1), ":set hls" enables highlighted search. 9.3.4. Customized display of time and date The default display format of time and date by the "ls -l" command depends on the locale (see Section 1.2.6, “Timestamps”     for value). The "$LANG" variable is referred first and it can be overridden by the "$LC_TIME" or "$LC_ALL" exported environment variables. The actual default display format for each locale depends on the     version of the standard C library (the libc6 package) used. I.e., different releases of Debian had different defaults. For iso-formats, see ISO 8601. If you really wish to customize this display format of time and     date beyond the locale, you should set the time style value by the "--time-style" argument or by the "$TIME_STYLE" value (see ls (1), date(1), "info coreutils 'ls invocation'"). Table 9.5. Display examples of time and date for the "ls -l" command with the time style value +--------------------------------------------------------------+ | time style | locale | display of time and date | | value | | | |--------------+--------------+--------------------------------| |iso |any |01-19 00:15 | |--------------+--------------+--------------------------------| |long-iso |any |2009-01-19 00:15 | |--------------+--------------+--------------------------------| |full-iso |any |2009-01-19 00:15:16.000000000 | | | |+0900 | |--------------+--------------+--------------------------------|     |locale |C |Jan 19 00:15 | |--------------+--------------+--------------------------------| |locale |en_US.UTF-8 |Jan 19 00:15 | |--------------+--------------+--------------------------------| |locale |es_ES.UTF-8 |ene 19 00:15 | |--------------+--------------+--------------------------------| |+%d.%m.%y |any |19.01.09 00:15 | |%H:%M | | | |--------------+--------------+--------------------------------| |+%d.%b.%y |C or |19.Jan.09 00:15 | |%H:%M |en_US.UTF-8 | | |--------------+--------------+--------------------------------| |+%d.%b.%y |es_ES.UTF-8 |19.ene.09 00:15 | |%H:%M | | | +--------------------------------------------------------------+ Tip     You can eliminate typing long option on commandline using command alias (see Section 1.5.9, “Command alias”): alias ls='ls --time-style=+%d.%m.%y %H:%M' 9.3.5. Colorized shell echo     Shell echo to most modern terminals can be colorized using ANSI escape code (see "/usr/share/doc/xterm/ctlseqs.txt.gz").     For example, try the following $ RED=$(printf "\x1b[31m")     $ NORMAL=$(printf "\x1b[0m") $ REVERSE=$(printf "\x1b[7m") $ echo "${RED}RED-TEXT${NORMAL} ${REVERSE}REVERSE-TEXT${NORMAL}" 9.3.6. Colorized commands Colorized commands are handy for inspecting their output in the     interactive environment. I include the following in my "~ /.bashrc". if [ "$TERM" != "dumb" ]; then eval "`dircolors -b`" alias ls='ls --color=always' alias ll='ls --color=always -l' alias la='ls --color=always -A' alias less='less -R' alias ls='ls --color=always'     alias grep='grep --color=always' alias egrep='egrep --color=always' alias fgrep='fgrep --color=always' alias zgrep='zgrep --color=always' else alias ll='ls -l' alias la='ls -A' fi The use of alias limits color effects to the interactive command usage. It has advantage over exporting environment variable     "export GREP_OPTIONS='--color=auto'" since color can be seen under pager programs such as less(1). If you wish to suppress color when piping to other programs, use "--color=auto" instead in the above example for "~/.bashrc". Tip     You can turn off these colorizing aliases in the interactive environment by invoking shell with "TERM=dumb bash". 9.3.7. Recording the editor activities for complex repeats     You can record the editor activities for complex repeats.     For Vim, as follows. * "qa": start recording typed characters into named register "a".     * … editor activities * "q": end recording typed characters. * "@a": execute the contents of register "a".     For Emacs, as follows. * "C-x (": start defining a keyboard macro. * … editor activities     * "C-x )": end defining a keyboard macro. * "C-x e": execute a keyboard macro. 9.3.8. Recording the graphics image of an X application     There are few ways to record the graphics image of an X application, including an xterm display. Table 9.6. List of graphics image manipulation tools +----------------------------------------------------------------+ | package | popcon |size | screen | command | |----------------+--------+-----+--------+-----------------------| |gnome-screenshot|V:18, |1134 |Wayland |screenshot application | | |I:173 | | |for GNOME | |----------------+--------+-----+--------+-----------------------| |flameshot |V:7, |3364 |Wayland |screenshot application | | |I:15 | | |on steroid | |----------------+--------+-----+--------+-----------------------|     |gimp |V:50, |19304|Wayland |screenshot in GUI menu | | |I:252 | |+ X | | |----------------+--------+-----+--------+-----------------------| |x11-apps |V:31, |2460 |X |xwd(1) | | |I:463 | | | | |----------------+--------+-----+--------+-----------------------| |imagemagick |I:317 |74 |X |import(1) | |----------------+--------+-----+--------+-----------------------| |scrot |V:5, |131 |X |scrot(1) | | |I:63 | | | | +----------------------------------------------------------------+ 9.3.9. Recording changes in configuration files     There are specialized tools to record changes in configuration files with help of DVCS and to make system snapshots on Btrfs. Table 9.7. List of packages which can record configuration history +---------------------------------------------------------------+ | package |popcon|size| description | |---------+------+----+-----------------------------------------| | |V:26, | |store configuration files and their | |etckeeper|I:30 |168 |metadata with Git (default), Mercurial, |     | | | |or GNU Bazaar | |---------+------+----+-----------------------------------------| |timeshift|V:5, |3506|system restore utility using rsync or | | |I:10 | |BTRFS snapshots | |---------+------+----+-----------------------------------------| |snapper |V:4, |2392|Linux filesystem snapshot management tool| | |I:5 | | | +---------------------------------------------------------------+     You may also think about local script Section 10.2.3, “Backup tips” approach. 9.4. Monitoring, controlling, and starting program activities     Program activities can be monitored and controlled using specialized tools. Table 9.8. List of tools for monitoring and controlling program activities +---------------------------------------------------------------+ | package |popcon|size | description | |---------+------+-----+----------------------------------------| |coreutils|V:880,|18307|nice(1): run a program with modified | | |I:999 | |scheduling priority | |---------+------+-----+----------------------------------------| |bsdutils |V:519,|356 |renice(1): modify the scheduling | | |I:999 | |priority of a running process | |---------+------+-----+----------------------------------------| |procps |V:766,|2389 |"/proc" filesystem utilities: ps(1), top| | |I:999 | |(1), kill(1), watch(1), … | |---------+------+-----+----------------------------------------| |psmisc |V:420,|908 |"/proc" filesystem utilities: killall | | |I:775 | |(1), fuser(1), peekfd(1), pstree(1) | |---------+------+-----+----------------------------------------| |time |V:7, |129 |time(1): run a program to report system | | |I:132 | |resource usages with respect to time | |---------+------+-----+----------------------------------------| |sysstat |V:148,|1904 |sar(1), iostat(1), mpstat(1), …: system | | |I:170 | |performance tools for Linux | |---------+------+-----+----------------------------------------| |isag |V:0, |109 |Interactive System Activity Grapher for | | |I:3 | |sysstat |     |---------+------+-----+----------------------------------------| |lsof |V:422,|482 |lsof(8): list files opened by a running | | |I:945 | |process using "-p" option | |---------+------+-----+----------------------------------------| |strace |V:12, |2897 |strace(1): trace system calls and | | |I:119 | |signals | |---------+------+-----+----------------------------------------| |ltrace |V:0, |330 |ltrace(1): trace library calls | | |I:16 | | | |---------+------+-----+----------------------------------------| |xtrace |V:0, |353 |xtrace(1): trace communication between | | |I:0 | |X11 client and server | |---------+------+-----+----------------------------------------| |powertop |V:18, |677 |powertop(1): information about system | | |I:217 | |power use | |---------+------+-----+----------------------------------------| |cron |V:872,|244 |run processes according to a schedule in| | |I:995 | |background from cron(8) daemon | |---------+------+-----+----------------------------------------| |anacron |V:396,|93 |cron-like command scheduler for systems | | |I:479 | |that don't run 24 hours a day | |---------+------+-----+----------------------------------------| | |V:101,| |at(1) or batch(1): run a job at a | |at |I:154 |158 |specified time or below certain load | | | | |level | +---------------------------------------------------------------+ Tip     The procps packages provide very basics of monitoring, controlling, and starting program activities. You should learn all of them. 9.4.1. Timing a process     Display time used by the process invoked by the command. # time some_command >/dev/null     real 0m0.035s # time on wall clock (elapsed real time) user 0m0.000s # time in user mode sys 0m0.020s # time in kernel mode 9.4.2. The scheduling priority     A nice value is used to control the scheduling priority for the process. Table 9.9. List of nice values for the scheduling priority +---------------------------------------------------------+ |nice value| scheduling priority | |----------+----------------------------------------------|     |19 |lowest priority process (nice) | |----------+----------------------------------------------| |0 |very high priority process for user | |----------+----------------------------------------------| |-20 |very high priority process for root (not-nice)| +---------------------------------------------------------+     # nice -19 top # very nice # nice --20 wodim -v -eject speed=2 dev=0,0 disk.img # very fast     Sometimes an extreme nice value does more harm than good to the system. Use this command carefully. 9.4.3. The ps command     The ps(1) command on a Debian system support both BSD and SystemV features and helps to identify the process activity statically. Table 9.10. List of ps command styles +------------------------------------------+ | style |typical command| feature |     |--------+---------------+-----------------| |BSD |ps aux |display %CPU %MEM| |--------+---------------+-----------------| |System V|ps -efH |display PPID | +------------------------------------------+     For the zombie (defunct) children process, you can kill them by the parent process ID identified in the "PPID" field.     The pstree(1) command display a tree of processes. 9.4.4. The top command     top(1) on the Debian system has rich features and helps to identify what process is acting funny dynamically. It is an interactive full screen program. You can get its usage     help press by pressing the "h"-key and terminate it by pressing the "q"-key. 9.4.5. Listing files opened by a process     You can list all files opened by a process with a process ID (PID), e.g. 1, by the following.     $ sudo lsof -p 1     PID=1 is usually the init program. 9.4.6. Tracing program activities You can trace program activity with strace(1), ltrace(1), or     xtrace(1) for system calls and signals, library calls, or communication between X11 client and server.     You can trace system calls of the ls command as the following.     $ sudo strace ls Tip     Use strace-graph script found in /usr/share/doc/strace/examples/ to make a nice tree view 9.4.7. Identification of processes using files or sockets     You can also identify processes using files by fuser(1), e.g. for "/var/log/mail.log" by the following. $ sudo fuser -v /var/log/mail.log     USER PID ACCESS COMMAND /var/log/mail.log: root 2946 F.... rsyslogd     You see that file "/var/log/mail.log" is open for writing by the rsyslogd(8) command.     You can also identify processes using sockets by fuser(1), e.g. for "smtp/tcp" by the following. $ sudo fuser -v smtp/tcp     USER PID ACCESS COMMAND smtp/tcp: Debian-exim 3379 F.... exim4     Now you know your system runs exim4(8) to handle TCP connections to SMTP port (25). 9.4.8. Repeating a command with a constant interval     watch(1) executes a program repeatedly with a constant interval while showing its output in fullscreen.     $ watch w     This displays who is logged on to the system updated every 2 seconds. 9.4.9. Repeating a command looping over files     There are several ways to repeat a command looping over files matching some condition, e.g. matching glob pattern "*.ext".     * Shell for-loop method (see Section 12.1.4, “Shell loops”):     for x in *.ext; do if [ -f "$x"]; then command "$x" ; fi; done     * find(1) and xargs(1) combination:     find . -type f -maxdepth 1 -name '*.ext' -print0 | xargs -0 -n 1 command     * find(1) with "-exec" option with a command:     find . -type f -maxdepth 1 -name '*.ext' -exec command '{}' \;     * find(1) with "-exec" option with a short shell script:     find . -type f -maxdepth 1 -name '*.ext' -exec sh -c "command '{}' && echo 'successful'" \; The above examples are written to ensure proper handling of funny     file names such as ones containing spaces. See Section 10.1.5, “Idioms for the selection of files” for more advance uses of find (1). 9.4.10. Starting a program from GUI For the command-line interface (CLI), the first program with the     matching name found in the directories specified in the $PATH environment variable is executed. See Section 1.5.3, “The "$PATH" variable”. For the graphical user interface (GUI) compliant to the freedesktop.org standards, the *.desktop files in the /usr/share/ applications/ directory provide necessary attributes for the GUI menu display of each program. Each package which is compliant to     Freedesktop.org's xdg menu system installs its menu data provided by "*.desktop" under "/usr/share/applications/". Modern desktop environments which are compliant to Freedesktop.org standard use these data to generate their menu using the xdg-utils package. See "/usr/share/doc/xdg-utils/README". For example, the chromium.desktop file defines attributes for the "Chromium Web Browser" such as "Name" for the program name,     "Exec" for the program execution path and arguments, "Icon" for the icon used, etc. (see the Desktop Entry Specification) as follows: [Desktop Entry] Version=1.0 Name=Chromium Web Browser GenericName=Web Browser Comment=Access the Internet Comment[fr]=Explorer le Web Exec=/usr/bin/chromium %U     Terminal=false X-MultipleArgs=false Type=Application Icon=chromium Categories=Network;WebBrowser; MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https; StartupWMClass=Chromium StartupNotify=true     This is an oversimplified description. The *.desktop files are scanned as follows.     The desktop environment sets $XDG_DATA_HOME and $XDG_DATA_DIR environment variables. For example, under the GNOME 3: * $XDG_DATA_HOME is unset. (The default value of $HOME/.local/ share is used.)     * $XDG_DATA_DIRS is set to /usr/share/gnome:/usr/local/share/:/ usr/share/.     So the base directories (see XDG Base Directory Specification) and the applications directories are as follows. * $HOME/.local/share/ → $HOME/.local/share/applications/ * /usr/share/gnome/ → /usr/share/gnome/applications/     * /usr/local/share/ → /usr/local/share/applications/ * /usr/share/ → /usr/share/applications/     The *.desktop files are scanned in these applications directories in this order. Tip     A user custom GUI menu entry can be created by adding a *.desktop file in the $HOME/.local/share/applications/ directory. Tip     The "Exec=..." line isn't parsed by the shell. Use the env(1) command if environment variables need to be set. Tip Similarly, if a *.desktop file is created in the autostart     directory under these base directories, the specified program in the *.desktop file is executed automatically when the desktop environment is started. See Desktop Application Autostart Specification. Tip Similarly, if a *.desktop file is created in the $HOME/Desktop     directory and the Desktop environment is configured to support the desktop icon launcher feature, the specified program in it is executed upon clicking the icon. Please note that the actual name of the $HOME/Desktop directory is locale dependent. See xdg-user-dirs-update(1). 9.4.11. Customizing program to be started     Some programs start another program automatically. Here are check points for customizing this process. * Application configuration menu: + GNOME3 desktop: "Settings" → "System" → "Details" → "Default Applications" + KDE desktop: "K" → "Control Center" → "KDE Components" → "Component Chooser" + Iceweasel browser: "Edit" → "Preferences" → "Applications" + mc(1): "/etc/mc/mc.ext"     * Environment variables such as "$BROWSER", "$EDITOR", "$VISUAL", and "$PAGER" (see environ(7)) * The update-alternatives(1) system for programs such as "editor", "view", "x-www-browser", "gnome-www-browser", and "www-browser" (see Section 1.4.7, “Setting a default text editor”) * the "~/.mailcap" and "/etc/mailcap" file contents which associate MIME type with program (see mailcap(5)) * The "~/.mime.types" and "/etc/mime.types" file contents which associate file name extension with MIME type (see run-mailcap (1)) Tip     update-mime(8) updates the "/etc/mailcap" file using "/etc/ mailcap.order" file (see mailcap.order(5)). Tip     The debianutils package provides sensible-browser(1), sensible-editor(1), and sensible-pager(1) which make sensible decisions on which editor, pager, and web browser to call, respectively. I recommend you to read these shell scripts. Tip In order to run a console application such as mutt under GUI as your preferred application, you should create an GUI application as following and set "/usr/local/bin/mutt-term" as your preferred     application to be started as described. # cat /usr/local/bin/mutt-term <> $HOME/tmp/out 2>&1     # run at 14:15 on the first of every month -- output mailed to paul 15 14 1 * * $HOME/bin/monthly # run at 22:00 on weekdays(1-5), annoy Joe. % for newline, last % for cc: 0 22 * * 1-5 mail -s "It's 10pm" joe%Joe,%%Where are your kids?%.%% 23 */2 1 2 * echo "run 23 minutes after 0am, 2am, 4am ..., on Feb 1" 5 4 * * sun echo "run at 04:05 every Sunday" # run at 03:40 on the first Monday of each month 40 3 1-7 * * [ "$(date +%a)" == "Mon" ] && command -args Tip     For the system not running continuously, install the anacron package to schedule periodic commands at the specified intervals as closely as machine-uptime permits. See anacron(8) and anacrontab(5). Tip For scheduled system maintenance scripts, you can run them     periodically from root account by placing such scripts in "/etc/ cron.hourly/", "/etc/cron.daily/", "/etc/cron.weekly/", or "/etc/ cron.monthly/". Execution timings of these scripts can be customized by "/etc/crontab" and "/etc/anacrontab". Systemd has low level capability to schedule programs to run     without cron daemon. For example, /lib/systemd/system/ apt-daily.timer and /lib/systemd/system/apt-daily.service set up daily apt download activities. See systemd.timer(5) . 9.4.15. Scheduling tasks on event Systemd can schedule program not only on the timer event but also     on the mount event. See Section 10.2.3.3, “Timer event triggered backup” and Section 10.2.3.2, “Mount event triggered backup” for examples. 9.4.16. Alt-SysRq key     Pressing Alt-SysRq (PrtScr) followed by one keys does the magic of rescuing control of the system. Table 9.12. List of notable SAK command keys +---------------------------------------------------------------+ | key following | description of action | | Alt-SysRq | | |------------------+--------------------------------------------| |k |kill all processes on the current virtual | | |console (SAK) | |------------------+--------------------------------------------|     |s |sync all mounted filesystems to avoid data | | |corruption | |------------------+--------------------------------------------| |u |remount all mounted filesystems read-only (u| | |mount) | |------------------+--------------------------------------------| |r |restore the keyboard from raw mode after X | | |crashes | +---------------------------------------------------------------+     See more on Linux kernel user’s and administrator’s guide » Linux Magic System Request Key Hacks Tip     From SSH terminal etc., you can use the Alt-SysRq feature by writing to the "/proc/sysrq-trigger". For example, "echo s > / proc/sysrq-trigger; echo u > /proc/sysrq-trigger" from the root shell prompt syncs and umounts all mounted filesystems.     The current (2021) Debian amd64 Linux kernel has /proc/sys/kernel /sysrq=438=0b110110110: * 2 = 0x2 - enable control of console logging level (ON) * 4 = 0x4 - enable control of keyboard (SAK, unraw) (ON) * 8 = 0x8 - enable debugging dumps of processes etc. (OFF) * 16 = 0x10 - enable sync command (ON)     * 32 = 0x20 - enable remount read-only (ON) * 64 = 0x40 - enable signaling of processes (term, kill, oom-kill) (OFF) * 128 = 0x80 - allow reboot/poweroff (ON) * 256 = 0x100 - allow nicing of all RT tasks (ON) 9.5. System maintenance tips 9.5.1. Who is on the system?     You can check who is on the system by the following. * who(1) shows who is logged on. * w(1) shows who is logged on and what they are doing.     * last(1) shows listing of last logged in user. * lastb(1) shows listing of last bad logged in users. Tip     "/var/run/utmp", and "/var/log/wtmp" hold such user information. See login(1) and utmp(5). 9.5.2. Warning everyone     You can send message to everyone who is logged on to the system with wall(1) by the following.     $ echo "We are shutting down in 1 hour" | wall 9.5.3. Hardware identification For the PCI-like devices (AGP, PCI-Express, CardBus, ExpressCard,     etc.), lspci(8) (probably with "-nn" option) is a good start for the hardware identification. Alternatively, you can identify the hardware by reading contents     of "/proc/bus/pci/devices" or browsing directory tree under "/sys /bus/pci" (see Section 1.2.12, “procfs and sysfs”). Table 9.13. List of hardware identification tools +---------------------------------------------------------------+ | package | popcon |size| description | |-----------+--------+----+-------------------------------------| |pciutils |V:249, |213 |Linux PCI Utilities: lspci(8) | | |I:991 | | | |-----------+--------+----+-------------------------------------| |usbutils |V:68, |325 |Linux USB utilities: lsusb(8) | | |I:869 | | | |-----------+--------+----+-------------------------------------| |nvme-cli |V:15, |1642|NVMe utilities for Linux: nvme(1) | | |I:22 | | | |-----------+--------+----+-------------------------------------|     |pcmciautils|V:6, |91 |PCMCIA utilities for Linux: pccardctl| | |I:10 | |(8) | |-----------+--------+----+-------------------------------------| |scsitools |V:0, I:2|346 |collection of tools for SCSI hardware| | | | |management: lsscsi(8) | |-----------+--------+----+-------------------------------------| |procinfo |V:0, I:9|132 |system information obtained from "/ | | | | |proc": lsdev(8) | |-----------+--------+----+-------------------------------------| |lshw |V:13, |919 |information about hardware | | |I:89 | |configuration: lshw(1) | |-----------+--------+----+-------------------------------------| |discover |V:40, |98 |hardware identification system: | | |I:958 | |discover(8) | +---------------------------------------------------------------+ 9.5.4. Hardware configuration Although most of the hardware configuration on modern GUI desktop     systems such as GNOME and KDE can be managed through accompanying GUI configuration tools, it is a good idea to know some basics methods to configure them. Table 9.14. List of hardware configuration tools +---------------------------------------------------------------+ | package |popcon|size | description | |-----------------+------+-----+--------------------------------| |console-setup |V:88, |428 |Linux console font and keytable | | |I:967 | |utilities | |-----------------+------+-----+--------------------------------| |x11-xserver-utils|V:302,|568 |X server utilities: xset(1), | | |I:528 | |xmodmap(1) | |-----------------+------+-----+--------------------------------| | | | |daemon to manage events | |acpid |V:84, |158 |delivered by the Advanced | | |I:148 | |Configuration and Power | | | | |Interface (ACPI) | |-----------------+------+-----+--------------------------------| |acpi |V:9, |47 |utility to display information | | |I:136 | |on ACPI devices | |-----------------+------+-----+--------------------------------| |sleepd |V:0, |86 |daemon to put a laptop to sleep | | |I:0 | |during inactivity |     |-----------------+------+-----+--------------------------------| | |V:178,| |hard disk access optimization | |hdparm |I:335 |256 |(see Section 9.6.9, | | | | |“Optimization of hard disk”) | |-----------------+------+-----+--------------------------------| |smartmontools |V:207,|2358 |control and monitor storage | | |I:250 | |systems using S.M.A.R.T. | |-----------------+------+-----+--------------------------------| |setserial |V:4, |103 |collection of tools for serial | | |I:6 | |port management | |-----------------+------+-----+--------------------------------| |memtest86+ |V:1, |12711|collection of tools for memory | | |I:21 | |hardware management | |-----------------+------+-----+--------------------------------| |scsitools |V:0, |346 |collection of tools for SCSI | | |I:2 | |hardware management | |-----------------+------+-----+--------------------------------| |setcd |V:0, |37 |compact disc drive access | | |I:0 | |optimization | |-----------------+------+-----+--------------------------------| |big-cursor |I:0 |26 |larger mouse cursors for X | +---------------------------------------------------------------+     Here, ACPI is a newer framework for the power management system than APM. Tip     CPU frequency scaling on modern system is governed by kernel modules such as acpi_cpufreq. 9.5.5. System and hardware time     The following sets system and hardware time to MM/DD hh:mm, CCYY. # date MMDDhhmmCCYY     # hwclock --utc --systohc # hwclock --show     Times are normally displayed in the local time on the Debian system but the hardware and system time usually use UTC(GMT).     If the hardware time is set to UTC, change the setting to "UTC= yes" in the "/etc/default/rcS".     The following reconfigure the timezone used by the Debian system.     # dpkg-reconfigure tzdata If you wish to update system time via network, consider to use     the NTP service with the packages such as ntp, ntpdate, and chrony. Tip     Under systemd, use systemd-timesyncd for the network time synchronization instead. See systemd-timesyncd(8).     See the following. * Managing Accurate Date and Time HOWTO     * NTP Public Services Project * The ntp-doc package Tip     ntptrace(8) in the ntp package can trace a chain of NTP servers back to the primary source. 9.5.6. The terminal configuration     There are several components to configure character console and ncurses(3) system features. * The "/etc/terminfo/*/*" file (terminfo(5))     * The "$TERM" environment variable (term(7)) * setterm(1), stty(1), tic(1), and toe(1) If the terminfo entry for xterm doesn't work with a non-Debian xterm, change your terminal type, "$TERM", from "xterm" to one of     the feature-limited versions such as "xterm-r6" when you log in to a Debian system remotely. See "/usr/share/doc/libncurses5/FAQ" for more. "dumb" is the lowest common denominator for "$TERM". 9.5.7. The sound infrastructure Device drivers for sound cards for current Linux are provided by     Advanced Linux Sound Architecture (ALSA). ALSA provides emulation mode for previous Open Sound System (OSS) for compatibility. Application softwares may be configured not only to access sound devices directly but also to access them via some standardized     sound server system. Currently, PulseAudio, JACK, and PipeWire are used as sound server system. See Debian wiki page on Sound for the latest situation. There is usually a common sound engine for each popular desktop     environment. Each sound engine used by the application can choose to connect to different sound servers. Tip     Use "cat /dev/urandom > /dev/audio" or speaker-test(1) to test speaker (^C to stop). Tip     If you can not get sound, your speaker may be connected to a muted output. Modern sound system has many outputs. alsamixer(1) in the alsa-utils package is useful to configure volume and mute settings. Table 9.15. List of sound packages +---------------------------------------------------------------+ | package |popcon|size| description | |-----------------+------+----+---------------------------------| |alsa-utils |V:330,|2605|utilities for configuring and | | |I:466 | |using ALSA | |-----------------+------+----+---------------------------------| | |V:1, | |OSS compatibility under ALSA | |oss-compat |I:17 |18 |preventing "/dev/dsp not found" | | | | |errors | |-----------------+------+----+---------------------------------| |pipewire |V:265,|120 |audio and video processing engine| | |I:319 | |multimedia server - metapackage | |-----------------+------+----+---------------------------------| | |V:274,| |audio and video processing engine| |pipewire-bin |I:319 |1631|multimedia server - audio server | | | | |and CLI programs | |-----------------+------+----+---------------------------------| | |V:105,| |audio and video processing engine| |pipewire-alsa |I:157 |206 |multimedia server - audio server | | | | |to replace ALSA |     |-----------------+------+----+---------------------------------| | |V:160,| |audio and video processing engine| |pipewire-pulse |I:214 |50 |multimedia server - audio server | | | | |to replace PulseAudio | |-----------------+------+----+---------------------------------| |pulseaudio |V:256,|6472|PulseAudio server | | |I:308 | | | |-----------------+------+----+---------------------------------| |libpulse0 |V:413,|975 |PulseAudio client library | | |I:580 | | | |-----------------+------+----+---------------------------------| |jackd |V:2, |9 |JACK Audio Connection Kit. (JACK)| | |I:18 | |server (low latency) | |-----------------+------+----+---------------------------------| |libjack0 |V:1, |326 |JACK Audio Connection Kit. (JACK)| | |I:9 | |library (low latency) | |-----------------+------+----+---------------------------------| |libgstreamer1.0-0|V:429,|4455|GStreamer: GNOME sound engine | | |I:597 | | | |-----------------+------+----+---------------------------------| |libphonon4qt5-4 |V:72, |594 |Phonon: KDE sound engine | | |I:162 | | | +---------------------------------------------------------------+ 9.5.8. Disabling the screen saver     For disabling the screen saver, use following commands. Table 9.16. List of commands for disabling the screen saver +---------------------------------------------------------------+ | environment | command | |----------------------------------------+----------------------| |The Linux console |setterm -powersave off| |----------------------------------------+----------------------|     |The X Window (turning off screensaver) |xset s off | |----------------------------------------+----------------------| |The X Window (disabling dpms) |xset -dpms | |----------------------------------------+----------------------| |The X Window (GUI configuration of |xscreensaver-command | |screen saver) |-prefs | +---------------------------------------------------------------+ 9.5.9. Disabling beep sounds     One can always unplug the PC speaker to disable beep sounds. Removing pcspkr kernel module does this for you.     The following prevents the readline(3) program used by bash(1) to beep when encountering an alert character (ASCII=7).     $ echo "set bell-style none">> ~/.inputrc 9.5.10. Memory usage     There are 2 resources available for you to get the memory usage situation. * The kernel boot message in the "/var/log/dmesg" contains the total exact size of available memory.     * free(1) and top(1) display information on memory resources on the running system.     Here is an example. # grep '\] Memory' /var/log/dmesg [ 0.004000] Memory: 990528k/1016784k available (1975k kernel code, 25868k reserved, 931k data, 296k init) $ free -k     total used free shared buffers cached Mem: 997184 976928 20256 0 129592 171932 -/+ buffers/cache: 675404 321780 Swap: 4545576 4 4545572     You may be wondering "dmesg tells me a free of 990 MB, and free -k says 320 MB is free. More than 600 MB missing …". Do not worry about the large size of "used" and the small size of     "free" in the "Mem:" line, but read the one under them (675404 and 321780 in the example above) and relax.     For my MacBook with 1GB=1048576k DRAM (video system steals some of this), I see the following. Table 9.17. List of memory sizes reported +----------------------------------------------------+ | report | size | |-------------------+--------------------------------| |Total size in dmesg|1016784k = 1GB - 31792k |     |-------------------+--------------------------------| |Free in dmesg |990528k | |-------------------+--------------------------------| |Total under shell |997184k | |-------------------+--------------------------------| |Free under shell |20256k (but effectively 321780k)| +----------------------------------------------------+ 9.5.11. System security and integrity check     Poor system maintenance may expose your system to external exploitation.     For system security and integrity check, you should start with the following. * The debsums package, see debsums(1) and Section 2.5.2, “Top level "Release" file and authenticity”. * The chkrootkit package, see chkrootkit(1).     * The clamav package family, see clamscan(1) and freshclam(1). * Debian security FAQ. * Securing Debian Manual. Table 9.18. List of tools for system security and integrity check +---------------------------------------------------------------+ | package |popcon|size | description | |----------+------+-----+---------------------------------------| |logcheck |V:6, |110 |daemon to mail anomalies in the system | | |I:7 | |logfiles to the administrator | |----------+------+-----+---------------------------------------| |debsums |V:5, |98 |utility to verify installed package | | |I:35 | |files against MD5 checksums | |----------+------+-----+---------------------------------------| |chkrootkit|V:8, |925 |rootkit detector | | |I:17 | | | |----------+------+-----+---------------------------------------| |clamav |V:9, |27455|anti-virus utility for Unix - | | |I:45 | |command-line interface | |----------+------+-----+---------------------------------------|     |tiger |V:1, |7800 |report system security vulnerabilities | | |I:2 | | | |----------+------+-----+---------------------------------------| |tripwire |V:1, |5016 |file and directory integrity checker | | |I:2 | | | |----------+------+-----+---------------------------------------| |john |V:1, |471 |active password cracking tool | | |I:9 | | | |----------+------+-----+---------------------------------------| |aide |V:1, |293 |Advanced Intrusion Detection | | |I:1 | |Environment - static binary | |----------+------+-----+---------------------------------------| |integrit |V:0, |2659 |file integrity verification program | | |I:0 | | | |----------+------+-----+---------------------------------------| |crack |V:0, |149 |password guessing program | | |I:1 | | | +---------------------------------------------------------------+     Here is a simple script to check for typical world writable incorrect file permissions.     # find / -perm 777 -a \! -type s -a \! -type l -a \! \( -type d -a -perm 1777 \) Caution     Since the debsums package uses MD5 checksums stored locally, it can not be fully trusted as the system security audit tool against malicious attacks. 9.6. Data storage tips Booting your system with Linux live CDs or debian-installer CDs     in rescue mode makes it easy for you to reconfigure data storage on your boot device. You may need to umount(8) some devices manually from the command     line before operating on them if they are automatically mounted by the GUI desktop system. 9.6.1. Disk space usage     The disk space usage can be evaluated by programs provided by the mount, coreutils, and xdu packages: * mount(8) reports all mounted filesystems (= disks).     * df(1) reports the disk space usage for the file system. * du(1) reports the disk space usage for the directory tree. Tip     You can feed the output of du(8) to xdu(1x) to produce its graphical and interactive presentation with "du -k . |xdu", "sudo du -k -x / |xdu", etc. 9.6.2. Disk partition configuration For disk partition configuration, although fdisk(8) has been     considered standard, parted(8) deserves some attention. "Disk partitioning data", "partition table", "partition map", and "disk label" are all synonyms. Older PCs use the classic Master Boot Record (MBR) scheme to hold     disk partitioning data in the first sector, i.e., LBA sector 0 (512 bytes). Recent PCs with Unified Extensible Firmware Interface (UEFI),     including Intel-based Macs, use GUID Partition Table (GPT) scheme to hold disk partitioning data not in the first sector.     Although fdisk(8) has been standard for the disk partitioning tool, parted(8) is replacing it. Table 9.19. List of disk partition management packages +---------------------------------------------------------------+ | package |popcon |size| description | |----------+-------+----+---------------------------------------| |util-linux|V:881, |5283|miscellaneous system utilities | | |I:999 | |including fdisk(8) and cfdisk(8) | |----------+-------+----+---------------------------------------| |parted |V:417, |122 |GNU Parted disk partition resizing | | |I:568 | |program |     |----------+-------+----+---------------------------------------| |gparted |V:15, |2175|GNOME partition editor based on | | |I:102 | |libparted | |----------+-------+----+---------------------------------------| |gdisk |V:338, |885 |partition editor for the GPT/MBR hybrid| | |I:511 | |disk | |----------+-------+----+---------------------------------------| |kpartx |V:22, |77 |program to create device mappings for | | |I:33 | |partitions | +---------------------------------------------------------------+ Caution     Although parted(8) claims to create and to resize filesystem too, it is safer to do such things using best maintained specialized tools such as mkfs(8) (mkfs.msdos(8), mkfs.ext2(8), mkfs.ext3(8), mkfs.ext4(8), …) and resize2fs(8). Note In order to switch between GPT and MBR, you need to erase first     few blocks of disk contents directly (see Section 9.8.6, “Clearing file contents”) and use "parted /dev/sdx mklabel gpt" or "parted /dev/sdx mklabel msdos" to set it. Please note "msdos" is use here for MBR. 9.6.3. Accessing partition using UUID Although reconfiguration of your partition or activation order of removable storage media may yield different names for partitions,     you can access them consistently. This is also helpful if you have multiple disks and your BIOS/UEFI doesn't give them consistent device names. * mount(8) with "-U" option can mount a block device using UUID, instead of using its file name such as "/dev/sda3".     * "/etc/fstab" (see fstab(5)) can use UUID. * Boot loaders (Section 3.1.2, “Stage 2: the boot loader”) may use UUID too. Tip     You can probe UUID of a block special device with blkid(8). You can also probe UUID and other information with "lsblk -f". 9.6.4. LVM2 LVM2 is a logical volume manager for the Linux kernel. With LVM2,     disk partitions can be created on logical volumes instead of the physical harddisks.     LVM requires the following. * device-mapper support in the Linux kernel (default for Debian kernels)     * the userspace device-mapper support library (libdevmapper* package) * the userspace LVM2 tools (lvm2 package)     Please start learning LVM2 from the following manpages. * lvm(8): Basics of LVM2 mechanism (list of all LVM2 commands) * lvm.conf(5): Configuration file for LVM2     * lvs(8): Report information about logical volumes * vgs(8): Report information about volume groups * pvs(8): Report information about physical volumes 9.6.5. Filesystem configuration     For ext4 filesystem, the e2fsprogs package provides the following. * mkfs.ext4(8) to create new ext4 filesystem * fsck.ext4(8) to check and to repair existing ext4 filesystem     * tune2fs(8) to configure superblock of ext4 filesystem * debugfs(8) to debug ext4 filesystem interactively. (It has undel command to recover deleted files.) The mkfs(8) and fsck(8) commands are provided by the e2fsprogs package as front-ends to various filesystem dependent programs     (mkfs.fstype and fsck.fstype). For ext4 filesystem, they are mkfs.ext4(8) and fsck.ext4(8) (they are symlinked to mke2fs(8) and e2fsck(8)).     Similar commands are available for each filesystem supported by Linux. Table 9.20. List of filesystem management packages +---------------------------------------------------------------+ | package |popcon|size| description | |--------------+------+----+------------------------------------| |e2fsprogs |V:767,|1499|utilities for the ext2/ext3/ext4 | | |I:999 | |filesystems | |--------------+------+----+------------------------------------| |btrfs-progs |V:44, |5078|utilities for the Btrfs filesystem | | |I:72 | | | |--------------+------+----+------------------------------------| |reiserfsprogs |V:12, |473 |utilities for the Reiserfs | | |I:25 | |filesystem | |--------------+------+----+------------------------------------| |zfsutils-linux|V:29, |1762|utilities for the OpenZFS filesystem| | |I:30 | | | |--------------+------+----+------------------------------------| |dosfstools |V:196,|315 |utilities for the FAT filesystem. | | |I:541 | |(Microsoft: MS-DOS, Windows) | |--------------+------+----+------------------------------------| |exfatprogs |V:29, |301 |utilities for the exFAT filesystem | | |I:371 | |maintained by Samsung. | |--------------+------+----+------------------------------------| |exfat-fuse |V:5, |73 |read/write exFAT filesystem |     | |I:120 | |(Microsoft) driver for FUSE. | |--------------+------+----+------------------------------------| |exfat-utils |V:4, |231 |utilities for the exFAT filesystem | | |I:106 | |maintained by the exfat-fuse author.| |--------------+------+----+------------------------------------| |xfsprogs |V:21, |3476|utilities for the XFS filesystem. | | |I:95 | |(SGI: IRIX) | |--------------+------+----+------------------------------------| | |V:197,| |read/write NTFS filesystem | |ntfs-3g |I:513 |1474|(Microsoft: Windows NT, …) driver | | | | |for FUSE. | |--------------+------+----+------------------------------------| |jfsutils |V:0, |1577|utilities for the JFS filesystem. | | |I:8 | |(IBM: AIX, OS/2) | |--------------+------+----+------------------------------------| |reiser4progs |V:0, |1367|utilities for the Reiser4 filesystem| | |I:2 | | | |--------------+------+----+------------------------------------| |hfsprogs |V:0, |394 |utilities for HFS and HFS Plus | | |I:4 | |filesystem. (Apple: Mac OS) | |--------------+------+----+------------------------------------| |zerofree |V:5, |25 |program to zero free blocks from | | |I:131 | |ext2/3/4 filesystems | +---------------------------------------------------------------+ Tip Ext4 filesystem is the default filesystem for the Linux system and strongly recommended to use it unless you have some specific reasons not to.     Btrfs status can be found at Debian wiki on btrfs and kernel.org wiki on btrfs. It is expected to be the next default filesystem after the ext4 filesystem. Some tools allow access to filesystem without Linux kernel support (see Section 9.8.2, “Manipulating files without mounting disk”). 9.6.6. Filesystem creation and integrity check The mkfs(8) command creates the filesystem on a Linux system. The     fsck(8) command provides the filesystem integrity check and repair on a Linux system.     Debian now defaults to no periodic fsck after filesystem creation. Caution     It is generally not safe to run fsck on mounted filesystems. Tip You can run the fsck(8) command safely on all filesystems including root filesystem on reboot by setting     "enable_periodic_fsck" in "/etc/mke2fs.conf" and the max mount count to 0 using "tune2fs -c0 /dev/partition_name". See mke2fs.conf(5) and tune2fs(8). Check files in "/var/log/fsck/" for the result of the fsck(8) command run from the boot script. 9.6.7. Optimization of filesystem by mount options     The basic static filesystem configuration is given by "/etc/ fstab". For example, «file system» «mount point» «type» «options» «dump» «pass» proc /proc proc defaults 0 0     UUID=709cbe4c-80c1-56db-8ab1-dbce3146d2f7 / ext4 errors=remount-ro 0 1 UUID=817bae6b-45d2-5aca-4d2a-1267ab46ac23 none swap sw 0 0 /dev/scd0 /media/cdrom0 udf,iso9660 user,noauto 0 0 Tip     UUID (see Section 9.6.3, “Accessing partition using UUID”) may be used to identify a block device instead of normal block device names such as "/dev/sda1", "/dev/sda2", …     Since Linux 2.6.30, the kernel defaults to the behavior provided by "relatime" option.     See fstab(5) and mount(8). 9.6.8. Optimization of filesystem via superblock     Characteristics of a filesystem can be optimized via its superblock using the tune2fs(8) command. * Execution of "sudo tune2fs -l /dev/hda1" displays the contents of the filesystem superblock on "/dev/hda1". * Execution of "sudo tune2fs -c 50 /dev/hda1" changes frequency of filesystem checks (fsck execution during boot-up) to every 50 boots on "/dev/hda1".     * Execution of "sudo tune2fs -j /dev/hda1" adds journaling capability to the filesystem, i.e. filesystem conversion from ext2 to ext3 on "/dev/hda1". (Do this on the unmounted filesystem.) * Execution of "sudo tune2fs -O extents,uninit_bg,dir_index / dev/hda1 && fsck -pf /dev/hda1" converts it from ext3 to ext4 on "/dev/hda1". (Do this on the unmounted filesystem.) Tip     Despite its name, tune2fs(8) works not only on the ext2 filesystem but also on the ext3 and ext4 filesystems. 9.6.9. Optimization of hard disk Warning     Please check your hardware and read manpage of hdparm(8) before playing with hard disk configuration because this may be quite dangerous for the data integrity. You can test disk access speed of a hard disk, e.g. "/dev/hda", by "hdparm -tT /dev/hda". For some hard disk connected with (E)     IDE, you can speed it up with "hdparm -q -c3 -d1 -u1 -m16 /dev/ hda" by enabling the "(E)IDE 32-bit I/O support", enabling the "using_dma flag", setting "interrupt-unmask flag", and setting the "multiple 16 sector I/O" (dangerous!). You can test write cache feature of a hard disk, e.g. "/dev/sda",     by "hdparm -W /dev/sda". You can disable its write cache feature with "hdparm -W 0 /dev/sda".     You may be able to read badly pressed CDROMs on modern high speed CD-ROM drive by slowing it down with "setcd -x 2". 9.6.10. Optimization of solid state drive     Solid state drive (SSD) is auto detected now.     Reduce unnecessary disk accesses to prevent disk wear out by mounting "tmpfs" on volatile data path in /etc/fstab. 9.6.11. Using SMART to predict hard disk failure     You can monitor and log your hard disk which is compliant to SMART with the smartd(8) daemon. 1. Enable SMART feature in BIOS. 2. Install the smartmontools package. 3. Identify your hard disk drives by listing them with df(1). + Let's assume a hard disk drive to be monitored as "/dev/ hda". 4. Check the output of "smartctl -a /dev/hda" to see if SMART     feature is actually enabled. + If not, enable it by "smartctl -s on -a /dev/hda". 5. Enable smartd(8) daemon to run by the following. + uncomment "start_smartd=yes" in the "/etc/default/ smartmontools" file. + restart the smartd(8) daemon by "sudo systemctl restart smartmontools". Tip     The smartd(8) daemon can be customized with the /etc/smartd.conf file including how to be notified of warnings. 9.6.12. Specify temporary storage directory via $TMPDIR Applications create temporary files normally under the temporary     storage directory "/tmp". If "/tmp" does not provide enough space, you can specify such temporary storage directory via the $TMPDIR variable for well-behaving programs. 9.6.13. Expansion of usable storage space via LVM For partitions created on Logical Volume Manager (LVM) (Linux feature) at install time, they can be resized easily by     concatenating extents onto them or truncating extents from them over multiple storage devices without major system reconfiguration. 9.6.14. Expansion of usable storage space by mounting another partition If you have an empty partition (e.g., "/dev/sdx"), you can format     it with mkfs.ext4(1) and mount(8) it to a directory where you need more space. (You need to copy original data contents.) $ sudo mv work-dir old-dir $ sudo mkfs.ext4 /dev/sdx     $ sudo mount -t ext4 /dev/sdx work-dir $ sudo cp -a old-dir/* work-dir $ sudo rm -rf old-dir Tip     You may alternatively mount an empty disk image file (see Section 9.7.5, “Making the empty disk image file”) as a loop device (see Section 9.7.3, “Mounting the disk image file”). The actual disk usage grows with the actual data stored. 9.6.15. Expansion of usable storage space by bind-mounting another directory If you have an empty directory (e.g., "/path/to/emp-dir") on     another partition with usable space, you can mount(8) it with "--bind" option to a directory (e.g., "work-dir") where you need more space.     $ sudo mount --bind /path/to/emp-dir work-dir 9.6.16. Expansion of usable storage space by overlay-mounting another directory If you have usable space in another partition (e.g., "/path/to/ empty" and "/path/to/work"), you can create a directory in it and     stack that on to an old directory (e.g., "/path/to/old") where you need space using the OverlayFS for Linux kernel 3.18 or newer (Debian Stretch 9.0 or newer).     $ sudo mount -t overlay overlay \ -olowerdir=/path/to/old-dir,upperdir=/path/to/empty,workdir=/path/to/work     Here, "/path/to/empty" and "/path/to/work" should be on the RW-enabled partition to write on "/path/to/old". 9.6.17. Expansion of usable storage space using symlink Caution     This is a deprecated method. Some software may not function well with "symlink to a directory". Instead, use the "mounting" approaches described in the above. If you have an empty directory (e.g., "/path/to/emp-dir") in     another partition with usable space, you can create a symlink to the directory with ln(8). $ sudo mv work-dir old-dir $ sudo mkdir -p /path/to/emp-dir     $ sudo ln -sf /path/to/emp-dir work-dir $ sudo cp -a old-dir/* work-dir $ sudo rm -rf old-dir Warning     Do not use "symlink to a directory" for directories managed by the system such as "/opt". Such a symlink may be overwritten when the system is upgraded. 9.7. The disk image     Here, we discuss manipulations of the disk image. 9.7.1. Making the disk image file The disk image file, "disk.img", of an unmounted device, e.g.,     the second SCSI or serial ATA drive "/dev/sdb", can be made using cp(1) or dd(1) by the following.     # cp /dev/sdb disk.img # dd if=/dev/sdb of=disk.img The disk image of the traditional PC's master boot record (MBR)     (see Section 9.6.2, “Disk partition configuration”) which reside on the first sector on the primary IDE disk can be made by using dd(1) by the following. # dd if=/dev/hda of=mbr.img bs=512 count=1     # dd if=/dev/hda of=mbr-nopart.img bs=446 count=1 # dd if=/dev/hda of=mbr-part.img skip=446 bs=1 count=66 * "mbr.img": The MBR with the partition table     * "mbr-nopart.img": The MBR without the partition table * "mbr-part.img": The partition table of the MBR only     If you have an SCSI or serial ATA device as the boot disk, substitute "/dev/hda" with "/dev/sda".     If you are making an image of a disk partition of the original disk, substitute "/dev/hda" with "/dev/hda1" etc. 9.7.2. Writing directly to the disk The disk image file, "disk.img" can be written to an unmounted     device, e.g., the second SCSI drive "/dev/sdb" with matching size, by the following.     # dd if=disk.img of=/dev/sdb Similarly, the disk partition image file, "partition.img" can be     written to an unmounted partition, e.g., the first partition of the second SCSI drive "/dev/sdb1" with matching size, by the following.     # dd if=partition.img of=/dev/sdb1 9.7.3. Mounting the disk image file The disk image "partition.img" containing a single partition     image can be mounted and unmounted by using the loop device as follows. # losetup --show -f partition.img /dev/loop0 # mkdir -p /mnt/loop0     # mount -t auto /dev/loop0 /mnt/loop0 ...hack...hack...hack # umount /dev/loop0 # losetup -d /dev/loop0     This can be simplified as follows. # mkdir -p /mnt/loop0     # mount -t auto -o loop partition.img /mnt/loop0 ...hack...hack...hack # umount partition.img     Each partition of the disk image "disk.img" containing multiple partitions can be mounted by using the loop device. # losetup --show -f -P disk.img /dev/loop0 # ls -l /dev/loop0* brw-rw---- 1 root disk 7, 0 Apr 2 22:51 /dev/loop0 brw-rw---- 1 root disk 259, 12 Apr 2 22:51 /dev/loop0p1 brw-rw---- 1 root disk 259, 13 Apr 2 22:51 /dev/loop0p14 brw-rw---- 1 root disk 259, 14 Apr 2 22:51 /dev/loop0p15 # fdisk -l /dev/loop0 Disk /dev/loop0: 2 GiB, 2147483648 bytes, 4194304 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: 6A1D9E28-C48C-2144-91F7-968B3CBC9BD1     Device Start End Sectors Size Type /dev/loop0p1 262144 4192255 3930112 1.9G Linux root (x86-64) /dev/loop0p14 2048 8191 6144 3M BIOS boot /dev/loop0p15 8192 262143 253952 124M EFI System Partition table entries are not in disk order. # mkdir -p /mnt/loop0p1 # mkdir -p /mnt/loop0p15 # mount -t auto /dev/loop0p1 /mnt/loop0p1 # mount -t auto /dev/loop0p15 /mnt/loop0p15 # mount |grep loop /dev/loop0p1 on /mnt/loop0p1 type ext4 (rw,relatime) /dev/loop0p15 on /mnt/loop0p15 type vfat (rw,relatime,fmask=0002,dmask=0002,allow_utime=0020,codepage=437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro) ...hack...hack...hack # umount /dev/loop0p1 # umount /dev/loop0p15 # losetup -d /dev/loop0 Alternatively, similar effects can be done by using the device     mapper devices created by kpartx(8) from the kpartx package as follows. # kpartx -a -v disk.img add map loop0p1 (253:0): 0 3930112 linear 7:0 262144 add map loop0p14 (253:1): 0 6144 linear 7:0 2048 add map loop0p15 (253:2): 0 253952 linear 7:0 8192 # fdisk -l /dev/loop0 Disk /dev/loop0: 2 GiB, 2147483648 bytes, 4194304 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: 6A1D9E28-C48C-2144-91F7-968B3CBC9BD1 Device Start End Sectors Size Type /dev/loop0p1 262144 4192255 3930112 1.9G Linux root (x86-64) /dev/loop0p14 2048 8191 6144 3M BIOS boot /dev/loop0p15 8192 262143 253952 124M EFI System     Partition table entries are not in disk order. # ls -l /dev/mapper/ total 0 crw------- 1 root root 10, 236 Apr 2 22:45 control lrwxrwxrwx 1 root root 7 Apr 2 23:19 loop0p1 -> ../dm-0 lrwxrwxrwx 1 root root 7 Apr 2 23:19 loop0p14 -> ../dm-1 lrwxrwxrwx 1 root root 7 Apr 2 23:19 loop0p15 -> ../dm-2 # mkdir -p /mnt/loop0p1 # mkdir -p /mnt/loop0p15 # mount -t auto /dev/mapper/loop0p1 /mnt/loop0p1 # mount -t auto /dev/mapper/loop0p15 /mnt/loop0p15 # mount |grep loop /dev/loop0p1 on /mnt/loop0p1 type ext4 (rw,relatime) /dev/loop0p15 on /mnt/loop0p15 type vfat (rw,relatime,fmask=0002,dmask=0002,allow_utime=0020,codepage=437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro) ...hack...hack...hack # umount /dev/mapper/loop0p1 # umount /dev/mapper/loop0p15 # kpartx -d disk.img 9.7.4. Cleaning a disk image file     A disk image file, "disk.img" can be cleaned of all removed files into clean sparse image "new.img" by the following. # mkdir old; mkdir new # mount -t auto -o loop disk.img old # dd bs=1 count=0 if=/dev/zero of=new.img seek=5G # mount -t auto -o loop new.img new     # cd old # cp -a --sparse=always ./ ../new/ # cd .. # umount new.img # umount disk.img     If "disk.img" is in ext2, ext3 or ext4, you can also use zerofree (8) from the zerofree package as follows. # losetup --show -f disk.img /dev/loop0     # zerofree /dev/loop0 # cp --sparse=always disk.img new.img # losetup -d /dev/loop0 9.7.5. Making the empty disk image file     The empty disk image "disk.img" which can grow up to 5GiB can be made using dd(1) as follows.     $ dd bs=1 count=0 if=/dev/zero of=disk.img seek=5G     Instead of using dd(1), specialized fallocate(8) may be used here.     You can create an ext4 filesystem on this disk image "disk.img" using the loop device as follows. # losetup --show -f disk.img /dev/loop0 # mkfs.ext4 /dev/loop0 ...hack...hack...hack     # losetup -d /dev/loop0 $ du --apparent-size -h disk.img 5.0G disk.img $ du -h disk.img 83M disk.img For "disk.img", its file size is 5.0 GiB and its actual disk     usage is mere 83MiB. This discrepancy is possible since ext4 can hold sparse file. Tip     The actual disk usage of sparse file grows with data which are written to it. Using similar operation on devices created by the loop device or the device mapper devices as Section 9.7.3, “Mounting the disk     image file”, you can partition this disk image "disk.img" using parted(8) or fdisk(8), and can create filesystem on it using mkfs.ext4(8), mkswap(8), etc. 9.7.6. Making the ISO9660 image file The ISO9660 image file, "cd.iso", from the source directory tree     at "source_directory" can be made using genisoimage(1) provided by cdrkit by the following.     # genisoimage -r -J -T -V volume_id -o cd.iso source_directory Similarly, the bootable ISO9660 image file, "cdboot.iso", can be     made from debian-installer like directory tree at "source_directory" by the following. # genisoimage -r -o cdboot.iso -V volume_id \     -b isolinux/isolinux.bin -c isolinux/boot.cat \ -no-emul-boot -boot-load-size 4 -boot-info-table source_directory     Here Isolinux boot loader (see Section 3.1.2, “Stage 2: the boot loader”) is used for booting.     You can calculate the md5sum value and make the ISO9660 image directly from the CD-ROM device as follows. $ isoinfo -d -i /dev/cdrom CD-ROM is in ISO 9660 format ...     Logical block size is: 2048 Volume size is: 23150592 ... # dd if=/dev/cdrom bs=2048 count=23150592 conv=notrunc,noerror | md5sum # dd if=/dev/cdrom bs=2048 count=23150592 conv=notrunc,noerror > cd.iso Warning     You must carefully avoid ISO9660 filesystem read ahead bug of Linux as above to get the right result. 9.7.7. Writing directly to the CD/DVD-R/RW Tip     DVD is only a large CD to wodim(1) provided by cdrkit.     You can find a usable device by the following.     # wodim --devices Then the blank CD-R is inserted to the CD drive, and the ISO9660     image file, "cd.iso" is written to this device, e.g., "/dev/hda", using wodim(1) by the following.     # wodim -v -eject dev=/dev/hda cd.iso     If CD-RW is used instead of CD-R, do this instead by the following.     # wodim -v -eject blank=fast dev=/dev/hda cd.iso Tip     If your desktop system mounts CDs automatically, unmount it by "sudo umount /dev/hda" from console before using wodim(1). 9.7.8. Mounting the ISO9660 image file     If "cd.iso" contains an ISO9660 image, then the following manually mounts it to "/cdrom".     # mount -t iso9660 -o ro,loop cd.iso /cdrom Tip     Modern desktop system may mount removable media such as ISO9660 formatted CD automatically (see Section 10.1.7, “Removable storage device”). 9.8. The binary data     Here, we discuss direct manipulations of the binary data on storage media. 9.8.1. Viewing and editing binary data     The most basic viewing method of binary data is to use "od -t x1" command. Table 9.21. List of packages which view and edit binary data +---------------------------------------------------------------+ | package |popcon|size | description | |---------------+------+-----+----------------------------------| |coreutils |V:880,|18307|basic package which has od(1) to | | |I:999 | |dump files (HEX, ASCII, OCTAL, …) | |---------------+------+-----+----------------------------------| |bsdmainutils |V:11, |17 |utility package which has hd(1) to| | |I:315 | |dump files (HEX, ASCII, OCTAL, …) | |---------------+------+-----+----------------------------------| |hexedit |V:0, |73 |binary editor and viewer (HEX, | | |I:9 | |ASCII) |     |---------------+------+-----+----------------------------------| |bless |V:0, |924 |full featured hexadecimal editor | | |I:2 | |(GNOME) | |---------------+------+-----+----------------------------------| |okteta |V:1, |1585 |full featured hexadecimal editor | | |I:12 | |(KDE4) | |---------------+------+-----+----------------------------------| |ncurses-hexedit|V:0, |130 |binary editor and viewer (HEX, | | |I:1 | |ASCII, EBCDIC) | |---------------+------+-----+----------------------------------| |beav |V:0, |137 |binary editor and viewer (HEX, | | |I:0 | |ASCII, EBCDIC, OCTAL, …) | +---------------------------------------------------------------+ Tip HEX is used as an acronym for hexadecimal format with radix 16.     OCTAL is for octal format with radix 8. ASCII is for American Standard Code for Information Interchange, i.e., normal English text code. EBCDIC is for Extended Binary Coded Decimal Interchange Code used on IBM mainframe operating systems. 9.8.2. Manipulating files without mounting disk     There are tools to read and write files without mounting disk. Table 9.22. List of packages to manipulate files without mounting disk +---------------------------------------------------------------+ |package |popcon |size| description | |--------+-------+----+-----------------------------------------|     |mtools |V:8, |390 |utilities for MSDOS files without | | |I:63 | |mounting them | |--------+-------+----+-----------------------------------------| |hfsutils|V:0, |184 |utilities for HFS and HFS+ files without | | |I:5 | |mounting them | +---------------------------------------------------------------+ 9.8.3. Data redundancy Software RAID systems offered by the Linux kernel provide data     redundancy in the kernel filesystem level to achieve high levels of storage reliability.     There are tools to add data redundancy to files in application program level to achieve high levels of storage reliability, too. Table 9.23. List of tools to add data redundancy to files +---------------------------------------------------------------+ | package |popcon|size| description | |----------+------+----+----------------------------------------| |par2 |V:9, |268 |Parity Archive Volume Set, for checking | | |I:94 | |and repair of files |     |----------+------+----+----------------------------------------| |dvdisaster|V:0, |1422|data loss/scratch/aging protection for | | |I:1 | |CD/DVD media | |----------+------+----+----------------------------------------| |dvbackup |V:0, |413 |backup tool using MiniDV camcorders | | |I:0 | |(providing rsbep(1)) | +---------------------------------------------------------------+ 9.8.4. Data file recovery and forensic analysis     There are tools for data file recovery and forensic analysis. Table 9.24. List of packages for data file recovery and forensic analysis +---------------------------------------------------------------+ | package |popcon|size| description | |-------------+------+----+-------------------------------------| |testdisk |V:2, |1413|utilities for partition scan and disk| | |I:28 | |recovery | |-------------+------+----+-------------------------------------| |magicrescue |V:0, |255 |utility to recover files by looking | | |I:2 | |for magic bytes | |-------------+------+----+-------------------------------------| |scalpel |V:0, |89 |frugal, high performance file carver | | |I:3 | | | |-------------+------+----+-------------------------------------| |myrescue |V:0, |83 |rescue data from damaged harddisks | | |I:2 | | | |-------------+------+----+-------------------------------------| |extundelete |V:0, |147 |utility to undelete files on the ext3| | |I:8 | |/4 filesystem | |-------------+------+----+-------------------------------------| |ext4magic |V:0, |233 |utility to undelete files on the ext3| | |I:4 | |/4 filesystem | |-------------+------+----+-------------------------------------|     |ext3grep |V:0, |293 |tool to help recover deleted files on| | |I:2 | |the ext3 filesystem | |-------------+------+----+-------------------------------------| |scrounge-ntfs|V:0, |50 |data recovery program for NTFS | | |I:2 | |filesystems | |-------------+------+----+-------------------------------------| |gzrt |V:0, |33 |gzip recovery toolkit | | |I:0 | | | |-------------+------+----+-------------------------------------| |sleuthkit |V:3, |1671|tools for forensics analysis. | | |I:24 | |(Sleuthkit) | |-------------+------+----+-------------------------------------| |autopsy |V:0, |1026|graphical interface to SleuthKit | | |I:1 | | | |-------------+------+----+-------------------------------------| |foremost |V:0, |102 |forensics application to recover data| | |I:5 | | | |-------------+------+----+-------------------------------------| |guymager |V:0, |1021|forensic imaging tool based on Qt | | |I:0 | | | |-------------+------+----+-------------------------------------| |dcfldd |V:0, |114 |enhanced version of dd for forensics | | |I:3 | |and security | +---------------------------------------------------------------+ Tip     You can undelete files on the ext2 filesystem using list_deleted_inodes and undel commands of debugfs(8) in the e2fsprogs package. 9.8.5. Splitting a large file into small files When a data is too big to backup as a single file, you can backup     its content after splitting it into, e.g. 2000MiB chunks and merge those chunks back into the original file later.     $ split -b 2000m large_file $ cat x* >large_file Caution     Please make sure you do not have any files starting with "x" to avoid name crashes. 9.8.6. Clearing file contents In order to clear the contents of a file such as a log file, do not use rm(1) to delete the file and then create a new empty     file, because the file may still be accessed in the interval between commands. The following is the safe way to clear the contents of the file.     $ :>file_to_be_cleared 9.8.7. Dummy files     The following commands create dummy or empty files. $ dd if=/dev/zero of=5kb.file bs=1k count=5     $ dd if=/dev/urandom of=7mb.file bs=1M count=7 $ touch zero.file $ : > alwayszero.file     You should find following files. * "5kb.file" is 5KB of zeros. * "7mb.file" is 7MB of random data.     * "zero.file" may be a 0 byte file. If it existed, its mtime is updated while its content and its length are kept. * "alwayszero.file" is always a 0 byte file. If it existed, its mtime is updated and its content is reset. 9.8.8. Erasing an entire hard disk     There are several ways to completely erase data from an entire hard disk like device, e.g., USB memory stick at "/dev/sda". Caution     Check your USB memory stick location with mount(8) first before executing commands here. The device pointed by "/dev/sda" may be SCSI hard disk or serial-ATA hard disk where your entire system resides.     Erase all the disk content by resetting data to 0 with the following.     # dd if=/dev/zero of=/dev/sda     Erase everything by overwriting with random data as follows.     # dd if=/dev/urandom of=/dev/sda     Erase everything by overwriting with random data very efficiently as follows.     # shred -v -n 1 /dev/sda     You may alternatively use badblocks(8) with -t random option. Since dd(1) is available from the shell of many bootable Linux     CDs such as Debian installer CD, you can erase your installed system completely by running an erase command from such media on the system hard disk, e.g., "/dev/hda", "/dev/sda", etc. 9.8.9. Erasing unused area of an hard disk Unused area on an hard disk (or USB memory stick), e.g. "/dev/     sdb1" may still contain erased data themselves since they are only unlinked from the filesystem. These can be cleaned by overwriting them. # mount -t auto /dev/sdb1 /mnt/foo # cd /mnt/foo # dd if=/dev/zero of=junk     dd: writing to `junk': No space left on device ... # sync # umount /dev/sdb1 Warning     This is usually good enough for your USB memory stick. But this is not perfect. Most parts of erased filenames and their attributes may be hidden and remain in the filesystem. 9.8.10. Undeleting deleted but still open files Even if you have accidentally deleted a file, as long as that     file is still being used by some application (read or write mode), it is possible to recover such a file.     For example, try the following $ echo foo > bar $ less bar $ ps aux | grep ' less[ ]' bozo 4775 0.0 0.0 92200 884 pts/8 S+ 00:18 0:00 less bar $ rm bar     $ ls -l /proc/4775/fd | grep bar lr-x------ 1 bozo bozo 64 2008-05-09 00:19 4 -> /home/bozo/bar (deleted) $ cat /proc/4775/fd/4 >bar $ ls -l -rw-r--r-- 1 bozo bozo 4 2008-05-09 00:25 bar $ cat bar foo     Execute on another terminal (when you have the lsof package installed) as follows. $ ls -li bar 2228329 -rw-r--r-- 1 bozo bozo 4 2008-05-11 11:02 bar $ lsof |grep bar|grep less less 4775 bozo 4r REG 8,3 4 2228329 /home/bozo/bar $ rm bar     $ lsof |grep bar|grep less less 4775 bozo 4r REG 8,3 4 2228329 /home/bozo/bar (deleted) $ cat /proc/4775/fd/4 >bar $ ls -li bar 2228302 -rw-r--r-- 1 bozo bozo 4 2008-05-11 11:05 bar $ cat bar foo 9.8.11. Searching all hardlinks     Files with hardlinks can be identified by "ls -li". $ ls -li total 0     2738405 -rw-r--r-- 1 root root 0 2008-09-15 20:21 bar 2738404 -rw-r--r-- 2 root root 0 2008-09-15 20:21 baz 2738404 -rw-r--r-- 2 root root 0 2008-09-15 20:21 foo Both "baz" and "foo" have link counts of "2" (>1) showing them to have hardlinks. Their inode numbers are common "2738404". This     means they are the same hardlinked file. If you do not happen to find all hardlinked files by chance, you can search it by the inode, e.g., "2738404" as the following.     # find /path/to/mount/point -xdev -inum 2738404 9.8.12. Invisible disk space consumption All deleted but open files consume disk space although they are     not visible from normal du(1). They can be listed with their size by the following.     # lsof -s -X / |grep deleted 9.9. Data encryption tips With physical access to your PC, anyone can easily gain root privilege and access all the files on your PC (see Section 4.6.4, “Securing the root password”). This means that login password     system can not secure your private and sensitive data against possible theft of your PC. You must deploy data encryption technology to do it. Although GNU privacy guard (see Section 10.3, “Data security infrastructure”) can encrypt files, it takes some user efforts.     Dm-crypt facilitates automatic data encryption via native Linux kernel modules with minimal user efforts using device-mapper. Table 9.25. List of data encryption utilities +---------------------------------------------------------------+ | package |popcon|size| description | |--------------+------+----+------------------------------------| |cryptsetup |V:19, |417 |utilities for encrypted block device| | |I:79 | |(dm-crypt / LUKS) | |--------------+------+----+------------------------------------| | |V:2, | |utilities for encrypted block device|     |cryptmount |I:3 |231 |(dm-crypt / LUKS) with focus on | | | | |mount/unmount by normal users | |--------------+------+----+------------------------------------| |fscrypt |V:0, |5520|utilities for Linux filesystem | | |I:1 | |encryption (fscrypt) | |--------------+------+----+------------------------------------| |libpam-fscrypt|V:0, |5519|PAM module for Linux filesystem | | |I:0 | |encryption (fscrypt) | +---------------------------------------------------------------+ Caution     Data encryption costs CPU time etc. Encrypted data becomes inaccessible if its password is lost. Please weigh its benefits and costs. Note     Entire Debian system can be installed on a encrypted disk by the debian-installer (lenny or newer) using dm-crypt/LUKS and initramfs. Tip     See Section 10.3, “Data security infrastructure” for user space encryption utility: GNU Privacy Guard. 9.9.1. Removable disk encryption with dm-crypt/LUKS You can encrypt contents of removable mass devices, e.g. USB     memory stick on "/dev/sdx", using dm-crypt/LUKS. You simply format it as the following. # fdisk /dev/sdx ... "n" "p" "1" "return" "return" "w" # cryptsetup luksFormat /dev/sdx1 ... # cryptsetup open /dev/sdx1 secret ...     # ls -l /dev/mapper/ total 0 crw-rw---- 1 root root 10, 60 2021-10-04 18:44 control lrwxrwxrwx 1 root root 7 2021-10-04 23:55 secret -> ../dm-0 # mkfs.vfat /dev/mapper/secret ... # cryptsetup close secret Then, it can be mounted just like normal one on to "/media/ username/disk_label", except for asking password (see     Section 10.1.7, “Removable storage device”) under modern desktop environment using the udisks2 package. The difference is that every data written to it is encrypted. The password entry may be automated using keyring (see Section 10.3.6, “Password keyring”). You may alternatively format media in different filesystem, e.g., ext4 with "mkfs.ext4 /dev/mapper/sdx1". If btrfs is used instead,     the udisks2-btrfs package needs to be installed. For these filesystems, the file ownership and permissions may need to be configured. 9.9.2. Mounting encrypted disk with dm-crypt/LUKS For example, an encrypted disk partition created with dm-crypt/     LUKS on "/dev/sdc5" by Debian Installer can be mounted onto "/ mnt" as follows: $ sudo cryptsetup open /dev/sdc5 ninja --type luks Enter passphrase for /dev/sdc5: **** $ sudo lvm lvm> lvscan inactive '/dev/ninja-vg/root' [13.52 GiB] inherit     inactive '/dev/ninja-vg/swap_1' [640.00 MiB] inherit ACTIVE '/dev/goofy/root' [180.00 GiB] inherit ACTIVE '/dev/goofy/swap' [9.70 GiB] inherit lvm> lvchange -a y /dev/ninja-vg/root lvm> exit Exiting. $ sudo mount /dev/ninja-vg/root /mnt 9.10. The kernel     Debian distributes modularized Linux kernel as packages for supported architectures.     If you are reading this documentation, you probably don't need to compile Linux kernel by yourself. 9.10.1. Kernel parameters     Many Linux features are configurable via kernel parameters as follows. * Kernel parameters initialized by the bootloader (see Section 3.1.2, “Stage 2: the boot loader”) * Kernel parameters changed by sysctl(8) at runtime for ones     accessible via sysfs (see Section 1.2.12, “procfs and sysfs”) * Module parameters set by arguments of modprobe(8) when a module is activated (see Section 9.7.3, “Mounting the disk image file”)     See "The Linux kernel user’s and administrator’s guide » The kernel’s command-line parameters" for the detail. 9.10.2. Kernel headers Most normal programs don't need kernel headers and in fact may break if you use them directly for compiling. They should be     compiled against the headers in "/usr/include/linux" and "/usr/ include/asm" provided by the libc6-dev package (created from the glibc source package) on the Debian system. Note For compiling some kernel-specific programs such as the kernel     modules from the external source and the automounter daemon (amd), you must include path to the corresponding kernel headers, e.g. "-I/usr/src/linux-particular-version/include/", to your command line. 9.10.3. Compiling the kernel and related modules     Debian has its own method of compiling the kernel and related modules. Table 9.26. List of key packages to be installed for the kernel recompilation on the Debian system +---------------------------------------------------------------+ | package |popcon|size | description | |----------------+------+-----+---------------------------------| |build-essential |I:480 |17 |essential packages for building | | | | |Debian packages: make, gcc, … | |----------------+------+-----+---------------------------------| |bzip2 |V:166,|112 |compress and decompress utilities| | |I:970 | |for bz2 files | |----------------+------+-----+---------------------------------| |libncurses5-dev |I:71 |6 |developer's libraries and docs | | | | |for ncurses | |----------------+------+-----+---------------------------------| |git |V:351,|46734|git: distributed revision control| | |I:549 | |system used by the Linux kernel |     |----------------+------+-----+---------------------------------| |fakeroot |V:29, |224 |provide fakeroot environment for | | |I:486 | |building package as non-root | |----------------+------+-----+---------------------------------| |initramfs-tools |V:430,|113 |tool to build an initramfs | | |I:989 | |(Debian specific) | |----------------+------+-----+---------------------------------| |dkms |V:74, |196 |dynamic kernel module support | | |I:162 | |(DKMS) (generic) | |----------------+------+-----+---------------------------------| |module-assistant|V:0, |406 |helper tool to make module | | |I:19 | |package (Debian specific) | |----------------+------+-----+---------------------------------| | |V:6, | |helper scripts for a Debian | |devscripts |I:40 |2658 |Package maintainer (Debian | | | | |specific) | +---------------------------------------------------------------+ If you use initrd in Section 3.1.2, “Stage 2: the boot loader”,     make sure to read the related information in initramfs-tools(8), update-initramfs(8), mkinitramfs(8) and initramfs.conf(5). Warning     Do not put symlinks to the directories in the source tree (e.g. " /usr/src/linux*") from "/usr/include/linux" and "/usr/include/ asm" when compiling the Linux kernel source. (Some outdated documents suggest this.) Note When compiling the latest Linux kernel on the Debian stable system, the use of backported latest tools from the Debian unstable may be needed. module-assistant(8) (or its short form m-a) helps users to build     and install module package(s) easily for one or more custom kernels. The dynamic kernel module support (DKMS) is a new distribution independent framework designed to allow individual kernel modules to be upgraded without changing the whole kernel. This is used for the maintenance of out-of-tree modules. This also makes it very easy to rebuild modules as you upgrade kernels. 9.10.4. Compiling the kernel source: Debian Kernel Team recommendation For building custom kernel binary packages from the upstream     kernel source, you should use the "deb-pkg" target provided by it. $ sudo apt-get build-dep linux $ cd /usr/src $ wget https://mirrors.edge.kernel.org/pub/linux/kernel/v6.x/linux-version.tar.xz $ tar --xz -xvf linux-version.tar.xz     $ cd linux-version $ cp /boot/config-version .config $ make menuconfig ... $ make deb-pkg Tip     The linux-source-version package provides the Linux kernel source with Debian patches as "/usr/src/linux-version.tar.bz2". For building specific binary packages from the Debian kernel     source package, you should use the "binary-arch_architecture_ featureset_flavour" targets in "debian/rules.gen". $ sudo apt-get build-dep linux     $ apt-get source linux $ cd linux-3.* $ fakeroot make -f debian/rules.gen binary-arch_i386_none_686     See further information: * Debian Wiki: KernelFAQ     * Debian Wiki: DebianKernel * Debian Linux Kernel Handbook: https:// kernel-handbook.debian.net 9.10.5. Hardware drivers and firmware The hardware driver is the code running on the main CPUs of the     target system. Most hardware drivers are available as free software now and are included in the normal Debian kernel packages in the main area. * GPU driver + Intel GPU driver (main)     + AMD/ATI GPU driver (main) + NVIDIA GPU driver (main for nouveau driver, and non-free for binary-only drivers supported by the vendor.) The firmware is the code or data loaded on the device attach to the target system (e.g., CPU microcode, rendering code running on GPU, or FPGA / CPLD data, …). Some firmware packages are     available as free software but many firmware packages are not available as free software since they contain sourceless binary data. Installing these firmware data is essential for the device to function as expected. * The firmware data packages containing data loaded to the volatile memory on the target device. + firmware-linux-free (main) + firmware-linux-nonfree (non-free-firmware) + firmware-linux-* (non-free-firmware) + *-firmware (non-free-firmware) + intel-microcode (non-free-firmware)     + amd64-microcode (non-free-firmware) * The firmware update program packages which update data on the non-volatile memory on the target device. + fwupd (main): Firmware update daemon which downloads firmware data from Linux Vendor Firmware Service. + gnome-firmware (main): GTK front end for fwupd + plasma-discover-backend-fwupd (main): Qt front end for fwupd Please note that access to non-free-firmware packages are provided by the official installation media to offer functional     installation experience to the user since Debian 12 Bookworm. The non-free-firmware area is described in Section 2.1.5, “Debian archive basics”. Please also note that the firmware data downloaded by fwupd from     Linux Vendor Firmware Service and loaded to the running Linux kernel may be non-free. 9.11. Virtualized system     Use of virtualized system enables us to run multiple instances of system simultaneously on a single hardware. Tip     See Debian wiki on SystemVirtualization. 9.11.1. Virtualization and emulation tools     There are several virtualization and emulation tool platforms. * Complete hardware emulation packages such as ones installed by the games-emulator metapackage * Mostly CPU level emulation with some I/O device emulations such as QEMU * Mostly CPU level virtualization with some I/O device emulations such as Kernel-based Virtual Machine (KVM) * OS level container virtualization with the kernel level support such as LXC (Linux Containers), Docker,     systemd-nspawn(1), ... * OS level filesystem access virtualization with the system library call override on the file path such as chroot * OS level filesystem access virtualization with the system library call override on the file ownership such as fakeroot * OS API emulation such as Wine * Interpreter level virtualization with its executable selection and run-time library overrides such as virtualenv and venv for Python The container virtualization uses Section 4.7.5, “Linux security     features” and is the backend technology of Section 7.7, “Sandbox” .     Here are some packages to help you to setup the virtualized system. Table 9.27. List of virtualization tools +---------------------------------------------------------------+ | package |popcon| size | description | |------------------+------+------+------------------------------| |coreutils |V:880,|18307 |GNU core utilities which | | |I:999 | |contain chroot(8) | |------------------+------+------+------------------------------| | |V:53, | |systemd container/nspawn tools| |systemd-container |I:61 |1330 |which contain systemd-nspawn | | | | |(1) | |------------------+------+------+------------------------------| | |V:5, | |specialized tool for executing| |schroot |I:7 |2579 |Debian binary packages in | | | | |chroot | |------------------+------+------+------------------------------| | |V:1, | |tool for building Debian | |sbuild |I:3 |243 |binary packages from Debian | | | | |sources | |------------------+------+------+------------------------------| |debootstrap |V:5, |314 |bootstrap a basic Debian | | |I:54 | |system (written in sh) | |------------------+------+------+------------------------------| |cdebootstrap |V:0, |115 |bootstrap a Debian system | | |I:1 | |(written in C) | |------------------+------+------+------------------------------| |cloud-image-utils |V:1, |66 |cloud image management | | |I:17 | |utilities | |------------------+------+------+------------------------------| |cloud-guest-utils |V:3, |71 |cloud guest utilities | | |I:13 | | | |------------------+------+------+------------------------------| | |V:11, | |Virtual Machine Manager: | |virt-manager |I:44 |2296 |desktop application for | | | | |managing virtual machines | |------------------+------+------+------------------------------| |libvirt-clients |V:46, |1241 |programs for the libvirt | | |I:65 | |library | |------------------+------+------+------------------------------| | |V:0, | |Incus: system container and | |incus |I:0 |56209 |virtual machine manager (for | | | | |Debian 13 "Trixie") | |------------------+------+------+------------------------------| | |V:0, | |LXD: system container and | |lxd |I:0 |52119 |virtual machine manager (for | | | | |Debian 12 "Bookworm") | |------------------+------+------+------------------------------| |podman |V:14, |41948 |podman: engine to run | | |I:16 | |OCI-based containers in Pods | |------------------+------+------+------------------------------| | |V:0, | |engine to run OCI-based | |podman-docker |I:0 |249 |containers in Pods - wrapper | | | | |for docker | |------------------+------+------+------------------------------| |docker.io |V:41, |150003|docker: Linux container | | |I:43 | |runtime |     |------------------+------+------+------------------------------| |games-emulator |I:0 |21 |games-emulator: Debian's | | | | |emulators for games | |------------------+------+------+------------------------------| |bochs |V:0, |6956 |Bochs: IA-32 PC emulator | | |I:0 | | | |------------------+------+------+------------------------------| |qemu |I:14 |97 |QEMU: fast generic processor | | | | |emulator | |------------------+------+------+------------------------------| |qemu-system |I:22 |66 |QEMU: full system emulation | | | | |binaries | |------------------+------+------+------------------------------| |qemu-user |V:1, |93760 |QEMU: user mode emulation | | |I:6 | |binaries | |------------------+------+------+------------------------------| |qemu-utils |V:12, |10635 |QEMU: utilities | | |I:106 | | | |------------------+------+------+------------------------------| | | | |KVM: full virtualization on | |qemu-system-x86 |V:33, |58140 |x86 hardware with the | | |I:91 | |hardware-assisted | | | | |virtualization | |------------------+------+------+------------------------------| |virtualbox |V:6, |130868|VirtualBox: x86 virtualization| | |I:8 | |solution on i386 and amd64 | |------------------+------+------+------------------------------| |gnome-boxes |V:1, |6691 |Boxes: Simple GNOME app to | | |I:7 | |access virtual systems | |------------------+------+------+------------------------------| |xen-tools |V:0, |719 |tools to manage debian XEN | | |I:2 | |virtual server | |------------------+------+------+------------------------------| | |V:13, | |Wine: Windows API | |wine |I:60 |132 |Implementation (standard | | | | |suite) | |------------------+------+------+------------------------------| | |V:1, | |DOSBox: x86 emulator with | |dosbox |I:15 |2696 |Tandy/Herc/CGA/EGA/VGA/SVGA | | | | |graphics, sound and DOS | |------------------+------+------+------------------------------| |lxc |V:9, |25890 |Linux containers user space | | |I:12 | |tools | |------------------+------+------+------------------------------| | | | |venv for creating virtual | |python3-venv |I:88 |6 |python environments (system | | | | |library) | |------------------+------+------+------------------------------| | |V:9, | |virtualenv for creating | |python3-virtualenv|I:50 |356 |isolated virtual python | | | | |environments | |------------------+------+------+------------------------------| | |V:3, | |pipx for installing python | |pipx |I:19 |3324 |applications in isolated | | | | |environments | +---------------------------------------------------------------+     See Wikipedia article Comparison of platform virtual machines for detail comparison of different platform virtualization solutions. 9.11.2. Virtualization work flow Note     Default Debian kernels support KVM since lenny.     Typical work flow for virtualization involves several steps. * Create an empty filesystem (a file tree or a disk image). + The file tree can be created by "mkdir -p /path/to/ chroot". + The raw disk image file can be created with dd(1) (see Section 9.7.1, “Making the disk image file” and Section 9.7.5, “Making the empty disk image file”). + qemu-img(1) can be used to create and convert disk image files supported by QEMU. + The raw and VMDK file format can be used as common format among virtualization tools. * Mount the disk image with mount(8) to the filesystem (optional). + For the raw disk image file, mount it as loop device or device mapper devices (see Section 9.7.3, “Mounting the disk image file”). + For disk images supported by QEMU, mount them as network     block device (see Section 9.11.3, “Mounting the virtual disk image file”). * Populate the target filesystem with required system data. + The use of programs such as debootstrap and cdebootstrap helps with this process (see Section 9.11.4, “Chroot system”). + Use installers of OSs under the full system emulation. * Run a program under a virtualized environment. + chroot provides basic virtualized environment enough to compile programs, run console applications, and run daemons in it. + QEMU provides cross-platform CPU emulation. + QEMU with KVM provides full system emulation by the hardware-assisted virtualization. + VirtualBox provides full system emulation on i386 and amd64 with or without the hardware-assisted virtualization. 9.11.3. Mounting the virtual disk image file     For the raw disk image file, see Section 9.7, “The disk image”. For other virtual disk image files, you can use qemu-nbd(8) to     export them using network block device protocol and mount them using the nbd kernel module. qemu-nbd(8) supports disk formats supported by QEMU: QEMU     supports following disk formats: raw, qcow2, qcow, vmdk, vdi, bochs, cow (user-mode Linux copy-on-write), parallels, dmg, cloop, vpc, vvfat (virtual VFAT), and host_device. The network block device can support partitions in the same way     as the loop device (see Section 9.7.3, “Mounting the disk image file”). You can mount the first partition of "disk.img" as follows. # modprobe nbd max_part=16 # qemu-nbd -v -c /dev/nbd0 disk.img     ... # mkdir /mnt/part1 # mount /dev/nbd0p1 /mnt/part1 Tip     You may export only the first partition of "disk.img" using "-P 1" option to qemu-nbd(8). 9.11.4. Chroot system If you wish to try a new Debian environment from a terminal console, I recommend you to use chroot. This enables you to run     console applications of Debian unstable and testing without usual risks associated and without rebooting. chroot(8) is the most basic way. Caution     Examples below assumes both parent system and chroot system share the same amd64 CPU architecture.     Although you can manually create a chroot(8) environment using debootstrap(1), this requires non-trivial efforts. The sbuild package to build Debian packages from source uses the     chroot environment managed by the schroot package. It comes with helper script sbuild-createchroot(1). Let's learn how it works by running it as follows. $ sudo mkdir -p /srv/chroot     $ sudo sbuild-createchroot -v --include=eatmydata,ccache unstable /srv/chroot/unstable-amd64-sbuild http://deb.debian.org/debian ... You see how debootstrap(8) populates system data for unstable     environment under "/srv/chroot/unstable-amd64-sbuild" for a minimal build system.     You can login to this environment using schroot(1).     $ sudo schroot -v -c chroot:unstable-amd64-sbuild     You see how a system shell running under unstable environment is created. Note     The "/usr/sbin/policy-rc.d" file which always exits with 101 prevents daemon programs to be started automatically on the Debian system. See "/usr/share/doc/init-system-helpers/ README.policy-rc.d.gz". Note Some programs under chroot may require access to more files from     the parent system to function than sbuild-createchroot provides as above. For example, "/sys", "/etc/passwd", "/etc/group", "/var /run/utmp", "/var/log/wtmp", etc. may need to be bind-mounted or copied. Tip The sbuild package helps to construct a chroot system and builds     a package inside the chroot using schroot as its backend. It is an ideal system to check build-dependencies. See more on sbuild at Debian wiki and sbuild configuration example in "Guide for Debian Maintainers". Tip The systemd-nspawn(1) command helps to run a command or OS in a     light-weight container in similar ways to chroot. It is more powerful since it uses namespaces to fully virtualize the the process tree, IPC, hostname, domain name and, optionally, networking and user databases. See systemd-nspawn. 9.11.5. Multiple desktop systems If you wish to try a new GUI Desktop environment of any OS, I recommend you to use QEMU or KVM on a Debian stable system to run     multiple desktop systems safely using virtualization. These enable you to run any desktop applications including ones of Debian unstable and testing without usual risks associated with them and without rebooting.     Since pure QEMU is very slow, it is recommended to accelerate it with KVM when the host system supports it. Virtual Machine Manager also known as virt-manager is a     convenient GUI tool for managing KVM virtual machines via libvirt. The virtual disk image "virtdisk.qcow2" containing a Debian     system for QEMU can be created using debian-installer: Small CDs as follows. $ wget https://cdimage.debian.org/debian-cd/5.0.3/amd64/iso-cd/debian-503-amd64-netinst.iso     $ qemu-img create -f qcow2 virtdisk.qcow2 5G $ qemu -hda virtdisk.qcow2 -cdrom debian-503-amd64-netinst.iso -boot d -m 256 ... Tip     Running other GNU/Linux distributions such as Ubuntu and Fedora under virtualization is a great way to learn configuration tips. Other proprietary OSs may be run nicely under this GNU/Linux virtualization, too.     See more tips at Debian wiki: SystemVirtualization. ---------------------------------------------------------------------     ^[2] More elaborate customization examples: "Vim Galore", "sensible.vim", "#vim Recommendations" ...     ^[3] vim-pathogen was popular. Chapter 10. Data management     Tools and tips for managing binary and text data on the Debian system are described. 10.1. Sharing, copying, and archiving Warning     The uncoordinated write access to actively accessed devices and files from multiple processes must not be done to avoid the race condition. File locking mechanisms using flock(1) may be used to avoid it.     The security of the data and its controlled sharing have several aspects. * The creation of data archive * The remote storage access * The duplication     * The tracking of the modification history * The facilitation of data sharing * The prevention of unauthorized file access * The detection of unauthorized file modification     These can be realized by using some combination of tools. * Archive and compression tools * Copy and synchronization tools * Network filesystems * Removable storage media     * The secure shell * The authentication system * Version control system tools * Hash and cryptographic encryption tools 10.1.1. Archive and compression tools     Here is a summary of archive and compression tools available on the Debian system. Table 10.1. List of archive and compression tools +---------------------------------------------------------------+ | package |popcon|size|extension|command| comment | |----------+------+----+---------+-------+----------------------| |tar |V:902,|3077|.tar |tar(1) |the standard archiver | | |I:999 | | | |(de facto standard) | |----------+------+----+---------+-------+----------------------| | |V:440,| | | |Unix System V style | |cpio |I:998 |1199|.cpio |cpio(1)|archiver, use with | | | | | | |find(1) | |----------+------+----+---------+-------+----------------------| | |V:172,| | | |archiver for the | |binutils |I:629 |144 |.ar |ar(1) |creation of static | | | | | | |libraries | |----------+------+----+---------+-------+----------------------| |fastjar |V:1, |183 |.jar |fastjar|archiver for Java (zip| | |I:13 | | |(1) |like) | |----------+------+----+---------+-------+----------------------| | |V:8, | | | |new POSIX standard | |pax |I:14 |170 |.pax |pax(1) |archiver, compromise | | | | | | |between tar and cpio | |----------+------+----+---------+-------+----------------------| | | | | |gzip |GNU LZ77 compression | |gzip |V:876,|252 |.gz |(1), |utility (de facto | | |I:999 | | |zcat |standard) | | | | | |(1), … | | |----------+------+----+---------+-------+----------------------| | | | | | |Burrows-Wheeler | | | | | | |block-sorting | | | | | |bzip2 |compression utility | |bzip2 |V:166,|112 |.bz2 |(1), |with higher | | |I:970 | | |bzcat |compression ratio than| | | | | |(1), … |gzip(1) (slower than | | | | | | |gzip with similar | | | | | | |syntax) | |----------+------+----+---------+-------+----------------------| | | | | | |LZMA compression | |lzma |V:1, |149 |.lzma |lzma(1)|utility with higher | | |I:16 | | | |compression ratio than| | | | | | |gzip(1) (deprecated) |     |----------+------+----+---------+-------+----------------------| | | | | | |XZ compression utility| | | | | | |with higher | | | | | |xz(1), |compression ratio than| |xz-utils |V:360,|1203|.xz |xzdec |bzip2(1) (slower than | | |I:980 | | |(1), … |gzip but faster than | | | | | | |bzip2; replacement for| | | | | | |LZMA compression | | | | | | |utility) | |----------+------+----+---------+-------+----------------------| | | | | |zstd |Zstandard fast | |zstd |V:193,|2158|.zstd |(1), |lossless compression | | |I:481 | | |zstdcat|utility | | | | | |(1), … | | |----------+------+----+---------+-------+----------------------| | | | | |7zr(1),|7-Zip file archiver | |p7zip |V:20, |8 |.7z |p7zip |with high compression | | |I:463 | | |(1) |ratio (LZMA | | | | | | |compression) | |----------+------+----+---------+-------+----------------------| | | | | | |7-Zip file archiver | | |V:110,| | |7z(1), |with high compression | |p7zip-full|I:480 |12 |.7z |7za(1) |ratio (LZMA | | | | | | |compression and | | | | | | |others) | |----------+------+----+---------+-------+----------------------| | | | | | |LZO compression | | | | | | |utility with higher | | | | | | |compression and | |lzop |V:15, |164 |.lzo |lzop(1)|decompression speed | | |I:142 | | | |than gzip(1) (lower | | | | | | |compression ratio than| | | | | | |gzip with similar | | | | | | |syntax) | |----------+------+----+---------+-------+----------------------| |zip |V:48, |616 |.zip |zip(1) |InfoZIP: DOS archive | | |I:380 | | | |and compression tool | |----------+------+----+---------+-------+----------------------| |unzip |V:105,|379 |.zip |unzip |InfoZIP: DOS unarchive| | |I:771 | | |(1) |and decompression tool| +---------------------------------------------------------------+ Warning     Do not set the "$TAPE" variable unless you know what to expect. It changes tar(1) behavior. * The gzipped tar(1) archive uses the file extension ".tgz" or ".tar.gz". * The xz-compressed tar(1) archive uses the file extension ".txz" or ".tar.xz". * Popular compression method in FOSS tools such as tar(1) has been moving as follows: gzip → bzip2 → xz * cp(1), scp(1) and tar(1) may have some limitation for special files. cpio(1) is most versatile.     * cpio(1) is designed to be used with find(1) and other commands and suitable for creating backup scripts since the file selection part of the script can be tested independently. * Internal structure of Libreoffice data files are ".jar" file which can be opened also by unzip. * The de-facto cross platform archive tool is zip. Use it as "zip -rX" to attain the maximum compatibility. Use also the "-s" option, if the maximum file size matters. 10.1.2. Copy and synchronization tools     Here is a summary of simple copy and backup tools available on the Debian system. Table 10.2. List of copy and synchronization tools +---------------------------------------------------------------+ | package |popcon|size |tool| function | |--------------+------+-----+----+------------------------------| | |V:880,| |GNU |locally copy files and | |coreutils |I:999 |18307|cp |directories ("-a" for | | | | | |recursive) | |--------------+------+-----+----+------------------------------| | |V:866,| | |remotely copy files and | |openssh-client|I:996 |4959 |scp |directories (client, "-r" for |     | | | | |recursive) | |--------------+------+-----+----+------------------------------| |openssh-server|V:730,|1804 |sshd|remotely copy files and | | |I:814 | | |directories (remote server) | |--------------+------+-----+----+------------------------------| |rsync |V:246,|781 | |1-way remote synchronization | | |I:552 | | |and backup | |--------------+------+-----+----+------------------------------| |unison |V:3, |14 | |2-way remote synchronization | | |I:15 | | |and backup | +---------------------------------------------------------------+     Copying files with rsync(8) offers richer features than others. * delta-transfer algorithm that sends only the differences between the source files and the existing files in the destination * quick check algorithm (by default) that looks for files that     have changed in size or in last-modified time * "--exclude" and "--exclude-from" options similar to tar(1) * "a trailing slash on the source directory" syntax that avoids creating an additional directory level at the destination. Tip     Version control system (VCS) tools in Table 10.14, “List of other version control system tools” can function as the multi-way copy and synchronization tools. 10.1.3. Idioms for the archive     Here are several ways to archive and unarchive the entire content of the directory "./source" using different tools.     GNU tar(1):     $ tar -cvJf archive.tar.xz ./source $ tar -xvJf archive.tar.xz     Alternatively, by the following.     $ find ./source -xdev -print0 | tar -cvJf archive.tar.xz --null -T -     cpio(1):     $ find ./source -xdev -print0 | cpio -ov --null > archive.cpio; xz archive.cpio $ zcat archive.cpio.xz | cpio -i 10.1.4. Idioms for the copy     Here are several ways to copy the entire content of the directory "./source" using different tools. * Local copy: "./source" directory → "/dest" directory     * Remote copy: "./source" directory at local host → "/dest" directory at "user@host.dom" host     rsync(8):     # cd ./source; rsync -aHAXSv . /dest # cd ./source; rsync -aHAXSv . user@host.dom:/dest     You can alternatively use "a trailing slash on the source directory" syntax.     # rsync -aHAXSv ./source/ /dest # rsync -aHAXSv ./source/ user@host.dom:/dest     Alternatively, by the following.     # cd ./source; find . -print0 | rsync -aHAXSv0 --files-from=- . /dest # cd ./source; find . -print0 | rsync -aHAXSv0 --files-from=- . user@host.dom:/dest     GNU cp(1) and openSSH scp(1):     # cd ./source; cp -a . /dest # cd ./source; scp -pr . user@host.dom:/dest     GNU tar(1):     # (cd ./source && tar cf - . ) | (cd /dest && tar xvfp - ) # (cd ./source && tar cf - . ) | ssh user@host.dom '(cd /dest && tar xvfp - )'     cpio(1):     # cd ./source; find . -print0 | cpio -pvdm --null --sparse /dest You can substitute "." with "foo" for all examples containing "."     to copy files from "./source/foo" directory to "/dest/foo" directory. You can substitute "." with the absolute path "/path/to/source/     foo" for all examples containing "." to drop "cd ./source;". These copy files to different locations depending on tools used as follows. * "/dest/foo": rsync(8), GNU cp(1), and scp(1)     * "/dest/path/to/source/foo": GNU tar(1), and cpio(1) Tip     rsync(8) and GNU cp(1) have option "-u" to skip files that are newer on the receiver. 10.1.5. Idioms for the selection of files find(1) is used to select files for archive and copy commands (see Section 10.1.3, “Idioms for the archive” and Section 10.1.4,     “Idioms for the copy”) or for xargs(1) (see Section 9.4.9, “Repeating a command looping over files”). This can be enhanced by using its command arguments.     Basic syntax of find(1) can be summarized as the following. * Its conditional arguments are evaluated from left to right. * This evaluation stops once its outcome is determined. * "Logical OR" (specified by "-o" between conditionals) has lower precedence than "logical AND" (specified by "-a" or nothing between conditionals). * "Logical NOT" (specified by "!" before a conditional) has higher precedence than "logical AND". * "-prune" always returns logical TRUE and, if it is a directory, searching of file is stopped beyond this point. * "-name" matches the base of the filename with shell glob (see     Section 1.5.6, “Shell glob”) but it also matches its initial "." with metacharacters such as "*" and "?". (New POSIX feature) * "-regex" matches the full path with emacs style BRE (see Section 1.6.2, “Regular expressions”) as default. * "-size" matches the file based on the file size (value precedented with "+" for larger, precedented with "-" for smaller) * "-newer" matches the file newer than the one specified in its argument. * "-print0" always returns logical TRUE and print the full filename (null terminated) on the standard output.     find(1) is often used with an idiomatic style as the following. # find /path/to \ -xdev -regextype posix-extended \     -type f -regex ".*\.cpio|.*~" -prune -o \ -type d -regex ".*/\.git" -prune -o \ -type f -size +99M -prune -o \ -type f -newer /path/to/timestamp -print0     This means to do following actions. 1. Search all files starting from "/path/to" 2. Globally limit its search within its starting filesystem and uses ERE (see Section 1.6.2, “Regular expressions”) instead 3. Exclude files matching regex of ".*\.cpio" or ".*~" from search by stop processing     4. Exclude directories matching regex of ".*/\.git" from search by stop processing 5. Exclude files larger than 99 Megabytes (units of 1048576 bytes) from search by stop processing 6. Print filenames which satisfy above search conditions and are newer than "/path/to/timestamp"     Please note the idiomatic use of "-prune -o" to exclude files in the above example. Note     For non-Debian Unix-like system, some options may not be supported by find(1). In such a case, please consider to adjust matching methods and replace "-print0" with "-print". You may need to adjust related commands too. 10.1.6. Archive media When choosing computer data storage media for important data archive, you should be careful about their limitations. For small     personal data backup, I use CD-R and DVD-R by the brand name company and store in a cool, shaded, dry, clean environment. (Tape archive media seem to be popular for professional use.) Note     A fire-resistant safe are meant for paper documents. Most of the computer data storage media have less temperature tolerance than paper. I usually rely on multiple secure encrypted copies stored in multiple secure locations.     Optimistic storage life of archive media seen on the net (mostly from vendor info). * 100+ years : Acid free paper with ink * 100 years : Optical storage (CD/DVD, CD/DVD-R)     * 30 years : Magnetic storage (tape, floppy) * 20 years : Phase change optical storage (CD-RW)     These do not count on the mechanical failures due to handling etc.     Optimistic write cycle of archive media seen on the net (mostly from vendor info). * 250,000+ cycles : Harddisk drive * 10,000+ cycles : Flash memory     * 1,000 cycles : CD/DVD-RW * 1 cycles : CD/DVD-R, paper Caution     Figures of storage life and write cycle here should not be used for decisions on any critical data storage. Please consult the specific product information provided by the manufacture. Tip     Since CD/DVD-R and paper have only 1 write cycle, they inherently prevent accidental data loss by overwriting. This is advantage! Tip     If you need fast and frequent backup of large amount of data, a hard disk on a remote host linked by a fast network connection, may be the only realistic option. Tip     If you use re-writable media for your backups, use of filesystem such as btrfs or zfs which supports read-only snapshots may be a good idea. 10.1.7. Removable storage device     Removable storage devices may be any one of the following. * USB flash drive * Hard disk drive     * Optical disc drive * Digital camera * Digital music player     They may be connected via any one of the following. * USB     * IEEE 1394 / FireWire * PC Card Modern desktop environments such as GNOME and KDE can mount these     removable devices automatically without a matching "/etc/fstab" entry. * udisks2 package provides a daemon and associated utilities to mount and unmount these devices.     * D-bus creates events to initiate automatic processes. * PolicyKit provides required privileges. Tip     Automounted devices may have the "uhelper=" mount option which is used by umount(8). Tip     Automounting under modern desktop environment happens only when those removable media devices are not listed in "/etc/fstab".     Mount point under modern desktop environment is chosen as "/media /username/disk_label" which can be customized by the following. * mlabel(1) for FAT filesystem     * genisoimage(1) with "-V" option for ISO9660 filesystem * tune2fs(1) with "-L" option for ext2/ext3/ext4 filesystem Tip     The choice of encoding may need to be provided as mount option (see Section 8.1.3, “Filename encoding”). Tip     The use of the GUI menu to unmount a filesystem may remove its dynamically generated device node such as "/dev/sdc". If you wish to keep its device node, unmount it with the umount(8) command from the shell prompt. 10.1.8. Filesystem choice for sharing data When sharing data with other system via removable storage device,     you should format it with common filesystem supported by both systems. Here is a list of filesystem choices. Table 10.3. List of filesystem choices for removable storage devices with typical usage scenarios +---------------------------------------------------------------+ |filesystem| typical usage scenario | | name | | |----------+----------------------------------------------------| |FAT12 |cross platform sharing of data on the floppy disk | | |(<32MiB) | |----------+----------------------------------------------------| |FAT16 |cross platform sharing of data on the small hard | | |disk like device (<2GiB) | |----------+----------------------------------------------------| | |cross platform sharing of data on the large hard | |FAT32 |disk like device (<8TiB, supported by newer than MS | | |Windows95 OSR2) | |----------+----------------------------------------------------| | |cross platform sharing of data on the large hard | |exFAT |disk like device (<512TiB, supported by WindowsXP, | | |Mac OS X Snow Leopard 10.6.5, and Linux kernel since| | |5.4 release) | |----------+----------------------------------------------------| | |cross platform sharing of data on the large hard |     |NTFS |disk like device (supported natively on MS Windows | | |NT and later version, and supported by NTFS-3G via | | |FUSE on Linux) | |----------+----------------------------------------------------| |ISO9660 |cross platform sharing of static data on CD-R and | | |DVD+/-R | |----------+----------------------------------------------------| |UDF |incremental data writing on CD-R and DVD+/-R (new) | |----------+----------------------------------------------------| |MINIX |space efficient unix file data storage on the floppy| | |disk | |----------+----------------------------------------------------| |ext2 |sharing of data on the hard disk like device with | | |older Linux systems | |----------+----------------------------------------------------| |ext3 |sharing of data on the hard disk like device with | | |older Linux systems | |----------+----------------------------------------------------| |ext4 |sharing of data on the hard disk like device with | | |current Linux systems | |----------+----------------------------------------------------| |btrfs |sharing of data on the hard disk like device with | | |current Linux systems with read-only snapshots | +---------------------------------------------------------------+ Tip     See Section 9.9.1, “Removable disk encryption with dm-crypt/LUKS” for cross platform sharing of data using device level encryption. The FAT filesystem is supported by almost all modern operating     systems and is quite useful for the data exchange purpose via removable hard disk like media. When formatting removable hard disk like devices for cross     platform sharing of data with the FAT filesystem, the following should be safe choices. * Partitioning them with fdisk(8), cfdisk(8) or parted(8) (see Section 9.6.2, “Disk partition configuration”) into a single primary partition and to mark it as the following. + Type "6" for FAT16 for media smaller than 2GB. + Type "c" for FAT32 (LBA) for larger media.     * Formatting the primary partition with mkfs.vfat(8) with the following. + Just its device name, e.g. "/dev/sda1" for FAT16 + The explicit option and its device name, e.g. "-F 32 /dev /sda1" for FAT32     When using the FAT or ISO9660 filesystems for sharing data, the following should be the safe considerations. * Archiving files into an archive file first using tar(1), or cpio(1) to retain the long filename, the symbolic link, the original Unix file permission and the owner information.     * Splitting the archive file into less than 2 GiB chunks with the split(1) command to protect it from the file size limitation. * Encrypting the archive file to secure its contents from the unauthorized access. Note For FAT filesystems by its design, the maximum file size is (2^32     - 1) bytes = (4GiB - 1 byte). For some applications on the older 32 bit OS, the maximum file size was even smaller (2^31 - 1) bytes = (2GiB - 1 byte). Debian does not suffer the latter problem. Note Microsoft itself does not recommend to use FAT for drives or     partitions of over 200 MB. Microsoft highlights its short comings such as inefficient disk space usage in their "Overview of FAT, HPFS, and NTFS File Systems". Of course, we should normally use the ext4 filesystem for Linux. Tip     For more on filesystems and accessing filesystems, please read "Filesystems HOWTO". 10.1.9. Sharing data via network     When sharing data with other system via network, you should use common service. Here are some hints. Table 10.4. List of the network service to chose with the typical usage scenario +---------------------------------------------------------------+ | network service | description of typical usage scenario | |-------------------+-------------------------------------------| |SMB/CIFS network |sharing files via "Microsoft Windows | |mounted filesystem |Network", see smb.conf(5) and The Official | |with Samba |Samba 3.x.x HOWTO and Reference Guide or | | |the samba-doc package | |-------------------+-------------------------------------------|     |NFS network mounted|sharing files via "Unix/Linux Network", see| |filesystem with the|exports(5) and Linux NFS-HOWTO | |Linux kernel | | |-------------------+-------------------------------------------| |HTTP service |sharing file between the web server/client | |-------------------+-------------------------------------------| | |sharing file between the web server/client | |HTTPS service |with encrypted Secure Sockets Layer (SSL) | | |or Transport Layer Security (TLS) | |-------------------+-------------------------------------------| |FTP service |sharing file between the FTP server/client | +---------------------------------------------------------------+ Although these filesystems mounted over network and file transfer     methods over network are quite convenient for sharing data, these may be insecure. Their network connection must be secured by the following. * Encrypt it with SSL/TLS * Tunnel it via SSH     * Tunnel it via VPN * Limit it behind the secure firewall     See also Section 6.5, “Other network application servers” and Section 6.6, “Other network application clients”. 10.2. Backup and recovery We all know that computers fail sometime or human errors cause     system and data damages. Backup and recovery operations are the essential part of successful system administration. All possible failure modes hit you some day. Tip     Keep your backup system simple and backup your system often. Having backup data is more important than how technically good your backup method is. 10.2.1. Backup and recovery policy     There are 3 key factors which determine actual backup and recovery policy. 1. Knowing what to backup and recover. + Data files directly created by you: data in "~/" + Data files created by applications used by you: data in " /var/" (except "/var/cache/", "/var/run/", and "/var/tmp/ ") + System configuration files: data in "/etc/" + Local programs: data in "/usr/local/" or "/opt/" + System installation information: a memo in plain text on key steps (partition, …) + Proven set of data: confirmed by experimental recovery operations in advance o Cron job as a user process: files in "/var/spool/cron /crontabs" directory and restart cron(8). See Section 9.4.14, “Scheduling tasks regularly” for cron (8) and crontab(1). o Systemd timer jobs as user processes: files in "~ /.config/systemd/user" directory. See systemd.timer (5) and systemd.service(5). o Autostart jobs as user processes: files in "~/.config /autostart" directory. See Desktop Application Autostart Specification. 2. Knowing how to backup and recover. + Secure storage of data: protection from overwrite and system failure + Frequent backup: scheduled backup + Redundant backup: data mirroring + Fool proof process: easy single command backup     3. Assessing risks and costs involved. + Risk of data when lost o Data should be at least on different disk partitions preferably on different disks and machines to withstand the filesystem corruption. Important data are best stored on a read-only filesystem. ^[4] + Risk of data when breached o Sensitive identity data such as "/etc/ssh/ ssh_host_*_key", "~/.gnupg/*", "~/.ssh/*", "~/.local/ share/keyrings/*", "/etc/passwd", "/etc/shadow", "popularity-contest.conf", "/etc/ppp/pap-secrets", and "/etc/exim4/passwd.client" should be backed up as encrypted. ^[5] (See Section 9.9, “Data encryption tips”.) o Never hard code system login password nor decryption passphrase in any script even on any trusted system. (See Section 10.3.6, “Password keyring”.) + Failure mode and their possibility o Hardware (especially HDD) will break o Filesystem may be corrupted and data in it may be lost o Remote storage system can't be trusted for security breaches o Weak password protection can be easily compromised o File permission system may be compromised + Required resources for backup: human, hardware, software, … o Automatic scheduled backup with cron job or systemd timer job Tip     You can recover debconf configuration data with "debconf-set-selections debconf-selections" and dpkg selection data with "dpkg --set-selection &2 ; exit 1     fi MSGID=$(notify-send -p "bkup.sh $DSTSV" "in progress ...") if [ ! -d "$DSTFS/$DSTSV" ]; then btrfs subvolume create "$DSTFS/$DSTSV" mkdir -p "$DSTSS" fi rsync -aHxS --delete --mkpath "${SRC}/" "${DSTFS}/${DSTSV}" btrfs subvolume snapshot -r "${DSTFS}/${DSTSV}" ${DSTSS}/$(date -u --iso=min) notify-send -r "$MSGID" "bkup.sh $DSTSV" "finished!"     Here, only the basic tool rsync(1) is used to facilitate system backup and the storage space is efficiently used by Btrfs. Tip     FYI: This author uses his own similar shell script "bss: Btrfs Subvolume Snapshot Utility" for his workstation. 10.2.3.1. GUI backup     Here is an example to setup the single GUI click backup. * Prepare a USB storage device to be used for backup. + Format a USB storage device with one partition in btrfs with its label name as "BKUP". This can be encrypted (see Section 9.9.1, “Removable disk encryption with dm-crypt/LUKS” ). + Plug this in to your system. The desktop system should automatically mount it as "/media/penguin/BKUP".     + Execute "sudo chown penguin:penguin /media/penguin/BKUP" to make it writable by the user. * Create "~/.local/share/applications/BKUP.desktop" following techniques written in Section 9.4.10, “Starting a program from GUI” as: [Desktop Entry] Name=bkss Comment=Backup and snapshot of ~/Documents Exec=/usr/local/bin/bkss.sh /home/penguin/Documents /media/penguin/BKUP Documents Type=Application     For each GUI click, your data is backed up from "~/Documents" to a USB storage device and a read-only snapshot is created. 10.2.3.2. Mount event triggered backup     Here is an example to setup for the automatic backup triggered by the mount event. * Prepare a USB storage device to be used for backup as in Section 10.2.3.1, “GUI backup”. * Create a systemd service unit file "~/.config/systemd/user/ back-BKUP.service" as: [Unit] Description=USB Disk backup Requires=media-%u-BKUP.mount After=media-%u-BKUP.mount     [Service] ExecStart=/usr/local/bin/bkss.sh %h/Documents /media/%u/BKUP Documents StandardOutput=append:%h/.cache/systemd-snap.log StandardError=append:%h/.cache/systemd-snap.log [Install] WantedBy=media-%u-BKUP.mount * Enable this systemd unit configuration with the following: $ systemctl --user enable bkup-BKUP.service     For each mount event, your data is backed up from "~/Documents" to a USB storage device and a read-only snapshot is created. Here, names of systemd mount units that systemd currently has in     memory can be asked to the service manager of the calling user with "systemctl --user list-units --type=mount". 10.2.3.3. Timer event triggered backup     Here is an example to setup for the automatic backup triggered by the timer event. * Prepare a USB storage device to be used for backup as in Section 10.2.3.1, “GUI backup”. * Create a systemd timer unit file "~/.config/systemd/user/ snap-Documents.timer" as: [Unit] Description=Run btrfs subvolume snapshot on timer Documentation=man:btrfs(1) [Timer] OnStartupSec=30 OnUnitInactiveSec=900 [Install] WantedBy=timers.target     * Create a systemd service unit file "~/.config/systemd/user/ snap-Documents.service" as: [Unit] Description=Run btrfs subvolume snapshot Documentation=man:btrfs(1) [Service] Type=oneshot Nice=15 ExecStart=/usr/local/bin/bkss.sh %h/Documents /media/%u/BKUP Documents IOSchedulingClass=idle CPUSchedulingPolicy=idle StandardOutput=append:%h/.cache/systemd-snap.log StandardError=append:%h/.cache/systemd-snap.log * Enable this systemd unit configuration with the following: $ systemctl --user enable snap-Documents.timer     For each timer event, your data is backed up from "~/Documents" to a USB storage device and a read-only snapshot is created. Here, names of systemd timer user units that systemd currently     has in memory can be asked to the service manager of the calling user with "systemctl --user list-units --type=timer". For the modern desktop system, this systemd approach can offer     more fine grained control than the traditional Unix ones using at (1), cron(8), or anacron(8). 10.3. Data security infrastructure     The data security infrastructure is provided by the combination of data encryption tool, message digest tool, and signature tool. Table 10.6. List of data security infrastructure tools +---------------------------------------------------------------+ | package |popcon|size | command | description | |---------------+------+-----+-----------+----------------------| | |V:554,| | |GNU Privacy Guard - | |gnupg |I:906 |885 |gpg(1) |OpenPGP encryption and| | | | | |signing tool | |---------------+------+-----+-----------+----------------------| | |V:893,| | |GNU Privacy Guard - | |gpgv |I:999 |922 |gpgv(1) |signature verification| | | | | |tool | |---------------+------+-----+-----------+----------------------| | |V:1, | | |extract just the | |paperkey |I:14 |58 |paperkey(1)|secret information out| | | | | |of OpenPGP secret keys| |---------------+------+-----+-----------+----------------------| | | | | |utilities for dm-crypt| |cryptsetup |V:19, |417 |cryptsetup |block device |     | |I:79 | |(8), … |encryption supporting | | | | | |LUKS | |---------------+------+-----+-----------+----------------------| |coreutils |V:880,|18307|md5sum(1) |compute and check MD5 | | |I:999 | | |message digest | |---------------+------+-----+-----------+----------------------| |coreutils |V:880,|18307|sha1sum(1) |compute and check SHA1| | |I:999 | | |message digest | |---------------+------+-----+-----------+----------------------| | |V:841,| |openssl |compute message digest| |openssl |I:995 |2111 |(1ssl) |with "openssl dgst" | | | | | |(OpenSSL) | |---------------+------+-----+-----------+----------------------| |libsecret-tools|V:0, |41 |secret-tool|store and retrieve | | |I:10 | |(1) |passwords (CLI) | |---------------+------+-----+-----------+----------------------| |seahorse |V:80, |7987 |seahorse(1)|key management tool | | |I:269 | | |(GNOME) | +---------------------------------------------------------------+ See Section 9.9, “Data encryption tips” on dm-crypt and fscrypt     which implement automatic data encryption infrastructure via Linux kernel modules. 10.3.1. Key management for GnuPG     Here are GNU Privacy Guard commands for the basic key management. Table 10.7. List of GNU Privacy Guard commands for the key management +---------------------------------------------------------------+ | command | description | |-------------------------+-------------------------------------| |gpg --gen-key |generate a new key | |-------------------------+-------------------------------------| |gpg --gen-revoke |generate revoke key for my_user_ID | |my_user_ID | | |-------------------------+-------------------------------------| |gpg --edit-key user_ID |edit key interactively, "help" for | | |help | |-------------------------+-------------------------------------| |gpg -o file --export |export all keys to file | |-------------------------+-------------------------------------|     |gpg --import file |import all keys from file | |-------------------------+-------------------------------------| |gpg --send-keys user_ID |send key of user_ID to keyserver | |-------------------------+-------------------------------------| |gpg --recv-keys user_ID |recv. key of user_ID from keyserver | |-------------------------+-------------------------------------| |gpg --list-keys user_ID |list keys of user_ID | |-------------------------+-------------------------------------| |gpg --list-sigs user_ID |list sig. of user_ID | |-------------------------+-------------------------------------| |gpg --check-sigs user_ID |check sig. of user_ID | |-------------------------+-------------------------------------| |gpg --fingerprint user_ID|check fingerprint of user_ID | |-------------------------+-------------------------------------| |gpg --refresh-keys |update local keyring | +---------------------------------------------------------------+     Here is the meaning of the trust code. Table 10.8. List of the meaning of the trust code +-------------------------------------------------+ |code| description of trust | |----+--------------------------------------------| |- |no owner trust assigned / not yet calculated| |----+--------------------------------------------| |e |trust calculation failed | |----+--------------------------------------------|     |q |not enough information for calculation | |----+--------------------------------------------| |n |never trust this key | |----+--------------------------------------------| |m |marginally trusted | |----+--------------------------------------------| |f |fully trusted | |----+--------------------------------------------| |u |ultimately trusted | +-------------------------------------------------+     The following uploads my key "1DD8D791" to the popular keyserver "hkp://keys.gnupg.net".     $ gpg --keyserver hkp://keys.gnupg.net --send-keys 1DD8D791     A good default keyserver set up in "~/.gnupg/gpg.conf" (or old location "~/.gnupg/options") contains the following.     keyserver hkp://keys.gnupg.net     The following obtains unknown keys from the keyserver.     $ gpg --list-sigs --with-colons | grep '^sig.*\[User ID not found\]' |\ cut -d ':' -f 5| sort | uniq | xargs gpg --recv-keys There was a bug in OpenPGP Public Key Server (pre version 0.9.6)     which corrupted key with more than 2 sub-keys. The newer gnupg (> 1.2.1-2) package can handle these corrupted subkeys. See gpg(1) under "--repair-pks-subkey-bug" option. 10.3.2. Using GnuPG on files     Here are examples for using GNU Privacy Guard commands on files. Table 10.9. List of GNU Privacy Guard commands on files +---------------------------------------------------------------+ | command | description | |-------------------------+-------------------------------------| |gpg -a -s file |sign file into ASCII armored file.asc| |-------------------------+-------------------------------------| |gpg --armor --sign file |, , | |-------------------------+-------------------------------------| |gpg --clearsign file |clear-sign message | |-------------------------+-------------------------------------| |gpg --clearsign file|mail|mail a clear-signed message to | |foo@example.org |foo@example.org | |-------------------------+-------------------------------------| |gpg --clearsign | | |--not-dash-escaped |clear-sign patchfile | |patchfile | | |-------------------------+-------------------------------------| |gpg --verify file |verify clear-signed file | |-------------------------+-------------------------------------| |gpg -o file.sig -b file |create detached signature | |-------------------------+-------------------------------------| |gpg -o file.sig |, , | |--detach-sign file | | |-------------------------+-------------------------------------| |gpg --verify file.sig |verify file with file.sig | |file | | |-------------------------+-------------------------------------|     |gpg -o crypt_file.gpg -r |public-key encryption intended for | |name -e file |name from file to binary | | |crypt_file.gpg | |-------------------------+-------------------------------------| |gpg -o crypt_file.gpg | | |--recipient name |, , | |--encrypt file | | |-------------------------+-------------------------------------| |gpg -o crypt_file.asc -a |public-key encryption intended for | |-r name -e file |name from file to ASCII armored | | |crypt_file.asc | |-------------------------+-------------------------------------| |gpg -o crypt_file.gpg -c |symmetric encryption from file to | |file |crypt_file.gpg | |-------------------------+-------------------------------------| |gpg -o crypt_file.gpg |, , | |--symmetric file | | |-------------------------+-------------------------------------| |gpg -o crypt_file.asc -a |symmetric encryption intended for | |-c file |name from file to ASCII armored | | |crypt_file.asc | |-------------------------+-------------------------------------| |gpg -o file -d |decryption | |crypt_file.gpg -r name | | |-------------------------+-------------------------------------| |gpg -o file --decrypt |, , | |crypt_file.gpg | | +---------------------------------------------------------------+ 10.3.3. Using GnuPG with Mutt Add the following to "~/.muttrc" to keep a slow GnuPG from     automatically starting, while allowing it to be used by typing "S" at the index menu.     macro index S ":toggle pgp_verify_sig\n" set pgp_verify_sig=no 10.3.4. Using GnuPG with Vim     The gnupg plugin let you run GnuPG transparently for files with extension ".gpg", ".asc", and ".pgp".^[6]     $ sudo aptitude install vim-scripts $ echo "packadd! gnupg" >> ~/.vim/vimrc 10.3.5. The MD5 sum     md5sum(1) provides utility to make a digest file using the method in rfc1321 and verifying each file with it. $ md5sum foo bar >baz.md5 $ cat baz.md5 d3b07384d113edec49eaa6238ad5ff00 foo     c157a79031e1c40f85931829bc5fc552 bar $ md5sum -c baz.md5 foo: OK bar: OK Note     The computation for the MD5 sum is less CPU intensive than the one for the cryptographic signature by GNU Privacy Guard (GnuPG). Usually, only the top level digest file is cryptographically signed to ensure data integrity. 10.3.6. Password keyring     On GNOME system, the GUI tool seahorse(1) manages passwords and stores them securely in the keyring ~/.local/share/keyrings/*.     secret-tool(1) can store password to the keyring from the command line.     Let's store passphrase used for LUKS/dm-crypt encrypted disk image     $ secret-tool store --label='LUKS passphrase for disk.img' LUKS my_disk.img Password: ********     This stored password can be retrieved and fed to other programs, e.g., cryptsetup(8). $ secret-tool lookup LUKS my_disk.img | \     cryptsetup open disk.img disk_img --type luks --keyring - $ sudo mount /dev/mapper/disk_img /mnt Tip     Whenever you need to provide password in a script, use secret-tool and avoid directly hardcoding the passphrase in it. 10.4. Source code merge tools     There are many merge tools for the source code. Following commands caught my eyes. Table 10.10. List of source code merge tools +---------------------------------------------------------------+ | package |popcon|size| command | description | |----------+------+----+-----------+----------------------------| |patch |V:97, |248 |patch(1) |apply a diff file to an | | |I:700 | | |original | |----------+------+----+-----------+----------------------------| |vim |V:95, |3743|vimdiff(1) |compare 2 files side by side| | |I:369 | | |in vim | |----------+------+----+-----------+----------------------------| |imediff |V:0, |200 |imediff(1) |interactive full screen 2/ | | |I:0 | | |3-way merge tool | |----------+------+----+-----------+----------------------------| |meld |V:7, |3536|meld(1) |compare and merge files | | |I:30 | | |(GTK) | |----------+------+----+-----------+----------------------------| |wiggle |V:0, |175 |wiggle(1) |apply rejected patches | | |I:0 | | | | |----------+------+----+-----------+----------------------------| |diffutils |V:862,|1735|diff(1) |compare files line by line | | |I:996 | | | | |----------+------+----+-----------+----------------------------| |diffutils |V:862,|1735|diff3(1) |compare and merges three | | |I:996 | | |files line by line | |----------+------+----+-----------+----------------------------| |quilt |V:2, |871 |quilt(1) |manage series of patches | | |I:22 | | | | |----------+------+----+-----------+----------------------------| |wdiff |V:7, |648 |wdiff(1) |display word differences | | |I:51 | | |between text files | |----------+------+----+-----------+----------------------------| |diffstat |V:13, |74 |diffstat(1)|produce a histogram of | | |I:121 | | |changes by the diff | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |combinediff|create a cumulative patch | | |I:119 | |(1) |from two incremental patches| |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |dehtmldiff |extract a diff from an HTML | | |I:119 | |(1) |page | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |filterdiff |extract or excludes diffs | | |I:119 | |(1) |from a diff file |     |----------+------+----+-----------+----------------------------| | |V:16, | |fixcvsdiff |fix diff files created by | |patchutils|I:119 |232 |(1) |CVS that patch(1) | | | | | |mis-interprets | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |flipdiff(1)|exchange the order of two | | |I:119 | | |patches | |----------+------+----+-----------+----------------------------| | |V:16, | | |show which files are | |patchutils|I:119 |232 |grepdiff(1)|modified by a patch matching| | | | | |a regex | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |interdiff |show differences between two| | |I:119 | |(1) |unified diff files | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |lsdiff(1) |show which files are | | |I:119 | | |modified by a patch | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |recountdiff|recompute counts and offsets| | |I:119 | |(1) |in unified context diffs | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |rediff(1) |fix offsets and counts of a | | |I:119 | | |hand-edited diff | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |splitdiff |separate out incremental | | |I:119 | |(1) |patches | |----------+------+----+-----------+----------------------------| |patchutils|V:16, |232 |unwrapdiff |demangle patches that have | | |I:119 | |(1) |been word-wrapped | |----------+------+----+-----------+----------------------------| | |V:0, | | |display differences and | |dirdiff |I:1 |167 |dirdiff(1) |merge changes between | | | | | |directory trees | |----------+------+----+-----------+----------------------------| |docdiff |V:0, |553 |docdiff(1) |compare two files word by | | |I:0 | | |word / char by char | |----------+------+----+-----------+----------------------------| |makepatch |V:0, |100 |makepatch |generate extended patch | | |I:0 | |(1) |files | |----------+------+----+-----------+----------------------------| |makepatch |V:0, |100 |applypatch |apply extended patch files | | |I:0 | |(1) | | +---------------------------------------------------------------+ 10.4.1. Extracting differences for source files The following procedures extract differences between two source     files and create unified diff files "file.patch0" or "file.patch1" depending on the file location.     $ diff -u file.old file.new > file.patch0 $ diff -u old/file new/file > file.patch1 10.4.2. Merging updates for source files The diff file (alternatively called patch file) is used to send a     program update. The receiving party applies this update to another file by the following.     $ patch -p0 file < file.patch0 $ patch -p1 file < file.patch1 10.4.3. Interactive merge     If you have two versions of a source code, you can perform 2-way merge interactively using imediff(1) by the following.     $ imediff -o file.merged file.old file.new     If you have three versions of a source code, you can perform 3-way merge interactively using imediff(1) by the following.     $ imediff -o file.merged file.yours file.base file.theirs 10.5. Git Git is the tool of choice these days for the version control     system (VCS) since Git can do everything for both local and remote source code management.     Debian provides free Git services via Debian Salsa service. Its documentation can be found at https://wiki.debian.org/Salsa .     Here are some Git related packages. Table 10.11. List of git related packages and commands +---------------------------------------------------------------+ | package |popcon|size | command | description | |----------------+------+-----+----------------+----------------| | | | | |Git, the fast, | | |V:351,| | |scalable, | |git |I:549 |46734|git(7) |distributed | | | | | |revision control| | | | | |system | |----------------+------+-----+----------------+----------------| | | | | |GUI Git | |gitk |V:5, |1838 |gitk(1) |repository | | |I:33 | | |browser with | | | | | |history | |----------------+------+-----+----------------+----------------| |git-gui |V:1, |2429 |git-gui(1) |GUI for Git (No | | |I:18 | | |history) | |----------------+------+-----+----------------+----------------| | | | | |send a | |git-email |V:0, |1087 |git-send-email |collection of | | |I:10 | |(1) |patches as email| | | | | |from the Git | |----------------+------+-----+----------------+----------------|     | |V:1, | |git-buildpackage|automate the | |git-buildpackage|I:9 |1988 |(1) |Debian packaging| | | | | |with the Git | |----------------+------+-----+----------------+----------------| | | | | |git | |dgit |V:0, |473 |dgit(1) |interoperability| | |I:1 | | |with the Debian | | | | | |archive | |----------------+------+-----+----------------+----------------| | |V:0, | | |interactive git | |imediff |I:0 |200 |git-ime(1) |commit split | | | | | |helper tool | |----------------+------+-----+----------------+----------------| |stgit |V:0, |601 |stg(1) |quilt on top of | | |I:0 | | |git (Python) | |----------------+------+-----+----------------+----------------| | | | | |official | |git-doc |I:12 |13208|N/A |documentation | | | | | |for Git | |----------------+------+-----+----------------+----------------| | | | | |"Git Magic", | |gitmagic |I:0 |721 |N/A |easier to | | | | | |understand guide| | | | | |for Git | +---------------------------------------------------------------+ 10.5.1. Configuration of Git client You may wish to set several global configuration in "~     /.gitconfig" such as your name and email address used by Git by the following.     $ git config --global user.name "Name Surname" $ git config --global user.email yourname@example.com     You may also customize the Git default behavior by the following. $ git config --global init.defaultBranch main     $ git config --global pull.rebase true $ git config --global push.default current     If you are too used to CVS or Subversion commands, you may wish to set several command aliases by the following.     $ git config --global alias.ci "commit -a" $ git config --global alias.co checkout     You can check your global configuration by the following.     $ git config --global --list 10.5.2. Basic Git commands     Git operation involves several data. * The working tree which holds user facing files and to which you make changes. + The changes to be recorded must be explicitly selected and staged to the index. This is git add and git rm commands. * The index which holds staged files. + Staged files will be committed to the local repository upon the subsequent request. This is git commit command. * The local repository which holds committed files. + Git records the linked history of the committed data and organizes them as branches in the repository. + The local repository can send data to the remote repository by git push command.     + The local repository can receive data from the remote repository by git fetch and git pull commands. o The git pull command performs git merge or git rebase command after git fetch command. o Here, git merge combines two separate branches of history at the end to a point. (This is default of git pull without customization and may be good for upstream people who publish branch to many people.) o Here, git rebase creates one single branch of sequential history of the remote branch one followed by the local branch one. (This is pull.rebase true customization case and may be good for rest of us.) * The remote repository which holds committed files. + The communication to the remote repository uses secure communication protocols such as SSH or HTTPS. The working tree is files outside of the .git/ directory. Files     inside of the .git/ directory hold the index, the local repository data, and some git configuration text files.     Here is an overview of main Git commands. Table 10.12. Main Git commands +---------------------------------------------------------------+ | Git command | function | |---------------+-----------------------------------------------| |git init |create the (local) repository | |---------------+-----------------------------------------------| |git clone URL |clone the remote repository to a local | | |repository with the working tree | |---------------+-----------------------------------------------| |git pull origin|update the local main branch by the remote | |main |repository origin | |---------------+-----------------------------------------------| |git add . |add file(s) in the working tree to the index | | |for pre-existing files in index only | |---------------+-----------------------------------------------| |git add -A . |add file(s) in the working tree to the index | | |for all files including removals | |---------------+-----------------------------------------------| |git rm filename|remove file(s) from the working tree and the | | |index | |---------------+-----------------------------------------------|     |git commit |commit staged changes in the index to the local| | |repository | |---------------+-----------------------------------------------| | |add all changes in the working tree to the | |git commit -a |index and commit them to the local repository | | |(add + commit) | |---------------+-----------------------------------------------| |git push -u |update the remote repository origin by the | |origin |local branch_name branch (initial invocation) | |branch_name | | |---------------+-----------------------------------------------| |git push origin|update the remote repository origin by the | |branch_name |local branch_name branch (subsequent | | |invocation) | |---------------+-----------------------------------------------| |git diff |show difference between treeish1 commit and | |treeish1 |treeish2 commit | |treeish2 | | |---------------+-----------------------------------------------| |gitk |GUI display of VCS repository branch history | | |tree | +---------------------------------------------------------------+ 10.5.3. Git tips     Here are some Git tips. Table 10.13. Git tips +---------------------------------------------------------------+ | Git command line | function | |------------------------+--------------------------------------| | |see complete Git history and operate | | |on them such as resetting HEAD to | |gitk --all |another commit, cheery-picking | | |patches, creating tags and branches | | |... | |------------------------+--------------------------------------| |git stash |get the clean working tree without | | |loosing data | |------------------------+--------------------------------------| |git remote -v |check settings for remote | |------------------------+--------------------------------------| |git branch -vv |check settings for branch | |------------------------+--------------------------------------| |git status |show working tree status | |------------------------+--------------------------------------| |git config -l |list git settings | |------------------------+--------------------------------------| |git reset --hard HEAD; |revert all working tree changes and | |git clean -x -d -f |clean them up completely | |------------------------+--------------------------------------| |git rm --cached filename|revert staged index changed by git add| | |filename | |------------------------+--------------------------------------| | |get reference log (useful for | |git reflog |recovering commits from the removed | | |branch) | |------------------------+--------------------------------------| |git branch |create a new branch from reflog | |new_branch_name HEAD@{6}|information | |------------------------+--------------------------------------| |git remote add |add a new_remote remote repository | |new_remote URL |pointed by URL | |------------------------+--------------------------------------| |git remote rename origin|rename the remote repository name from| |upstream |origin to upstream | |------------------------+--------------------------------------| |git branch -u upstream/ |set the remote tracking to the remote | |branch_name |repository upstream and its branch | | |name branch_name. | |------------------------+--------------------------------------| |git remote set-url | | |origin https://foo/ |change URL of origin | |bar.git | | |------------------------+--------------------------------------|     |git remote set-url |disable push to upstream (Edit .git/ | |--push upstream DISABLED|config to re-enable) | |------------------------+--------------------------------------| |git remote update |fetch updates of all remote branches | |upstream |in the upstream repository | |------------------------+--------------------------------------| |git fetch upstream |create a local (possibly orphan) | |foo:upstream-foo |upstream-foo branch as a copy of foo | | |branch in the upstream repository | |------------------------+--------------------------------------| |git checkout -b |make a new topic_branch and push it to| |topic_branch ; git push |origin | |-u topic_branch origin | | |------------------------+--------------------------------------| |git branch -m oldname |rename local branch name | |newname | | |------------------------+--------------------------------------| |git push -d origin |remove remote branch (new method) | |branch_to_be_removed | | |------------------------+--------------------------------------| |git push origin |remove remote branch (old method) | |:branch_to_be_removed | | |------------------------+--------------------------------------| |git checkout --orphan |create a new unconnected branch | |unconnected | | |------------------------+--------------------------------------| |git rebase -i origin/ |reorder/drop/squish commits from | |main |origin/main to clean branch history | |------------------------+--------------------------------------| |git reset HEAD^; git |squash last 2 commits into one | |commit --amend | | |------------------------+--------------------------------------| |git checkout |squash entire topic_branch into a | |topic_branch ; git merge|commit | |--squash topic_branch | | |------------------------+--------------------------------------| |git fetch --unshallow | | |--update-head-ok origin |convert a shallow clone to the full | |'+refs/heads/*:refs/ |clone of all branches | |heads/*' | | |------------------------+--------------------------------------| | |split the last commit into a series of| |git ime |file-by-file smaller commits etc. | | |(imediff package required) | |------------------------+--------------------------------------| | |repack the local repository into | |git repack -a -d; git |single pack (this may limit chance of | |prune |lost data recovery from erased branch | | |etc.) | +---------------------------------------------------------------+ Warning     Do not use the tag string with spaces in it even if some tools such as gitk(1) allow you to use it. It may choke some other git commands. Caution If a local branch which has been pushed to remote repository is     rebased or squashed, pushing this branch has risks and requires --force option. This is usually not an acceptable for main branch but may be acceptable for a topic branch before merging to main branch. Caution     Invoking a git subcommand directly as "git-xyz" from the command line has been deprecated since early 2006. Tip     If there is a executable file git-foo in the path specified by $PATH, entering "git foo" without hyphen to the command line invokes this git-foo. This is a feature of the git command. 10.5.4. Git references     See the following. * manpage: git(1) (/usr/share/doc/git-doc/git.html) * Git User's Manual (/usr/share/doc/git-doc/user-manual.html) * A tutorial introduction to git (/usr/share/doc/git-doc/ gittutorial.html)     * A tutorial introduction to git: part two (/usr/share/doc/ git-doc/gittutorial-2.html) * Everyday GIT With 20 Commands Or So (/usr/share/doc/git-doc/ giteveryday.html) * Git Magic (/usr/share/doc/gitmagic/html/index.html) 10.5.5. Other version control systems The version control systems (VCS) is sometimes known as the     revision control system (RCS), or the software configuration management (SCM).     Here is a summary of the notable other non-Git VCS on the Debian system. Table 10.14. List of other version control system tools +---------------------------------------------------------------+ | package |popcon|size | tool | VCS type | comment | |----------+------+-----+----------+-----------+----------------| |mercurial |V:5, |2019 |Mercurial |distributed|DVCS in Python | | |I:32 | | | |and some C | |----------+------+-----+----------+-----------+----------------| | |V:0, | | | |DVCS with smart | |darcs |I:5 |34070|Darcs |distributed|algebra of | | | | | | |patches (slow) | |----------+------+-----+----------+-----------+----------------| | | | | | |DVCS influenced | |bzr |I:8 |28 |GNU Bazaar|distributed|by tla written | | | | | | |in Python | | | | | | |(historic) | |----------+------+-----+----------+-----------+----------------| | |V:0, | | | |DVCS mainly by | |tla |I:1 |1022 |GNU arch |distributed|Tom Lord | | | | | | |(historic) | |----------+------+-----+----------+-----------+----------------|     | | | | | |"CVS done | |subversion|V:13, |4837 |Subversion|remote |right", newer | | |I:72 | | | |standard remote | | | | | | |VCS (historic) | |----------+------+-----+----------+-----------+----------------| | |V:4, | | | |previous | |cvs |I:30 |4753 |CVS |remote |standard remote | | | | | | |VCS (historic) | |----------+------+-----+----------+-----------+----------------| | | | | | |GUI display of | |tkcvs |V:0, |1498 |CVS, … |remote |VCS (CVS, | | |I:1 | | | |Subversion, RCS)| | | | | | |repository tree | |----------+------+-----+----------+-----------+----------------| | |V:2, | | | |"Unix SCCS done | |rcs |I:13 |564 |RCS |local |right" | | | | | | |(historic) | |----------+------+-----+----------+-----------+----------------| | |V:0, | | | |clone of the | |cssc |I:1 |2044 |CSSC |local |Unix SCCS | | | | | | |(historic) | +---------------------------------------------------------------+ --------------------------------------------------------------------- ^[4] A write-once media such as CD/DVD-R can prevent overwrite accidents. (See Section 9.8, “The binary data” for how to write     to the storage media from the shell commandline. GNOME desktop GUI environment gives you easy access via menu: "Places→CD/DVD Creator".)     ^[5] Some of these data can not be regenerated by entering the same input string to the system.     ^[6] If you use "~/.vimrc" instead of "~/.vim/vimrc", please substitute accordingly. Chapter 11. Data conversion     Tools and tips for converting data formats on the Debian system are described.     Standard based tools are in very good shape but support for proprietary data formats are limited. 11.1. Text data conversion tools     Following packages for the text data conversion caught my eyes. Table 11.1. List of text data conversion tools +---------------------------------------------------------------+ |package |popcon|size | keyword | description | |--------+------+-----+-----------+-----------------------------| | |V:917,| | |text encoding converter | |libc6 |I:999 |12988|charset |between locales by iconv(1) | | | | | |(fundamental) | |--------+------+-----+-----------+-----------------------------| | |V:2, | | |text encoding converter | |recode |I:18 |602 |charset+eol|between locales (versatile, | | | | | |more aliases and features) | |--------+------+-----+-----------+-----------------------------| |konwert |V:1, |134 |charset |text encoding converter | | |I:48 | | |between locales (fancy) | |--------+------+-----+-----------+-----------------------------|     |nkf |V:0, |360 |charset |character set translator for | | |I:9 | | |Japanese | |--------+------+-----+-----------+-----------------------------| |tcs |V:0, |518 |charset |character set translator | | |I:0 | | | | |--------+------+-----+-----------+-----------------------------| |unaccent|V:0, |35 |charset |replace accented letters by | | |I:0 | | |their unaccented equivalent | |--------+------+-----+-----------+-----------------------------| | |V:1, | | |text format converter between| |tofrodos|I:17 |51 |eol |DOS and Unix: fromdos(1) and | | | | | |todos(1) | |--------+------+-----+-----------+-----------------------------| | |V:0, | | |text format converter between| |macutils|I:0 |312 |eol |Macintosh and Unix: frommac | | | | | |(1) and tomac(1) | +---------------------------------------------------------------+ 11.1.1. Converting a text file with iconv Tip     iconv(1) is provided as a part of the libc6 package and it is always available on practically all Unix-like systems to convert the encoding of characters.     You can convert encodings of a text file with iconv(1) by the following.     $ iconv -f encoding1 -t encoding2 input.txt >output.txt Encoding values are case insensitive and ignore "-" and "_" for     matching. Supported encodings can be checked by the "iconv -l" command. Table 11.2. List of encoding values and their usage +---------------------------------------------------------------+ | encoding | usage | | value | | |-----------+---------------------------------------------------| |ASCII |American Standard Code for Information Interchange,| | |7 bit code w/o accented characters | |-----------+---------------------------------------------------| |UTF-8 |current multilingual standard for all modern OSs | |-----------+---------------------------------------------------| |ISO-8859-1 |old standard for western European languages, ASCII | | |+ accented characters | |-----------+---------------------------------------------------| |ISO-8859-2 |old standard for eastern European languages, ASCII | | |+ accented characters | |-----------+---------------------------------------------------| |ISO-8859-15|old standard for western European languages, | | |ISO-8859-1 with euro sign | |-----------+---------------------------------------------------| | |code page 850, Microsoft DOS characters with | |CP850 |graphics for western European languages, ISO-8859-1| | |variant | |-----------+---------------------------------------------------| |CP932 |code page 932, Microsoft Windows style Shift-JIS | | |variant for Japanese |     |-----------+---------------------------------------------------| |CP936 |code page 936, Microsoft Windows style GB2312, GBK | | |or GB18030 variant for Simplified Chinese | |-----------+---------------------------------------------------| |CP949 |code page 949, Microsoft Windows style EUC-KR or | | |Unified Hangul Code variant for Korean | |-----------+---------------------------------------------------| |CP950 |code page 950, Microsoft Windows style Big5 variant| | |for Traditional Chinese | |-----------+---------------------------------------------------| |CP1251 |code page 1251, Microsoft Windows style encoding | | |for the Cyrillic alphabet | |-----------+---------------------------------------------------| |CP1252 |code page 1252, Microsoft Windows style ISO-8859-15| | |variant for western European languages | |-----------+---------------------------------------------------| |KOI8-R |old Russian UNIX standard for the Cyrillic alphabet| |-----------+---------------------------------------------------| |ISO-2022-JP|standard encoding for Japanese email which uses | | |only 7 bit codes | |-----------+---------------------------------------------------| |eucJP |old Japanese UNIX standard 8 bit code and | | |completely different from Shift-JIS | |-----------+---------------------------------------------------| |Shift-JIS |JIS X 0208 Appendix 1 standard for Japanese (see | | |CP932) | +---------------------------------------------------------------+ Note     Some encodings are only supported for the data conversion and are not used as locale values (Section 8.1, “The locale”). For character sets which fit in single byte such as ASCII and     ISO-8859 character sets, the character encoding means almost the same thing as the character set. For character sets with many characters such as JIS X 0213 for     Japanese or Universal Character Set (UCS, Unicode, ISO-10646-1) for practically all languages, there are many encoding schemes to fit them into the sequence of the byte data. * EUC and ISO/IEC 2022 (also known as JIS X 0202) for Japanese     * UTF-8, UTF-16/UCS-2 and UTF-32/UCS-4 for Unicode     For these, there are clear differentiations between the character set and the character encoding.     The code page is used as the synonym to the character encoding tables for some vendor specific ones. Note Please note most encoding systems share the same code with ASCII for the 7 bit characters. But there are some exceptions. If you     are converting old Japanese C programs and URLs data from the casually-called shift-JIS encoding format to UTF-8 format, use "CP932" as the encoding name instead of "shift-JIS" to get the expected results: 0x5C → "\" and 0x7E → "~". Otherwise, these are converted to wrong characters. Tip     recode(1) may be used too and offers more than the combined functionality of iconv(1), fromdos(1), todos(1), frommac(1), and tomac(1). For more, see "info recode". 11.1.2. Checking file to be UTF-8 with iconv     You can check if a text file is encoded in UTF-8 with iconv(1) by the following.     $ iconv -f utf8 -t utf8 input.txt >/dev/null || echo "non-UTF-8 found" Tip     Use "--verbose" option in the above example to find the first non-UTF-8 character. 11.1.3. Converting file names with iconv Here is an example script to convert encoding of file names from     ones created under older OS to modern UTF-8 ones in a single directory. #!/bin/sh ENCDN=iso-8859-1     for x in *; do mv "$x" "$(echo "$x" | iconv -f $ENCDN -t utf-8)" done The "$ENCDN" variable specifies the original encoding used for     file names under older OS as in Table 11.2, “List of encoding values and their usage”. For more complicated case, please mount a filesystem (e.g. a partition on a disk drive) containing such file names with proper     encoding as the mount(8) option (see Section 8.1.3, “Filename encoding”) and copy its entire contents to another filesystem mounted as UTF-8 with "cp -a" command. 11.1.4. EOL conversion     The text file format, specifically the end-of-line (EOL) code, is dependent on the platform. Table 11.3. List of EOL styles for different platforms +------------------------------------------------------+ | platform |EOL code|control|decimal|hexadecimal| |-----------------+--------+-------+-------+-----------|     |Debian (unix) |LF |^J |10 |0A | |-----------------+--------+-------+-------+-----------| |MSDOS and Windows|CR-LF |^M^J |13 10 |0D 0A | |-----------------+--------+-------+-------+-----------| |Apple's Macintosh|CR |^M |13 |0D | +------------------------------------------------------+     The EOL format conversion programs, fromdos(1), todos(1), frommac (1), and tomac(1), are quite handy. recode(1) is also useful. Note     Some data on the Debian system, such as the wiki page data for the python-moinmoin package, use MSDOS style CR-LF as the EOL code. So the above rule is just a general rule. Note     Most editors (eg. vim, emacs, gedit, …) can handle files in MSDOS style EOL transparently. Tip     The use of "sed -e '/\r$/!s/$/\r/'" instead of todos(1) is better when you want to unify the EOL style to the MSDOS style from the mixed MSDOS and Unix style. (e.g., after merging 2 MSDOS style files with diff3(1).) This is because todos adds CR to all lines. 11.1.5. TAB conversion     There are few popular specialized programs to convert the tab codes. Table 11.4. List of TAB conversion commands from bsdmainutils and coreutils packages +-----------------------------------------------+ | function |bsdmainutils|coreutils|     |------------------------+------------+---------| |expand tab to spaces |"col -x" |expand | |------------------------+------------+---------| |unexpand tab from spaces|"col -h" |unexpand | +-----------------------------------------------+     indent(1) from the indent package completely reformats whitespaces in the C program. Editor programs such as vim and emacs can be used for TAB     conversion, too. For example with vim, you can expand TAB with ":set expandtab" and ":%retab" command sequence. You can revert this with ":set noexpandtab" and ":%retab!" command sequence. 11.1.6. Editors with auto-conversion Intelligent modern editors such as the vim program are quite     smart and copes well with any encoding systems and any file formats. You should use these editors under the UTF-8 locale in the UTF-8 capable console for the best compatibility. An old western European Unix text file, "u-file.txt", stored in     the latin1 (iso-8859-1) encoding can be edited simply with vim by the following.     $ vim u-file.txt This is possible since the auto detection mechanism of the file     encoding in vim assumes the UTF-8 encoding first and, if it fails, assumes it to be latin1.     An old Polish Unix text file, "pu-file.txt", stored in the latin2 (iso-8859-2) encoding can be edited with vim by the following.     $ vim '+e ++enc=latin2 pu-file.txt'     An old Japanese unix text file, "ju-file.txt", stored in the eucJP encoding can be edited with vim by the following.     $ vim '+e ++enc=eucJP ju-file.txt' An old Japanese MS-Windows text file, "jw-file.txt", stored in     the so called shift-JIS encoding (more precisely: CP932) can be edited with vim by the following.     $ vim '+e ++enc=CP932 ++ff=dos jw-file.txt' When a file is opened with "++enc" and "++ff" options, ":w" in the Vim command line stores it in the original format and     overwrite the original file. You can also specify the saving format and the file name in the Vim command line, e.g., ":w ++enc =utf8 new.txt". Please refer to the mbyte.txt "multi-byte text support" in vim     on-line help and Table 11.2, “List of encoding values and their usage” for locale values used with "++enc".     The emacs family of programs can perform the equivalent functions. 11.1.7. Plain text extraction The following reads a web page into a text file. This is very     useful when copying configurations off the Web or applying basic Unix text tools such as grep(1) on the web page.     $ w3m -dump https://www.remote-site.com/help-info.html >textfile     Similarly, you can extract plain text data from other formats using the following. Table 11.5. List of tools to extract plain text data +---------------------------------------------------------------+ | package |popcon|size| keyword | function | |---------+------+----+---------------+-------------------------| | |V:15, | | |HTML to text converter | |w3m |I:187 |2837|html→text |with the "w3m -dump" | | | | | |command | |---------+------+----+---------------+-------------------------| |html2text|V:3, |243 |html→text |advanced HTML to text | | |I:53 | | |converter (ISO 8859-1) | |---------+------+----+---------------+-------------------------| | |V:25, | | |HTML to text converter | |lynx |I:344 |1948|html→text |with the "lynx -dump" | | | | | |command | |---------+------+----+---------------+-------------------------| | |V:3, | | |HTML to text converter | |elinks |I:20 |1654|html→text |with the "elinks -dump" | | | | | |command | |---------+------+----+---------------+-------------------------|     | |V:3, | | |HTML to text converter | |links |I:28 |2314|html→text |with the "links -dump" | | | | | |command | |---------+------+----+---------------+-------------------------| | |V:1, | | |HTML to text converter | |links2 |I:12 |5492|html→text |with the "links2 -dump" | | | | | |command | |---------+------+----+---------------+-------------------------| |catdoc |V:14, |686 |MSWord→text,TeX|convert MSWord files to | | |I:155 | | |plain text or TeX | |---------+------+----+---------------+-------------------------| |antiword |V:1, |589 |MSWord→text,ps |convert MSWord files to | | |I:7 | | |plain text or ps | |---------+------+----+---------------+-------------------------| |unhtml |V:0, |40 |html→text |remove the markup tags | | |I:0 | | |from an HTML file | |---------+------+----+---------------+-------------------------| |odt2txt |V:2, |60 |odt→text |converter from | | |I:40 | | |OpenDocument Text to text| +---------------------------------------------------------------+ 11.1.8. Highlighting and formatting plain text data     You can highlight and format plain text data by the following. Table 11.6. List of tools to highlight plain text data +---------------------------------------------------------------+ | package |popcon| size | keyword | description | |----------------+------+------+----------+---------------------| | | | | |Vim MACRO to convert | | |V:18, | | |source code to HTML | |vim-runtime |I:395 |36525 |highlight |with ":source | | | | | |$VIMRUNTIME/syntax/ | | | | | |html.vim" | |----------------+------+------+----------+---------------------| | |V:0, | | |converter for the C | |cxref |I:0 |1190 |c→html |program to latex and | | | | | |HTML (C language) | |----------------+------+------+----------+---------------------| | |V:0, | | |convert many source | |src2tex |I:0 |622 |highlight |codes to TeX (C | | | | | |language) | |----------------+------+------+----------+---------------------| | | | | |convert many source | | | | | |codes to HTML, XHTML,| | |V:0, | | |LaTeX, Texinfo, ANSI | |source-highlight|I:5 |2114 |highlight |color escape | | | | | |sequences and DocBook| | | | | |files with highlight | | | | | |(C++) | |----------------+------+------+----------+---------------------| | | | | |convert many source | | |V:0, | | |codes to HTML, XHTML,| |highlight |I:5 |1371 |highlight |RTF, LaTeX, TeX or |     | | | | |XSL-FO files with | | | | | |highlight (C++) | |----------------+------+------+----------+---------------------| | |V:0, | | |generic colouriser | |grc |I:5 |208 |text→color|for everything | | | | | |(Python) | |----------------+------+------+----------+---------------------| |pandoc |V:9, |194495|text→any |general markup | | |I:45 | | |converter (Haskell) | |----------------+------+------+----------+---------------------| | |V:14, | | |ReStructured Text | |python3-docutils|I:51 |1804 |text→any |document formatter to| | | | | |XML (Python) | |----------------+------+------+----------+---------------------| | |V:0, | | |Markdown text | |markdown |I:9 |58 |text→html |document formatter to| | | | | |(X)HTML (Perl) | |----------------+------+------+----------+---------------------| | |V:0, | | |AsciiDoc text | |asciidoctor |I:7 |98 |text→any |document formatter to| | | | | |XML/HTML (Ruby) | |----------------+------+------+----------+---------------------| | | | | |ReStructured Text | |python3-sphinx |V:6, |2756 |text→any |based document | | |I:24 | | |publication system | | | | | |(Python) | |----------------+------+------+----------+---------------------| | |V:0, | | |Markdown based static| |hugo |I:5 |78678 |text→html |site publication | | | | | |system (Go) | +---------------------------------------------------------------+ 11.2. XML data     The Extensible Markup Language (XML) is a markup language for documents containing structured information.     See introductory information at XML.COM. * "What is XML?" * "What Is XSLT?"     * "What Is XSL-FO?" * "What Is XLink?" 11.2.1. Basic hints for XML XML text looks somewhat like HTML. It enables us to manage     multiple formats of output for a document. One easy XML system is the docbook-xsl package, which is used here.     Each XML file starts with standard XML declaration as the following.         The basic syntax for one XML element is marked up as the following.     content     XML element with empty content is marked up in the following short form.         The "attribute="value"" in the above examples are optional.     The comment section in XML is marked up as the following.         Other than adding markups, XML requires minor conversion to the content using predefined entities for following characters. Table 11.7. List of predefined entities for XML +------------------------------------------------+ |predefined entity|character to be converted into| |-----------------+------------------------------| |" |" : quote | |-----------------+------------------------------|     |' |' : apostrophe | |-----------------+------------------------------| |< |< : less-than | |-----------------+------------------------------| |> |> : greater-than | |-----------------+------------------------------| |& |& : ampersand | +------------------------------------------------+ Caution     "<" or "&" can not be used in attributes or elements. Note     When SGML style user defined entities, e.g. "&some-tag;", are used, the first definition wins over others. The entity definition is expressed in "". Note     As long as the XML markup are done consistently with certain set of the tag name (either some data as content or attribute value), conversion to another XML is trivial task using Extensible Stylesheet Language Transformations (XSLT). 11.2.2. XML processing     There are many tools available to process XML files such as the Extensible Stylesheet Language (XSL). Basically, once you create well formed XML file, you can convert     it to any format using Extensible Stylesheet Language Transformations (XSLT). The Extensible Stylesheet Language for Formatting Objects (XSL-FO) is supposed to be solution for formatting. The fop     package is new to the Debian main archive due to its dependence to the Java programing language. So the LaTeX code is usually generated from XML using XSLT and the LaTeX system is used to create printable file such as DVI, PostScript, and PDF. Table 11.8. List of XML tools +---------------------------------------------------------------+ | package |popcon|size |keyword| description | |-----------+------+-----+-------+------------------------------| |docbook-xml|I:403 |2134 |xml |XML document type definition | | | | | |(DTD) for DocBook | |-----------+------+-----+-------+------------------------------| | |V:13, | |xml/ |XSL stylesheets for processing| |docbook-xsl|I:146 |14851|xslt |DocBook XML to various output | | | | | |formats with XSLT | |-----------+------+-----+-------+------------------------------| | |V:16, | | |XSLT command line processor | |xsltproc |I:79 |162 |xslt |(XML→ XML, HTML, plain text, |     | | | | |etc.) | |-----------+------+-----+-------+------------------------------| |xmlto |V:1, |130 |xml/ |XML-to-any converter with XSLT| | |I:14 | |xslt | | |-----------+------+-----+-------+------------------------------| |fop |V:0, |284 |xml/ |convert Docbook XML files to | | |I:11 | |xsl-fo |PDF | |-----------+------+-----+-------+------------------------------| | |V:2, | |xml/ |convert Docbook files to DVI, | |dblatex |I:10 |4636 |xslt |PostScript, PDF documents with| | | | | |XSLT | |-----------+------+-----+-------+------------------------------| |dbtoepub |V:0, |37 |xml/ |DocBook XML to .epub converter| | |I:0 | |xslt | | +---------------------------------------------------------------+ Since XML is subset of Standard Generalized Markup Language     (SGML), it can be processed by the extensive tools available for SGML, such as Document Style Semantics and Specification Language (DSSSL). Table 11.9. List of DSSSL tools +---------------------------------------------------------------+ | package |popcon|size|keyword| description | |-------------+------+----+-------+-----------------------------| |openjade |V:1, |1061|dsssl |ISO/IEC 10179:1996 standard | | |I:26 | | |DSSSL processor (latest) | |-------------+------+----+-------+-----------------------------| | | | | |DSSSL stylesheets for | |docbook-dsssl|V:0, |2605|xml/ |processing DocBook XML to | | |I:13 | |dsssl |various output formats with |     | | | | |DSSSL | |-------------+------+----+-------+-----------------------------| | | | | |utilities for DocBook files | | |V:0, | |xml/ |including conversion to other| |docbook-utils|I:9 |287 |dsssl |formats (HTML, RTF, PS, man, | | | | | |PDF) with docbook2* commands | | | | | |with DSSSL | |-------------+------+----+-------+-----------------------------| |sgml2x |V:0, |90 |SGML/ |converter from SGML and XML | | |I:0 | |dsssl |using DSSSL stylesheets | +---------------------------------------------------------------+ Tip     GNOME's yelp is sometimes handy to read DocBook XML files directly since it renders decently on X. 11.2.3. The XML data extraction     You can extract HTML or XML data from other formats using followings. Table 11.10. List of XML data extraction tools +---------------------------------------------------------------+ | package |popcon|size| keyword | description | |---------+------+----+---------------+-------------------------| |man2html |V:0, |142 |manpage→html |converter from manpage to| | |I:1 | | |HTML (CGI support) | |---------+------+----+---------------+-------------------------| |doclifter|I:0 |472 |troff→xml |converter from troff to | | | | | |DocBook XML | |---------+------+----+---------------+-------------------------| |texi2html|V:0, |1847|texi→html |converter from Texinfo to| | |I:5 | | |HTML | |---------+------+----+---------------+-------------------------|     |info2www |V:1, |74 |info→html |converter from GNU info | | |I:2 | | |to HTML (CGI support) | |---------+------+----+---------------+-------------------------| | |V:0, | | |document converter from | |wv |I:4 |733 |MSWord→any |Microsoft Word to HTML, | | | | | |LaTeX, etc. | |---------+------+----+---------------+-------------------------| |unrtf |V:0, |148 |rtf→html |document converter from | | |I:3 | | |RTF to HTML, etc | |---------+------+----+---------------+-------------------------| | |V:0, | | |WordPerfect 5.0 and 5.1 | |wp2x |I:0 |200 |WordPerfect→any|files to TeX, LaTeX, | | | | | |troff, GML and HTML | +---------------------------------------------------------------+ 11.2.4. The XML data lint     For non-XML HTML files, you can convert them to XHTML which is an instance of well formed XML. XHTML can be processed by XML tools.     Syntax of XML files and goodness of URLs found in them may be checked. Table 11.11. List of XML pretty print tools +---------------------------------------------------------------+ | package |popcon|size| function | description | |-------------+------+----+--------------+----------------------| | | | | |command line XML tool | |libxml2-utils|V:21, |180 |xml↔html↔xhtml|with xmllint(1) | | |I:213 | | |(syntax check, | | | | | |reformat, lint, …) | |-------------+------+----+--------------+----------------------|     |tidy |V:1, |75 |xml↔html↔xhtml|HTML syntax checker | | |I:9 | | |and reformatter | |-------------+------+----+--------------+----------------------| |weblint-perl |V:0, |32 |lint |syntax and minimal | | |I:1 | | |style checker for HTML| |-------------+------+----+--------------+----------------------| | |V:0, | | |fast link checker and | |linklint |I:0 |343 |link check |web site maintenance | | | | | |tool | +---------------------------------------------------------------+     Once proper XML is generated, you can use XSLT technology to extract data based on the mark-up context etc. 11.3. Type setting     The Unix troff program originally developed by AT&T can be used for simple typesetting. It is usually used to create manpages. TeX created by Donald Knuth is a very powerful type setting tool     and is the de facto standard. LaTeX originally written by Leslie Lamport enables a high-level access to the power of TeX. Table 11.12. List of type setting tools +---------------------------------------------------------------+ |package|popcon|size |keyword| description | |-------+------+-----+-------+----------------------------------|     |texlive|V:2, |56 |(La)TeX|TeX system for typesetting, | | |I:35 | | |previewing and printing | |-------+------+-----+-------+----------------------------------| |groff |V:2, |20720|troff |GNU troff text-formatting system | | |I:36 | | | | +---------------------------------------------------------------+ 11.3.1. roff typesetting Traditionally, roff is the main Unix text processing system. See     roff(7), groff(7), groff(1), grotty(1), troff(1), groff_mdoc(7), groff_man(7), groff_ms(7), groff_me(7), groff_mm(7), and "info groff".     You can read or print a good tutorial and reference on "-me" macro in "/usr/share/doc/groff/" by installing the groff package. Tip     "groff -Tascii -me -" produces plain text output with ANSI escape code. If you wish to get manpage like output with many "^H" and "_", use "GROFF_NO_SGR=1 groff -Tascii -me -" instead. Tip     To remove "^H" and "_" from a text file generated by groff, filter it by "col -b -x". 11.3.2. TeX/LaTeX The TeX Live software distribution offers a complete TeX system.     The texlive metapackage provides a decent selection of the TeX Live packages which should suffice for the most common tasks.     There are many references available for TeX and LaTeX. * The teTeX HOWTO: The Linux-teTeX Local Guide * tex(1) * latex(1) * texdoc(1)     * texdoctk(1) * "The TeXbook", by Donald E. Knuth, (Addison-Wesley) * "LaTeX - A Document Preparation System", by Leslie Lamport, (Addison-Wesley) * "The LaTeX Companion", by Goossens, Mittelbach, Samarin, (Addison-Wesley) This is the most powerful typesetting environment. Many SGML processors use this as their back end text processor. Lyx     provided by the lyx package and GNU TeXmacs provided by the texmacs package offer nice WYSIWYG editing environment for LaTeX while many use Emacs and Vim as the choice for the source editor.     There are many online resources available. * The TEX Live Guide - TEX Live 2007 ("/usr/share/doc/ texlive-doc-base/english/texlive-en/live.html") (texlive-doc-base package)     * A Simple Guide to Latex/Lyx * Word Processing Using LaTeX When documents become bigger, sometimes TeX may cause errors. You     must increase pool size in "/etc/texmf/texmf.cnf" (or more appropriately edit "/etc/texmf/texmf.d/95NonPath" and run update-texmf(8)) to fix this. Note The TeX source of "The TeXbook" is available at www.ctan.org tex-archive site for texbook.tex. This file contains most of the     required macros. I heard that you can process this document with tex(1) after commenting lines 7 to 10 and adding "\input manmac \ proofmodefalse". It's strongly recommended to buy this book (and all other books from Donald E. Knuth) instead of using the online version but the source is a great example of TeX input! 11.3.3. Pretty print a manual page     You can print a manual page in PostScript nicely by one of the following commands.     $ man -Tps some_manpage | lpr 11.3.4. Creating a manual page     Although writing a manual page (manpage) in the plain troff format is possible, there are few helper packages to create it. Table 11.13. List of packages to help creating the manpage +---------------------------------------------------------------+ | package |popcon|size| keyword | description | |--------------+------+----+------------+-----------------------| | |V:0, | | |converter from DocBook | |docbook-to-man|I:8 |191 |SGML→manpage|SGML into roff man | | | | | |macros | |--------------+------+----+------------+-----------------------|     |help2man |V:0, |542 |text→manpage|automatic manpage | | |I:7 | | |generator from --help | |--------------+------+----+------------+-----------------------| |info2man |V:0, |134 |info→manpage|converter from GNU info| | |I:0 | | |to POD or man pages | |--------------+------+----+------------+-----------------------| |txt2man |V:0, |112 |text→manpage|convert flat ASCII text| | |I:0 | | |to man page format | +---------------------------------------------------------------+ 11.4. Printable data Printable data is expressed in the PostScript format on the     Debian system. Common Unix Printing System (CUPS) uses Ghostscript as its rasterizer backend program for non-PostScript printers.     Printable data may also be expressed in the PDF format on the recent Debian system. PDF files can displayed and its form entries may be filled using     GUI viewer tools such as Evince and Okular (see Section 7.4, “GUI applications”); and modern browsers such as Chromium. PDF files can be edited using some graphics tools such as     LibreOffice, Scribus, and Inkscape (see Section 11.6, “Graphic data tools”). Tip     You can read a PDF file with GIMP and convert it into PNG format using higher than 300 dpi resolution. This may be used as a background image for LibreOffice to produce a desirable altered printout with minimum efforts. 11.4.1. Ghostscript     The core of printable data manipulation is the Ghostscript PostScript (PS) interpreter which generates raster image. Table 11.14. List of Ghostscript PostScript interpreters +---------------------------------------------------------------+ | package |popcon|size | description | |----------------+------+-----+---------------------------------| |ghostscript |V:161,|179 |The GPL Ghostscript PostScript/ | | |I:583 | |PDF interpreter | |----------------+------+-----+---------------------------------| |ghostscript-x |V:2, |87 |GPL Ghostscript PostScript/PDF | | |I:38 | |interpreter - X display support |     |----------------+------+-----+---------------------------------| |libpoppler102 |V:16, |4274 |PDF rendering library forked from| | |I:129 | |the xpdf PDF viewer | |----------------+------+-----+---------------------------------| |libpoppler-glib8|V:260,|484 |PDF rendering library (GLib-based| | |I:485 | |shared library) | |----------------+------+-----+---------------------------------| |poppler-data |V:134,|13086|CMaps for PDF rendering library | | |I:607 | |(for CJK support: Adobe-*) | +---------------------------------------------------------------+ Tip     "gs -h" can display the configuration of Ghostscript. 11.4.2. Merge two PS or PDF files     You can merge two PostScript (PS) or Portable Document Format (PDF) files using gs(1) of Ghostscript.     $ gs -q -dNOPAUSE -dBATCH -sDEVICE=pswrite -sOutputFile=bla.ps -f foo1.ps foo2.ps $ gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=bla.pdf -f foo1.pdf foo2.pdf Note     The PDF, which is a widely used cross-platform printable data format, is essentially the compressed PS format with few additional features and extensions. Tip     For command line, psmerge(1) and other commands from the psutils package are useful for manipulating PostScript documents. pdftk (1) from the pdftk package is useful for manipulating PDF documents, too. 11.4.3. Printable data utilities     The following packages for the printable data utilities caught my eyes. Table 11.15. List of printable data utilities +---------------------------------------------------------------+ | package |popcon|size| keyword | description | |-------------+------+----+-------------+-----------------------| | |V:152,| | |PDF utilities: pdftops,| |poppler-utils|I:471 |728 |pdf→ps,text,…|pdfinfo, pdfimages, | | | | | |pdftotext, pdffonts | |-------------+------+----+-------------+-----------------------| |psutils |V:4, |219 |ps→ps |PostScript document | | |I:67 | | |conversion tools | |-------------+------+----+-------------+-----------------------| |poster |V:0, |57 |ps→ps |create large posters | | |I:3 | | |out of PostScript pages| |-------------+------+----+-------------+-----------------------| | |V:1, | |text→ps, |convert ASCII text to | |enscript |I:14 |2130|html, rtf |PostScript, HTML, RTF | | | | | |or Pretty-Print | |-------------+------+----+-------------+-----------------------| | |V:0, | | |'Anything to | |a2ps |I:10 |3979|text→ps |PostScript' converter | | | | | |and pretty-printer | |-------------+------+----+-------------+-----------------------| |pdftk |I:37 |28 |pdf→pdf |PDF document conversion| | | | | |tool: pdftk | |-------------+------+----+-------------+-----------------------| |html2ps |V:0, |261 |html→ps |converter from HTML to | | |I:2 | | |PostScript | |-------------+------+----+-------------+-----------------------| |gnuhtml2latex|V:0, |27 |html→latex |converter from html to | | |I:0 | | |latex |     |-------------+------+----+-------------+-----------------------| | |V:0, | | |convert documents from | |latex2rtf |I:4 |495 |latex→rtf |LaTeX to RTF which can | | | | | |be read by MS Word | |-------------+------+----+-------------+-----------------------| | | | | |converter from | |ps2eps |V:2, |95 |ps→eps |PostScript to EPS | | |I:42 | | |(Encapsulated | | | | | |PostScript) | |-------------+------+----+-------------+-----------------------| | |V:0, | | |Text to PostScript | |e2ps |I:0 |109 |text→ps |converter with Japanese| | | | | |encoding support | |-------------+------+----+-------------+-----------------------| |impose+ |V:0, |118 |ps→ps |PostScript utilities | | |I:0 | | | | |-------------+------+----+-------------+-----------------------| | | | | |pretty print many | | | | | |source codes (C, C++, | |trueprint |V:0, |149 |text→ps |Java, Pascal, Perl, | | |I:0 | | |Pike, Sh, and Verilog) | | | | | |to PostScript. (C | | | | | |language) | |-------------+------+----+-------------+-----------------------| | |V:0, | | |converter from PDF to | |pdf2svg |I:3 |32 |pdf→svg |Scalable vector | | | | | |graphics format | |-------------+------+----+-------------+-----------------------| |pdftoipe |V:0, |65 |pdf→ipe |converter from PDF to | | |I:0 | | |IPE's XML format | +---------------------------------------------------------------+ 11.4.4. Printing with CUPS Both lp(1) and lpr(1) commands offered by Common Unix Printing     System (CUPS) provides options for customized printing the printable data.     You can print 3 copies of a file collated using one of the following commands.     $ lp -n 3 -o Collate=True filename     $ lpr -#3 -o Collate=True filename You can further customize printer operation by using printer     option such as "-o number-up=2", "-o page-set=even", "-o page-set =odd", "-o scaling=200", "-o natural-scaling=200", etc., documented at Command-Line Printing and Options. 11.5. The mail data conversion     The following packages for the mail data conversion caught my eyes. Table 11.16. List of packages to help mail data conversion +---------------------------------------------------------------+ | package |popcon|size|keyword| description | |---------+------+----+-------+---------------------------------| |sharutils|V:2, |1415|mail |shar(1), unshar(1), uuencode(1), | | |I:36 | | |uudecode(1) | |---------+------+----+-------+---------------------------------| |mpack |V:1, |108 |MIME |encoding and decoding of MIME | | |I:11 | | |messages: mpack(1) and munpack(1)|     |---------+------+----+-------+---------------------------------| | |V:0, | | |unpacking MIME attachments of | |tnef |I:6 |110 |ms-tnef|type "application/ms-tnef" which | | | | | |is a Microsoft only format | |---------+------+----+-------+---------------------------------| | | | | |encoder and decoder for the | |uudeview |V:0, |105 |mail |following formats: uuencode, | | |I:3 | | |xxencode, BASE64, quoted | | | | | |printable, and BinHex | +---------------------------------------------------------------+ Tip     The Internet Message Access Protocol version 4 (IMAP4) server may be used to move mails out from proprietary mail systems if the mail client software can be configured to use IMAP4 server too. 11.5.1. Mail data basics Mail (SMTP) data should be limited to series of 7 bit data. So binary data and 8 bit text data are encoded into 7 bit format     with the Multipurpose Internet Mail Extensions (MIME) and the selection of the charset (see Table 11.2, “List of encoding values and their usage”). The standard mail storage format is mbox formatted according to     RFC2822 (updated RFC822). See mbox(5) (provided by the mutt package). For European languages, "Content-Transfer-Encoding: quoted-printable" with the ISO-8859-1 charset is usually used for     mail since there are not much 8 bit characters. If European text is encoded in UTF-8, "Content-Transfer-Encoding: quoted-printable" is likely to be used since it is mostly 7 bit data. For Japanese, traditionally "Content-Type: text/plain; charset= ISO-2022-JP" is usually used for mail to keep text in 7 bits. But     older Microsoft systems may send mail data in Shift-JIS without proper declaration. If Japanese text is encoded in UTF-8, Base64 is likely to be used since it contains many 8 bit data. The situation of other Asian languages is similar. Note     If your non-Unix mail data is accessible by a non-Debian client software which can talk to the IMAP4 server, you may be able to move them out by running your own IMAP4 server. Note     If you use other mail storage formats, moving them to mbox format is the good first step. The versatile client program such as mutt (1) may be handy for this.     You can split mailbox contents to each message using procmail(1) and formail(1). Each mail message can be unpacked using munpack(1) from the mpack     package (or other specialized tools) to obtain the MIME encoded contents. 11.6. Graphic data tools Although GUI programs such as gimp(1) are very powerful, command     line tools such as imagemagick(1) are quite useful for automating image manipulation via scripts. The de facto image file format of the digital camera is the     Exchangeable Image File Format (EXIF) which is the JPEG image file format with additional metadata tags. It can hold information such as date, time, and camera settings. The Lempel-Ziv-Welch (LZW) lossless data compression patent has     been expired. Graphics Interchange Format (GIF) utilities which use the LZW compression method are now freely available on the Debian system. Tip     Any digital camera or scanner with removable recording media works with Linux through USB storage readers since it follows the Design rule for Camera Filesystem and uses FAT filesystem. See Section 10.1.7, “Removable storage device”. 11.6.1. Graphic data tools (metapackage) The following metapackages are good starting points for searching     graphics data tools using aptitude(8). "Packages overview for Debian PhotoTools Maintainers" can be another starting point. Table 11.17. List of graphics data tools (metapackage) +---------------------------------------------------------------+ | package |popcon|size|keyword| description | |------------------------+------+----+-------+------------------| |design-desktop-graphics |I:0 |13 |svg, |metapackage for | | | | |jpeg, …|graphics designers|     |------------------------+------+----+-------+------------------| | | | |svg, |metapackage for | |education-graphics |I:0 |30 |jpeg, …|teaching graphics | | | | | |and pictural art. | |------------------------+------+----+-------+------------------| |open-font-design-toolkit|I:0 |9 |ttf, |metapackage for | | | | |ps, … |open font design | +---------------------------------------------------------------+ Tip     Search more image tools using regex "~Gworks-with::image" in aptitude(8) (see Section 2.2.6, “Search method options with aptitude”). 11.6.2. Graphic data tools (GUI)     The following packages for the GUI graphics data conversion, editing, and organization tools caught my eyes. Table 11.18. List of graphics data tools (GUI) +---------------------------------------------------------------+ | package |popcon|size | keyword | description | |----------------+------+-----+----------+----------------------| |gimp |V:50, |19304|image |GNU Image Manipulation| | |I:252 | |(bitmap) |Program | |----------------+------+-----+----------+----------------------| | |V:12, | |image |GTK-based X11 frontend| |xsane |I:144 |2339 |(bitmap) |for SANE (Scanner | | | | | |Access Now Easy) | |----------------+------+-----+----------+----------------------| |scribus |V:1, |31345|ps/pdf/SVG|Scribus DTP editor | | |I:16 | |/… | | |----------------+------+-----+----------+----------------------| |libreoffice-draw|V:72, |10312|image |LibreOffice office | | |I:430 | |(vector) |suite - drawing | |----------------+------+-----+----------+----------------------| |inkscape |V:15, |99800|image |SVG (Scalable Vector | | |I:112 | |(vector) |Graphics) editor | |----------------+------+-----+----------+----------------------| |dia |V:2, |3741 |image |diagram editor (Gtk) | | |I:22 | |(vector) | | |----------------+------+-----+----------+----------------------| | |V:0, | |image |Facility for | |xfig |I:11 |7849 |(vector) |Interactive Generation| | | | | |of figures under X11 | |----------------+------+-----+----------+----------------------| |gocr |V:0, |540 |image→text|free OCR software | | |I:7 | | | | |----------------+------+-----+----------+----------------------| |eog |V:64, |7770 |image |Eye of GNOME graphics | | |I:277 | |(Exif) |viewer program | |----------------+------+-----+----------+----------------------| |gthumb |V:3, |5032 |image |image viewer and | | |I:16 | |(Exif) |browser (GNOME) | |----------------+------+-----+----------+----------------------| |geeqie |V:4, |2522 |image |image viewer using GTK| | |I:15 | |(Exif) | | |----------------+------+-----+----------+----------------------|     |shotwell |V:17, |6263 |image |digital photo | | |I:255 | |(Exif) |organizer (GNOME) | |----------------+------+-----+----------+----------------------| |gwenview |V:33, |11755|image |image viewer (KDE) | | |I:106 | |(Exif) | | |----------------+------+-----+----------+----------------------| |kamera |I:105 |998 |image |digital camera support| | | | |(Exif) |for KDE applications | |----------------+------+-----+----------+----------------------| | |V:1, | |image |digital photo | |digikam |I:9 |293 |(Exif) |management application| | | | | |for KDE | |----------------+------+-----+----------+----------------------| | |V:4, | |image |virtual lighttable and| |darktable |I:13 |30554|(Exif) |darkroom for | | | | | |photographers | |----------------+------+-----+----------+----------------------| |hugin |V:0, |5208 |image |panorama photo | | |I:8 | |(Exif) |stitcher | |----------------+------+-----+----------+----------------------| |librecad |V:1, |8963 |DXF, ... |2D CAD data editor | | |I:15 | | | | |----------------+------+-----+----------+----------------------| |freecad |I:18 |36 |DXF, ... |3D CAD data editor | |----------------+------+-----+----------+----------------------| | |V:3, | |blend, |3D content editor for | |blender |I:28 |84492|TIFF, |animation etc | | | | |VRML, … | | |----------------+------+-----+----------+----------------------| |mm3d |V:0, |3881 |ms3d, obj,|OpenGL based 3D model | | |I:0 | |dxf, … |editor | |----------------+------+-----+----------+----------------------| | |V:0, | | |font editor for PS, | |fontforge |I:6 |3993 |ttf, ps, …|TrueType and OpenType | | | | | |fonts | |----------------+------+-----+----------+----------------------| | |V:0, | | |program for | |xgridfit |I:0 |806 |ttf |gridfitting and | | | | | |hinting TrueType fonts| +---------------------------------------------------------------+ 11.6.3. Graphic data tools (CLI)     The following packages for the CLI graphics data conversion, editing, and organization tools caught my eyes. Table 11.19. List of graphics data tools (CLI) +---------------------------------------------------------------+ | package |popcon|size| keyword | description | |-----------------+------+----+-----------------+---------------| | | | | |image | |imagemagick |I:317 |74 |image(bitmap) |manipulation | | | | | |programs | |-----------------+------+----+-----------------+---------------| | | | | |image | |graphicsmagick |V:1, |5565|image(bitmap) |manipulation | | |I:11 | | |programs (fork | | | | | |of imagemagick)| |-----------------+------+----+-----------------+---------------| | |V:28, | | |graphics | |netpbm |I:326 |8526|image(bitmap) |conversion | | | | | |tools | |-----------------+------+----+-----------------+---------------| | | | | |convert High | | | | | |Efficiency | | | | | |Image File | | |V:0, | | |Format (HEIF) | |libheif-examples |I:2 |191 |heif→jpeg(bitmap)|to JPEG, PNG, | | | | | |or Y4M formats | | | | | |with | | | | | |heif-convert(1)| | | | | |command | |-----------------+------+----+-----------------+---------------| | | | | |convert MS | | | | | |Windows icons | |icoutils |V:7, |221 |png↔ico(bitmap) |and cursors to | | |I:50 | | |and from PNG | | | | | |formats | | | | | |(favicon.ico) | |-----------------+------+----+-----------------+---------------| | | | | |PostScript and | | |V:2, | |ps/pdf→image |PDF files to | |pstoedit |I:52 |1011|(vector) |editable vector| | | | | |graphics | | | | | |converter (SVG)| |-----------------+------+----+-----------------+---------------| | | | | |Windows | | | | | |metafile | |libwmf-bin |V:7, |151 |Windows/image |(vector | | |I:119 | |(vector) |graphics data) | | | | | |conversion | | | | | |tools | |-----------------+------+----+-----------------+---------------| | | | | |convert XFig | |fig2sxd |V:0, |151 |fig→sxd(vector) |files to | | |I:0 | | |OpenOffice.org | | | | | |Draw format | |-----------------+------+----+-----------------+---------------| | | | | |post-processing| |unpaper |V:2, |412 |image→image |tool for | | |I:17 | | |scanned pages | | | | | |for OCR | |-----------------+------+----+-----------------+---------------| | | | | |free OCR | | |V:7, | | |software based | |tesseract-ocr |I:33 |2228|image→text |on the HP's | | | | | |commercial OCR | | | | | |engine | |-----------------+------+----+-----------------+---------------| | | | | |OCR engine | | | | | |data: |     |tesseract-ocr-eng|V:7, |4032|image→text |tesseract-ocr | | |I:34 | | |language files | | | | | |for English | | | | | |text | |-----------------+------+----+-----------------+---------------| |ocrad |V:0, |587 |image→text |free OCR | | |I:3 | | |software | |-----------------+------+----+-----------------+---------------| | | | | |command-line | | |V:2, | | |utility to show| |exif |I:42 |339 |image(Exif) |EXIF | | | | | |information in | | | | | |JPEG files | |-----------------+------+----+-----------------+---------------| | | | | |EXIF/IPTC | |exiv2 |V:2, |275 |image(Exif) |metadata | | |I:27 | | |manipulation | | | | | |tool | |-----------------+------+----+-----------------+---------------| | |V:1, | | |transform | |exiftran |I:14 |69 |image(Exif) |digital camera | | | | | |jpeg images | |-----------------+------+----+-----------------+---------------| | | | | |utility to read| | |V:0, | | |Exif tags from | |exiftags |I:3 |292 |image(Exif) |a digital | | | | | |camera JPEG | | | | | |file | |-----------------+------+----+-----------------+---------------| | |V:0, | | |read metadata | |exifprobe |I:3 |499 |image(Exif) |from digital | | | | | |pictures | |-----------------+------+----+-----------------+---------------| | |V:1, | | |decode raw | |dcraw |I:12 |583 |image(Raw)→ppm |digital camera | | | | | |images | |-----------------+------+----+-----------------+---------------| | | | | |find visually | |findimagedupes |V:0, |77 |image→fingerprint|similar or | | |I:1 | | |duplicate | | | | | |images | |-----------------+------+----+-----------------+---------------| | | | | |merge images to| |ale |V:0, |839 |image→image |increase | | |I:0 | | |fidelity or | | | | | |create mosaics | |-----------------+------+----+-----------------+---------------| | |V:0, | | |generate static| |imageindex |I:1 |145 |image(Exif)→html |HTML galleries | | | | | |from images | |-----------------+------+----+-----------------+---------------| | |V:0, | | |universal | |outguess |I:1 |230 |jpeg,png |Steganographic | | | | | |tool | |-----------------+------+----+-----------------+---------------| |jpegoptim |V:0, |59 |jpeg |optimize JPEG | | |I:7 | | |files | |-----------------+------+----+-----------------+---------------| | |V:3, | | |optimize PNG | |optipng |I:43 |213 |png |files, lossless| | | | | |compression | |-----------------+------+----+-----------------+---------------| | |V:0, | | |optimize PNG | |pngquant |I:9 |61 |png |files, lossy | | | | | |compression | +---------------------------------------------------------------+ 11.7. Miscellaneous data conversion There are many other programs for converting data. Following     packages caught my eyes using regex "~Guse::converting" in aptitude(8) (see Section 2.2.6, “Search method options with aptitude”). Table 11.20. List of miscellaneous data conversion tools +---------------------------------------------------------------+ | package |popcon|size | keyword | description | |---------+------+-----+---------+------------------------------| | |V:1, | |rpm/ |converter for the foreign | |alien |I:19 |163 |tgz→deb |package into the Debian | | | | | |package |     |---------+------+-----+---------+------------------------------| | | | | |converter from "Electric Book"| |freepwing|V:0, |424 |EB→EPWING|(popular in Japan) to a single| | |I:0 | | |JIS X 4081 format (a subset of| | | | | |the EPWING V1) | |---------+------+-----+---------+------------------------------| |calibre |V:6, |63385|any→EPUB |e-book converter and library | | |I:28 | | |management | +---------------------------------------------------------------+     You can also extract data from RPM format with the following.     $ rpm2cpio file.src.rpm | cpio --extract Chapter 12. Programming I provide some pointers for people to learn programming on the     Debian system enough to trace the packaged source code. Here are notable packages and corresponding documentation packages for programming. Online references are available by typing "man name" after installing manpages and manpages-dev packages. Online references for the GNU tools are available by typing "info program_name"     after installing the pertinent documentation packages. You may need to include the contrib and non-free archives in addition to the main archive since some GFDL documentations are not considered to be DFSG compliant.     Please consider to use version control system tools. See Section 10.5, “Git”. Warning     Do not use "test" as the name of an executable test file. "test" is a shell builtin. Caution     You should install software programs directly compiled from source into "/usr/local" or "/opt" to avoid collision with system programs. Tip     Code examples of creating "Song 99 Bottles of Beer" should give you good ideas of practically all the programming languages. 12.1. The shell script     The shell script is a text file with the execution bit set and contains the commands in the following format.     #!/bin/sh ... command lines     The first line specifies the shell interpreter which read and execute this file contents. Reading shell scripts is the best way to understand how a Unix-like system works. Here, I give some pointers and reminders     for shell programming. See "Shell Mistakes" (https:// www.greenend.org.uk/rjk/2001/04/shell.html) to learn from mistakes. Unlike shell interactive mode (see Section 1.5, “The simple shell     command” and Section 1.6, “Unix-like text processing”), shell scripts frequently use parameters, conditionals, and loops. 12.1.1. POSIX shell compatibility     Many system scripts may be interpreted by any one of POSIX shells (see Table 1.13, “List of shell programs”). * The default non-interactive POSIX shell "/usr/bin/sh" is a symlink pointing to /usr/bin/dash and used by many system     programs. * The default interactive POSIX shell is /usr/bin/bash. Avoid writing a shell script with bashisms or zshisms to make it     portable among all POSIX shells. You can check it using checkbashisms(1). Table 12.1. List of typical bashisms +---------------------------------------------------------------+ | Good: POSIX | Avoid: bashism | |-------------------------------+-------------------------------| |if [ "$foo" = "$bar" ] ; then …|if [ "$foo" == "$bar" ] ; then | | |… | |-------------------------------+-------------------------------|     |diff -u file.c.orig file.c |diff -u file.c{.orig,} | |-------------------------------+-------------------------------| |mkdir /foobar /foobaz |mkdir /foo{bar,baz} | |-------------------------------+-------------------------------| |funcname() { … } |function funcname() { … } | |-------------------------------+-------------------------------| |octal format: "\377" |hexadecimal format: "\xff" | +---------------------------------------------------------------+     The "echo" command must be used with following cares since its implementation differs among shell builtin and external commands. * Avoid using any command options except "-n".     * Avoid using escape sequences in the string since their handling varies. Note     Although "-n" option is not really POSIX syntax, it is generally accepted. Tip     Use the "printf" command instead of the "echo" command if you need to embed escape sequences in the output string. 12.1.2. Shell parameters     Special shell parameters are frequently used in the shell script. Table 12.2. List of shell parameters +-----------------------------------------------------------+ |shell parameter| value | |---------------+-------------------------------------------| |$0 |name of the shell or shell script | |---------------+-------------------------------------------| |$1 |first (1st) shell argument | |---------------+-------------------------------------------| |$9 |ninth (9th) shell argument | |---------------+-------------------------------------------|     |$# |number of positional parameters | |---------------+-------------------------------------------| |"$*" |"$1 $2 $3 $4 … " | |---------------+-------------------------------------------| |"$@" |"$1" "$2" "$3" "$4" … | |---------------+-------------------------------------------| |$? |exit status of the most recent command | |---------------+-------------------------------------------| |$$ |PID of this shell script | |---------------+-------------------------------------------| |$! |PID of most recently started background job| +-----------------------------------------------------------+     Basic parameter expansions to remember are as follows. Table 12.3. List of shell parameter expansions +---------------------------------------------------------------+ | parameter |value if var | value if var is not set | | expression form | is set | | |-----------------+-------------+-------------------------------| |${var:-string} |"$var" |"string" |     |-----------------+-------------+-------------------------------| |${var:+string} |"string" |"null" | |-----------------+-------------+-------------------------------| |${var:=string} |"$var" |"string" (and run "var=string")| |-----------------+-------------+-------------------------------| |${var:?string} |"$var" |echo "string" to stderr (and | | | |exit with error) | +---------------------------------------------------------------+     Here, the colon ":" in all of these operators is actually optional. * with ":" = operator test for exist and not null     * without ":" = operator test for exist only Table 12.4. List of key shell parameter substitutions +----------------------------------------------------------+ |parameter substitution form| result | |---------------------------+------------------------------| |${var%suffix} |remove smallest suffix pattern|     |---------------------------+------------------------------| |${var%%suffix} |remove largest suffix pattern | |---------------------------+------------------------------| |${var#prefix} |remove smallest prefix pattern| |---------------------------+------------------------------| |${var##prefix} |remove largest prefix pattern | +----------------------------------------------------------+ 12.1.3. Shell conditionals     Each command returns an exit status which can be used for conditional expressions. * Success: 0 ("True")     * Error: non 0 ("False") Note     "0" in the shell conditional context means "True", while "0" in the C conditional context means "False". Note     "[" is the equivalent of the test command, which evaluates its arguments up to "]" as a conditional expression.     Basic conditional idioms to remember are the following. * "command && if_success_run_this_command_too || true"     * "command || if_not_success_run_this_command_too || true" * A multi-line script snippet as the following if [ conditional_expression ]; then if_success_run_this_command     else if_not_success_run_this_command fi Here trailing "|| true" was needed to ensure this shell script     does not exit at this line accidentally when shell is invoked with "-e" flag. Table 12.5. List of file comparison operators in the conditional expression +---------------------------------------------------------------+ | equation | condition to return logical true | |------------+--------------------------------------------------| |-e file |file exists | |------------+--------------------------------------------------| |-d file |file exists and is a directory | |------------+--------------------------------------------------| |-f file |file exists and is a regular file | |------------+--------------------------------------------------|     |-w file |file exists and is writable | |------------+--------------------------------------------------| |-x file |file exists and is executable | |------------+--------------------------------------------------| |file1 -nt |file1 is newer than file2 (modification) | |file2 | | |------------+--------------------------------------------------| |file1 -ot |file1 is older than file2 (modification) | |file2 | | |------------+--------------------------------------------------| |file1 -ef |file1 and file2 are on the same device and the | |file2 |same inode number | +---------------------------------------------------------------+ Table 12.6. List of string comparison operators in the conditional expression +------------------------------------------------------+ | equation | condition to return logical true | |------------+-----------------------------------------| |-z str |the length of str is zero | |------------+-----------------------------------------| |-n str |the length of str is non-zero |     |------------+-----------------------------------------| |str1 = str2 |str1 and str2 are equal | |------------+-----------------------------------------| |str1 != str2|str1 and str2 are not equal | |------------+-----------------------------------------| |str1 < str2 |str1 sorts before str2 (locale dependent)| |------------+-----------------------------------------| |str1 > str2 |str1 sorts after str2 (locale dependent) | +------------------------------------------------------+     Arithmetic integer comparison operators in the conditional expression are "-eq", "-ne", "-lt", "-le", "-gt", and "-ge". 12.1.4. Shell loops     There are several loop idioms to use in POSIX shell. * "for x in foo1 foo2 … ; do command ; done" loops by assigning items from the list "foo1 foo2 …" to variable "x" and executing "command". * "while condition ; do command ; done" repeats "command" while "condition" is true.     * "until condition ; do command ; done" repeats "command" while "condition" is not true. * "break" enables to exit from the loop. * "continue" enables to resume the next iteration of the loop. Tip     The C-language like numeric iteration can be realized by using seq(1) as the "foo1 foo2 …" generator. Tip     See Section 9.4.9, “Repeating a command looping over files”. 12.1.5. Shell environment variables Some popular environment variables for the normal shell command     prompt may not be available under the execution environment of your script. * For "$USER", use "$(id -un)" * For "$UID", use "$(id -u)"     * For "$HOME", use "$(getent passwd "$(id -u)"|cut -d ":" -f 6) " (this works also on Section 4.5.2, “The modern centralized system management”) 12.1.6. The shell command-line processing sequence     The shell processes a script roughly as the following sequence. * The shell reads a line. * The shell groups a part of the line as one token if it is within "…" or '…'. * The shell splits other part of a line into tokens by the following. + Whitespaces: space tab newline + Metacharacters: < > | ; & ( ) * The shell checks the reserved word for each token to adjust its behavior if not within "…" or '…'. + reserved word: if then elif else fi for in while unless do done case esac * The shell expands alias if not within "…" or '…'. * The shell expands tilde if not within "…" or '…'. + "~" → current user's home directory + "~user" → user's home directory * The shell expands parameter to its value if not within '…'.     + parameter: "$PARAMETER" or "${PARAMETER}" * The shell expands command substitution if not within '…'. + "$( command )" → the output of "command" + "` command `" → the output of "command" * The shell expands pathname glob to matching file names if not within "…" or '…'. + * → any characters + ? → one character + […] → any one of the characters in "…" * The shell looks up command from the following and execute it. + function definition + builtin command + executable file in "$PATH" * The shell goes to the next line and repeats this process again from the top of this sequence.     Single quotes within double quotes have no effect. Executing "set -x" in the shell or invoking the shell with "-x"     option make the shell to print all of commands executed. This is quite handy for debugging. 12.1.7. Utility programs for shell script In order to make your shell program as portable as possible     across Debian systems, it is a good idea to limit utility programs to ones provided by essential packages. * "aptitude search ~E" lists essential packages.     * "dpkg -L package_name |grep '/man/man.*/'" lists manpages for commands offered by package_name package. Table 12.7. List of packages containing small utility programs for shell scripts +---------------------------------------------------------------+ | package | popcon |size | description | |-------------+---------+-----+---------------------------------| |dash |V:884, |191 |small and fast POSIX-compliant | | |I:997 | |shell for sh | |-------------+---------+-----+---------------------------------| |coreutils |V:880, |18307|GNU core utilities | | |I:999 | | | |-------------+---------+-----+---------------------------------| |grep |V:782, |1266 |GNU grep, egrep and fgrep | | |I:999 | | | |-------------+---------+-----+---------------------------------| |sed |V:790, |987 |GNU sed |     | |I:999 | | | |-------------+---------+-----+---------------------------------| |mawk |V:442, |285 |small and fast awk | | |I:997 | | | |-------------+---------+-----+---------------------------------| |debianutils |V:907, |224 |miscellaneous utilities specific | | |I:999 | |to Debian | |-------------+---------+-----+---------------------------------| |bsdutils |V:519, |356 |basic utilities from 4.4BSD-Lite | | |I:999 | | | |-------------+---------+-----+---------------------------------| |bsdextrautils|V:596, |339 |extra utilities from 4.4BSD-Lite | | |I:713 | | | |-------------+---------+-----+---------------------------------| |moreutils |V:15, |231 |additional Unix utilities | | |I:38 | | | +---------------------------------------------------------------+ Tip     Although moreutils may not exist outside of Debian, it offers interesting small programs. Most notable one is sponge(8) which is quite useful when you wish to overwrite original file.     See Section 1.6, “Unix-like text processing” for examples. 12.2. Scripting in interpreted languages Table 12.8. List of interpreter related packages +---------------------------------------------------------------+ | package |popcon|size| documentation | |-------------------------+------+----+-------------------------| | |V:884,| |sh: small and fast | |dash |I:997 |191 |POSIX-compliant shell for| | | | |sh | |-------------------------+------+----+-------------------------| |bash |V:838,|7175|sh: "info bash" provided | | |I:999 | |by bash-doc | |-------------------------+------+----+-------------------------| |mawk |V:442,|285 |AWK: small and fast awk | | |I:997 | | | |-------------------------+------+----+-------------------------| |gawk |V:285,|2906|AWK: "info gawk" provided| | |I:349 | |by gawk-doc | |-------------------------+------+----+-------------------------| | | | |Perl: perl(1) and html | |perl |V:707,|673 |pages provided by | | |I:989 | |perl-doc and | | | | |perl-doc-html | |-------------------------+------+----+-------------------------| | |V:2, | |Perl extension for the |     |libterm-readline-gnu-perl|I:29 |380 |GNU ReadLine/History | | | | |Library: perlsh(1) | |-------------------------+------+----+-------------------------| |libreply-perl |V:0, |171 |REPL for Perl: reply(1) | | |I:0 | | | |-------------------------+------+----+-------------------------| |libdevel-repl-perl |V:0, |237 |REPL for Perl: re.pl(1) | | |I:0 | | | |-------------------------+------+----+-------------------------| | |V:718,| |Python: python3(1) and | |python3 |I:953 |81 |html pages provided by | | | | |python3-doc | |-------------------------+------+----+-------------------------| | |V:25, | |Tcl: tcl(3) and detail | |tcl |I:218 |21 |manual pages provided by | | | | |tcl-doc | |-------------------------+------+----+-------------------------| | |V:20, | |Tk: tk(3) and detail | |tk |I:211 |21 |manual pages provided by | | | | |tk-doc | |-------------------------+------+----+-------------------------| |ruby |V:86, |29 |Ruby: ruby(1), erb(1), | | |I:208 | |irb(1), rdoc(1), ri(1) | +---------------------------------------------------------------+ When you wish to automate a task on Debian, you should script it     with an interpreted language first. The guide line for the choice of the interpreted language is: * Use dash, if the task is a simple one which combines CLI programs with a shell program. * Use python3, if the task isn't a simple one and you are     writing it from scratch. * Use perl, tcl, ruby, ... if there is an existing code using one of these languages on Debian which needs to be touched up to do the task. If the resulting code is too slow, you can rewrite only the     critical portion for the execution speed in a compiled language and call it from the interpreted language. 12.2.1. Debugging interpreted language codes     Most interpreters offer basic syntax check and code tracing functionalities. * “dash -n script.sh” - Syntax check of a Shell script * “dash -x script.sh” - Trace a Shell script * “python -m py_compile script.py” - Syntax check of a Python script     * “python -mtrace --trace script.py” - Trace a Python script * “perl -I ../libpath -c script.pl” - Syntax check of a Perl script * “perl -d:Trace script.pl” - Trace a Perl script     For testing code for dash, try Section 9.1.4, “Readline wrapper” which accommodates bash-like interactive environment. For testing code for perl, try REPL environment for Perl which     accommodates Python-like REPL (=READ + EVAL + PRINT + LOOP) environment for Perl. 12.2.2. GUI program with the shell script The shell script can be improved to create an attractive GUI     program. The trick is to use one of so-called dialog programs instead of dull interaction using echo and read commands. Table 12.9. List of dialog programs +---------------------------------------------------------------+ | package |popcon|size| description | |---------+------+----+-----------------------------------------| |x11-utils|V:192,|651 |xmessage(1): display a message or query | | |I:566 | |in a window (X) | |---------+------+----+-----------------------------------------| |whiptail |V:284,|56 |displays user-friendly dialog boxes from | | |I:996 | |shell scripts (newt) | |---------+------+----+-----------------------------------------|     |dialog |V:11, |1227|displays user-friendly dialog boxes from | | |I:99 | |shell scripts (ncurses) | |---------+------+----+-----------------------------------------| |zenity |V:76, |183 |display graphical dialog boxes from shell| | |I:363 | |scripts (GTK) | |---------+------+----+-----------------------------------------| |ssft |V:0, |75 |Shell Scripts Frontend Tool (wrapper for | | |I:0 | |zenity, kdialog, and dialog with gettext)| |---------+------+----+-----------------------------------------| |gettext |V:56, |5818|"/usr/bin/gettext.sh": translate message | | |I:259 | | | +---------------------------------------------------------------+     Here is an example of GUI program to demonstrate how easy it is just with a shell script.     This script uses zenity to select a file (default /etc/motd) and display it.     GUI launcher for this script can be created following Section 9.4.10, “Starting a program from GUI”. #!/bin/sh -e # Copyright (C) 2021 Osamu Aoki , Public Domain # vim:set sw=2 sts=2 et: DATA_FILE=$(zenity --file-selection --filename="/etc/motd" --title="Select a file to check") || \ ( echo "E: File selection error" >&2 ; exit 1 ) # Check size of archive     if ( file -ib "$DATA_FILE" | grep -qe '^text/' ) ; then zenity --info --title="Check file: $DATA_FILE" --width 640 --height 400 \ --text="$(head -n 20 "$DATA_FILE")" else zenity --info --title="Check file: $DATA_FILE" --width 640 --height 400 \ --text="The data is MIME=$(file -ib "$DATA_FILE")" fi This kind of approach to GUI program with the shell script is     useful only for simple choice cases. If you are to write any program with complexities, please consider writing it on more capable platform. 12.2.3. Custom actions for GUI filer GUI filer programs can be extended to perform some popular     actions on selected files using additional extension packages. They can also made to perform very specific custom actions by adding your specific scripts. * For GNOME, see NautilusScriptsHowto. * For KDE, see Creating Dolphin Service Menus.     * For Xfce, see Thunar - Custom Actions and https:// help.ubuntu.com/community/ThunarCustomActions. * For LXDE, see Custom Actions. 12.2.4. Perl short script madness In order to process data, sh needs to spawn sub-process running     cut, grep, sed, etc., and is slow. On the other hand, perl has internal capabilities to process data, and is fast. So many system maintenance scripts on Debian use perl.     Let's think following one-liner AWK script snippet and its equivalents in Perl.     awk '($2=="1957") { print $3 }' |     This is equivalent to any one of the following lines.     perl -ne '@f=split; if ($f[1] eq "1957") { print "$f[2]\n"}' |     perl -ne 'if ((@f=split)[1] eq "1957") { print "$f[2]\n"}' |     perl -ne '@f=split; print $f[2] if ( $f[1]==1957 )' |     perl -lane 'print $F[2] if $F[1] eq "1957"' |     perl -lane 'print$F[2]if$F[1]eq+1957' |     The last one is a riddle. It took advantage of following Perl features. * The whitespace is optional. * The automatic conversion exists from number to the string.     * Perl execution tricks via command line options: perlrun(1) * Perl special variables: perlvar(1)     This flexibility is the strength of Perl. At the same time, this allows us to create cryptic and tangled codes. So be careful. 12.3. Coding in compiled languages Table 12.10. List of compiler related packages +---------------------------------------------------------------+ | package |popcon |size | description | |----------------+-------+-----+--------------------------------| |gcc |V:167, |36 |GNU C compiler | | |I:550 | | | |----------------+-------+-----+--------------------------------| |libc6-dev |V:248, |12053|GNU C Library: Development | | |I:567 | |Libraries and Header Files | |----------------+-------+-----+--------------------------------| |g++ |V:56, |13 |GNU C++ compiler | | |I:501 | | | |----------------+-------+-----+--------------------------------| |libstdc++-10-dev|V:14, |17537|GNU Standard C++ Library v3 | | |I:165 | |(development files) | |----------------+-------+-----+--------------------------------| |cpp |V:334, |18 |GNU C preprocessor | | |I:727 | | | |----------------+-------+-----+--------------------------------| |gettext |V:56, |5818 |GNU Internationalization | | |I:259 | |utilities | |----------------+-------+-----+--------------------------------| |glade |V:0, |1204 |GTK User Interface Builder | | |I:5 | | | |----------------+-------+-----+--------------------------------| |valac |V:0, |725 |C# like language for the GObject| | |I:4 | |system |     |----------------+-------+-----+--------------------------------| |flex |V:7, |1243 |LEX-compatible fast lexical | | |I:73 | |analyzer generator | |----------------+-------+-----+--------------------------------| |bison |V:7, |3116 |YACC-compatible parser generator| | |I:80 | | | |----------------+-------+-----+--------------------------------| |susv2 |I:0 |16 |fetch "The Single UNIX | | | | |Specifications v2" | |----------------+-------+-----+--------------------------------| |susv3 |I:0 |16 |fetch "The Single UNIX | | | | |Specifications v3" | |----------------+-------+-----+--------------------------------| |susv4 |I:0 |16 |fetch "The Single UNIX | | | | |Specifications v4" | |----------------+-------+-----+--------------------------------| |golang |I:20 |11 |Go programming language compiler| |----------------+-------+-----+--------------------------------| |rustc |V:3, |8860 |Rust systems programming | | |I:14 | |language | |----------------+-------+-----+--------------------------------| |haskell-platform|I:1 |12 |Standard Haskell libraries and | | | | |tools | |----------------+-------+-----+--------------------------------| |gfortran |V:6, |15 |GNU Fortran 95 compiler | | |I:62 | | | |----------------+-------+-----+--------------------------------| |fpc |I:2 |103 |Free Pascal | +---------------------------------------------------------------+ Here, Section 12.3.3, “Flex — a better Lex” and Section 12.3.4,     “Bison — a better Yacc” are included to indicate how compiler-like program can be written in C language by compiling higher level description into C language. 12.3.1. C     You can set up proper environment to compile programs written in the C programming language by the following.     # apt-get install glibc-doc manpages-dev libc6-dev gcc build-essential The libc6-dev package, i.e., GNU C Library, provides C standard     library which is collection of header files and library routines used by the C programming language.     See references for C as the following. * "info libc" (C library function reference) * gcc(1) and "info gcc"     * each_C_library_function_name(3) * Kernighan & Ritchie, "The C Programming Language", 2nd edition (Prentice Hall) 12.3.2. Simple C program (gcc)     A simple example "example.c" can compiled with a library "libm" into an executable "run_example" by the following. $ cat > example.c << EOF #include #include #include int main(int argc, char **argv, char **envp){ double x; char y[11]; x=sqrt(argc+7.5);     strncpy(y, argv[0], 10); /* prevent buffer overflow */ y[10] = '\0'; /* fill to make sure string ends with '\0' */ printf("%5i, %5.3f, %10s, %10s\n", argc, x, y, argv[1]); return 0; } EOF $ gcc -Wall -g -o run_example example.c -lm $ ./run_example 1, 2.915, ./run_exam, (null) $ ./run_example 1234567890qwerty 2, 3.082, ./run_exam, 1234567890qwerty Here, "-lm" is needed to link library "/usr/lib/libm.so" from the     libc6 package for sqrt(3). The actual library is in "/lib/" with filename "libm.so.6", which is a symlink to "libm-2.7.so".     Look at the last parameter in the output text. There are more than 10 characters even though "%10s" is specified. The use of pointer memory operation functions without boundary     checks, such as sprintf(3) and strcpy(3), is deprecated to prevent buffer overflow exploits that leverage the above overrun effects. Instead, use snprintf(3) and strncpy(3). 12.3.3. Flex — a better Lex     Flex is a Lex-compatible fast lexical analyzer generator.     Tutorial for flex(1) can be found in "info flex".     Many simple examples can be found under "/usr/share/doc/flex/ examples/". ^[7] 12.3.4. Bison — a better Yacc     Several packages provide a Yacc-compatible lookahead LR parser or LALR parser generator in Debian. Table 12.11. List of Yacc-compatible LALR parser generators +---------------------------------------------------------------+ |package| popcon |size| description | |-------+--------+----+-----------------------------------------| |bison |V:7, |3116|GNU LALR parser generator |     | |I:80 | | | |-------+--------+----+-----------------------------------------| |byacc |V:0, I:4|258 |Berkeley LALR parser generator | |-------+--------+----+-----------------------------------------| |btyacc |V:0, I:0|243 |backtracking parser generator based on | | | | |byacc | +---------------------------------------------------------------+     Tutorial for bison(1) can be found in "info bison". You need to provide your own "main()" and "yyerror()". "main()"     calls "yyparse()" which calls "yylex()", usually created with Flex.     Here is an example to create a simple terminal calculator program.     Let's create example.y: /* calculator source for bison */ %{ #include extern int yylex(void); extern int yyerror(char *); %} /* declare tokens */ %token NUMBER %token OP_ADD OP_SUB OP_MUL OP_RGT OP_LFT OP_EQU %% calc: | calc exp OP_EQU { printf("Y: RESULT = %d\n", $2); } ; exp: factor | exp OP_ADD factor { $$ = $1 + $3; } | exp OP_SUB factor { $$ = $1 - $3; }     ; factor: term | factor OP_MUL term { $$ = $1 * $3; } ; term: NUMBER | OP_LFT exp OP_RGT { $$ = $2; } ; %% int main(int argc, char **argv) { yyparse(); } int yyerror(char *s) { fprintf(stderr, "error: '%s'\n", s); }     Let's create, example.l: /* calculator source for flex */ %{ #include "example.tab.h" %} %% [0-9]+ { printf("L: NUMBER = %s\n", yytext); yylval = atoi(yytext); return NUMBER; }     "+" { printf("L: OP_ADD\n"); return OP_ADD; } "-" { printf("L: OP_SUB\n"); return OP_SUB; } "*" { printf("L: OP_MUL\n"); return OP_MUL; } "(" { printf("L: OP_LFT\n"); return OP_LFT; } ")" { printf("L: OP_RGT\n"); return OP_RGT; } "=" { printf("L: OP_EQU\n"); return OP_EQU; } "exit" { printf("L: exit\n"); return YYEOF; } /* YYEOF = 0 */ . { /* ignore all other */ } %%     Then execute as follows from the shell prompt to try this: $ bison -d example.y $ flex example.l $ gcc -lfl example.tab.c lex.yy.c -o example $ ./example 1 + 2 * ( 3 + 1 ) = L: NUMBER = 1 L: OP_ADD L: NUMBER = 2 L: OP_MUL     L: OP_LFT L: NUMBER = 3 L: OP_ADD L: NUMBER = 1 L: OP_RGT L: OP_EQU Y: RESULT = 9 exit L: exit 12.4. Static code analysis tools     Lint like tools can help automatic static code analysis.     Indent like tools can help human code reviews by reformatting source codes consistently.     Ctags like tools can help human code reviews by generating an index (or tag) file of names found in source codes. Tip Configuring your favorite editor (emacs or vim) to use     asynchronous lint engine plugins helps your code writing. These plugins are getting very powerful by taking advantage of Language Server Protocol. Since they are moving fast, using their upstream code instead of Debian package may be a good option. Table 12.12. List of tools for static code analysis +---------------------------------------------------------------+ | package |popcon|size | description | |---------------+------+-----+----------------------------------| |vim-ale |I:0 |2591 |Asynchronous Lint Engine for Vim 8| | | | |and NeoVim | |---------------+------+-----+----------------------------------| |vim-syntastic |I:3 |1379 |Syntax checking hacks for vim | |---------------+------+-----+----------------------------------| |elpa-flycheck |V:0, |808 |modern on-the-fly syntax checking | | |I:1 | |for Emacs | |---------------+------+-----+----------------------------------| |elpa-relint |V:0, |147 |Emacs Lisp regexp mistake finder | | |I:0 | | | |---------------+------+-----+----------------------------------| |cppcheck-gui |V:0, |7224 |tool for static C/C++ code | | |I:1 | |analysis (GUI) | |---------------+------+-----+----------------------------------| |shellcheck |V:2, |18987|lint tool for shell scripts | | |I:13 | | | |---------------+------+-----+----------------------------------| |pyflakes3 |V:2, |20 |passive checker of Python 3 | | |I:15 | |programs | |---------------+------+-----+----------------------------------| |pylint |V:4, |2018 |Python code static checker | | |I:20 | | | |---------------+------+-----+----------------------------------| |perl |V:707,|673 |interpreter with internal static | | |I:989 | |code checker: B::Lint(3perl) | |---------------+------+-----+----------------------------------| |rubocop |V:0, |3247 |Ruby static code analyzer | | |I:0 | | | |---------------+------+-----+----------------------------------| |clang-tidy |V:2, |21 |clang-based C++ linter tool |     | |I:11 | | | |---------------+------+-----+----------------------------------| |splint |V:0, |2320 |tool for statically checking C | | |I:2 | |programs for bugs | |---------------+------+-----+----------------------------------| |flawfinder |V:0, |205 |tool to examine C/C++ source code | | |I:0 | |and looks for security weaknesses | |---------------+------+-----+----------------------------------| |black |V:3, |660 |uncompromising Python code | | |I:13 | |formatter | |---------------+------+-----+----------------------------------| |perltidy |V:0, |2493 |Perl script indenter and | | |I:4 | |reformatter | |---------------+------+-----+----------------------------------| |indent |V:0, |431 |C language source code formatting | | |I:7 | |program | |---------------+------+-----+----------------------------------| |astyle |V:0, |785 |Source code indenter for C, C++, | | |I:2 | |Objective-C, C#, and Java | |---------------+------+-----+----------------------------------| |bcpp |V:0, |111 |C(++) beautifier | | |I:0 | | | |---------------+------+-----+----------------------------------| |xmlindent |V:0, |53 |XML stream reformatter | | |I:1 | | | |---------------+------+-----+----------------------------------| |global |V:0, |1908 |Source code search and browse | | |I:2 | |tools | |---------------+------+-----+----------------------------------| |exuberant-ctags|V:2, |341 |build tag file indexes of source | | |I:20 | |code definitions | |---------------+------+-----+----------------------------------| |universal-ctags|V:1, |3386 |build tag file indexes of source | | |I:11 | |code definitions | +---------------------------------------------------------------+ 12.5. Debug Debug is important part of programming activities. Knowing how to     debug programs makes you a good Debian user who can produce meaningful bug reports. Table 12.13. List of debug packages +-------------------------------------------------------+ |package| popcon |size | documentation |     |-------+----------+-----+------------------------------| |gdb |V:14, I:96|11637|"info gdb" provided by gdb-doc| |-------+----------+-----+------------------------------| |ddd |V:0, I:7 |4105 |"info ddd" provided by ddd-doc| +-------------------------------------------------------+ 12.5.1. Basic gdb execution     Primary debugger on Debian is gdb(1) which enables you to inspect a program while it executes.     Let's install gdb and related programs by the following.     # apt-get install gdb gdb-doc build-essential devscripts     Good tutorial of gdb can be found: * “info gdb”     * “Debugging with GDB” in /usr/share/doc/gdb-doc/html/gdb/ index.html * “tutorial on the web”     Here is a simple example of using gdb(1) on a "program" compiled with the "-g" option to produce debugging information. $ gdb program (gdb) b 1 # set break point at line 1 (gdb) run args # run program with args (gdb) next # next line ...     (gdb) step # step forward ... (gdb) p parm # print parm ... (gdb) p parm=12 # set value to 12 ... (gdb) quit Tip     Many gdb(1) commands can be abbreviated. Tab expansion works as in the shell. 12.5.2. Debugging the Debian package Since all installed binaries should be stripped on the Debian system by default, most debugging symbols are removed in the normal package. In order to debug Debian packages with gdb(1), *-dbgsym packages need to be installed (e.g. coreutils-dbgsym in     the case of coreutils). The source packages generate *-dbgsym packages automatically along with normal binary packages and those debug packages are placed separately in debian-debug archive. Please refer to articles on Debian Wiki for more information. If a package to be debugged does not provide its *-dbgsym     package, you need to install it after rebuilding it by the following. $ mkdir /path/new ; cd /path/new $ sudo apt-get update $ sudo apt-get dist-upgrade     $ sudo apt-get install fakeroot devscripts build-essential $ apt-get source package_name $ cd package_name* $ sudo apt-get build-dep ./     Fix bugs if needed. Bump package version to one which does not collide with official Debian versions, e.g. one appended with "+debug1" when     recompiling existing package version, or one appended with "~pre1" when compiling unreleased package version by the following.     $ dch -i     Compile and install packages with debug symbols by the following. $ export DEB_BUILD_OPTIONS="nostrip noopt"     $ debuild $ cd .. $ sudo debi package_name*.changes     You need to check build scripts of the package and ensure to use "CFLAGS=-g -Wall" for compiling binaries. 12.5.3. Obtaining backtrace     When you encounter program crash, reporting bug report with cut-and-pasted backtrace information is a good idea.     The backtrace can be obtained by gdb(1) using one of the following approaches: * Crash-in-GDB approach: + Run the program from GDB. + Crash the program. + Type "bt" at the GDB prompt. * Crash-first approach: + Update the “/etc/security/limits.conf” file to include the following:     * soft core unlimited + Type "ulimit -c unlimited" to the shell prompt. + Run the program from this shell prompt. + Crash the program to produce a core dump file. + Load the core dump file to GDB as "gdb gdb ./ program_binary core" . + Type "bt" at the GDB prompt. For infinite loop or frozen keyboard situation, you can force to     crash the program by pressing Ctrl-\ or Ctrl-C or executing “kill -ABRT PID”. (See Section 9.4.12, “Killing a process”) Tip Often, you see a backtrace where one or more of the top lines are in "malloc()" or "g_malloc()". When this happens, chances are     your backtrace isn't very useful. The easiest way to find some useful information is to set the environment variable "$MALLOC_CHECK_" to a value of 2 (malloc(3)). You can do this while running gdb by doing the following. $ MALLOC_CHECK_=2 gdb hello 12.5.4. Advanced gdb commands Table 12.14. List of advanced gdb commands +---------------------------------------------------------------+ | command | description for command objectives | |-------------------+-------------------------------------------| |(gdb) thread apply |get a backtrace for all threads for | |all bt |multi-threaded program | |-------------------+-------------------------------------------| |(gdb) bt full |get parameters came on the stack of | | |function calls |     |-------------------+-------------------------------------------| |(gdb) thread apply |get a backtrace and parameters as the | |all bt full |combination of the preceding options | |-------------------+-------------------------------------------| |(gdb) thread apply |get a backtrace and parameters for top 10 | |all bt full 10 |calls to cut off irrelevant output | |-------------------+-------------------------------------------| |(gdb) set logging |write log of gdb output to a file (the | |on |default is "gdb.txt") | +---------------------------------------------------------------+ 12.5.5. Check dependency on libraries     Use ldd(1) to find out a program's dependency on libraries by the followings. $ ldd /usr/bin/ls librt.so.1 => /lib/librt.so.1 (0x4001e000)     libc.so.6 => /lib/libc.so.6 (0x40030000) libpthread.so.0 => /lib/libpthread.so.0 (0x40153000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)     For ls(1) to work in a `chroot`ed environment, the above libraries must be available in your `chroot`ed environment.     See Section 9.4.6, “Tracing program activities”. 12.5.6. Dynamic call tracing tools There are several dynamic call tracing tools available in Debian.     See Section 9.4, “Monitoring, controlling, and starting program activities”. 12.5.7. Debugging X Errors     If a GNOME program preview1 has received an X error, you should see a message as follows.     The program 'preview1' received an X Window System error. If this is the case, you can try running the program with     "--sync", and break on the "gdk_x_error" function in order to obtain a backtrace. 12.5.8. Memory leak detection tools     There are several memory leak detection tools available in Debian. Table 12.15. List of memory leak detection tools +---------------------------------------------------------------+ | package |popcon|size | description | |--------------+------+-----+-----------------------------------| |libc6-dev |V:248,|12053|mtrace(1): malloc debugging | | |I:567 | |functionality in glibc | |--------------+------+-----+-----------------------------------| |valgrind |V:6, |78191|memory debugger and profiler | | |I:37 | | | |--------------+------+-----+-----------------------------------| |electric-fence|V:0, |73 |malloc(3) debugger |     | |I:3 | | | |--------------+------+-----+-----------------------------------| |libdmalloc5 |V:0, |390 |debug memory allocation library | | |I:2 | | | |--------------+------+-----+-----------------------------------| | |V:0, | |library to detect buffer overruns | |duma |I:0 |296 |and under-runs in C and C++ | | | | |programs | |--------------+------+-----+-----------------------------------| |leaktracer |V:0, |56 |memory-leak tracer for C++ programs| | |I:1 | | | +---------------------------------------------------------------+ 12.5.9. Disassemble binary     You can disassemble binary code with objdump(1) by the following.     $ objdump -m i386 -b binary -D /usr/lib/grub/x86_64-pc/stage1 Note     gdb(1) may be used to disassemble code interactively. 12.6. Build tools Table 12.16. List of build tool packages +---------------------------------------------------------------+ | package | popcon |size | documentation | |-----------+--------+-----+------------------------------------| |make |V:151, |1592 |"info make" provided by make-doc | | |I:555 | | | |-----------+--------+-----+------------------------------------| |autoconf |V:31, |2025 |"info autoconf" provided by | | |I:230 | |autoconf-doc | |-----------+--------+-----+------------------------------------| |automake |V:30, |1837 |"info automake" provided by | | |I:228 | |automake1.10-doc | |-----------+--------+-----+------------------------------------|     |libtool |V:25, |1213 |"info libtool" provided by | | |I:212 | |libtool-doc | |-----------+--------+-----+------------------------------------| |cmake |V:17, |36607|cmake(1) cross-platform, open-source| | |I:115 | |make system | |-----------+--------+-----+------------------------------------| |ninja-build|V:6, |428 |ninja(1) small build system closest | | |I:41 | |in spirit to Make | |-----------+--------+-----+------------------------------------| |meson |V:3, |3759 |meson(1) high productivity build | | |I:22 | |system on top of ninja | |-----------+--------+-----+------------------------------------| |xutils-dev |V:0, I:9|1484 |imake(1), xmkmf(1), etc. | +---------------------------------------------------------------+ 12.6.1. Make Make is a utility to maintain groups of programs. Upon execution of make(1), make read the rule file, "Makefile", and updates a     target if it depends on prerequisite files that have been modified since the target was last modified, or if the target does not exist. The execution of these updates may occur concurrently.     The rule file syntax is the following. target: [ prerequisites ... ]     [TAB] command1 [TAB] -command2 # ignore errors [TAB] @command3 # suppress echoing Here "[TAB]" is a TAB code. Each line is interpreted by the shell     after make variable substitution. Use "\" at the end of a line to continue the script. Use "$$" to enter "$" for environment values for a shell script.     Implicit rules for the target and prerequisites can be written, for example, by the following.     %.o: %.c header.h Here, the target contains the character "%" (exactly one of     them). The "%" can match any nonempty substring in the actual target filenames. The prerequisites likewise use "%" to show how their names relate to the actual target name. Table 12.17. List of make automatic variables +---------------------------------------------------------+ |automatic variable| value | |------------------+--------------------------------------| |$@ |target | |------------------+--------------------------------------|     |$< |first prerequisite | |------------------+--------------------------------------| |$? |all newer prerequisites | |------------------+--------------------------------------| |$^ |all prerequisites | |------------------+--------------------------------------| |$* |"%" matched stem in the target pattern| +---------------------------------------------------------+ Table 12.18. List of make variable expansions +--------------------------------------+ |variable expansion| description | |------------------+-------------------|     |foo1 := bar |one-time expansion | |------------------+-------------------| |foo2 = bar |recursive expansion| |------------------+-------------------| |foo3 += bar |append | +--------------------------------------+     Run "make -p -f/dev/null" to see automatic internal rules. 12.6.2. Autotools     Autotools is a suite of programming tools designed to assist in making source code packages portable to many Unix-like systems. * Autoconf is a tool to produce a shell script "configure" from "configure.ac". + "configure" is used later to produce "Makefile" from "Makefile.in" template.     * Automake is a tool to produce "Makefile.in" from "Makefile.am". * Libtool is a shell script to address the software portability problem when compiling shared libraries from source code. 12.6.2.1. Compile and install a program Warning     Do not overwrite system files with your compiled programs when installing them. Debian does not touch files in "/usr/local/" or "/opt". So if you     compile a program from source, install it into "/usr/local/" so it does not interfere with Debian. $ cd src     $ ./configure --prefix=/usr/local $ make # this compiles program $ sudo make install # this installs the files in the system 12.6.2.2. Uninstall program If you have the original source and if it uses autoconf(1)/     automake(1) and if you can remember how you configured it, execute as follows to uninstall the program.     $ ./configure all-of-the-options-you-gave-it $ sudo make uninstall Alternatively, if you are absolutely sure that the install     process puts files only under "/usr/local/" and there is nothing important there, you can erase all its contents by the following.     # find /usr/local -type f -print0 | xargs -0 rm -f If you are not sure where files are installed, you should     consider using checkinstall(8) from the checkinstall package, which provides a clean path for the uninstall. It now supports to create a Debian package with "-D" option. 12.6.3. Meson     The software build system has been evolving: * Autotools on the top of Make has been the de facto standard for the portable build infrastructure since 1990s. This is extremely slow. * CMake initially released in 2000 improved speed significantly but was originally built on the top of inherently slow Make.     (Now Ninja can be its backend.) * Ninja initially released in 2012 is meant to replace Make for the further improved build speed and is designed to have its input files generated by a higher-level build system. * Meson initially released in 2013 is the new popular and fast higher-level build system which uses Ninja as its backend.     See documents found at "The Meson Build system" and "The Ninja build system". 12.7. Web     Basic interactive dynamic web pages can be made as follows. * Queries are presented to the browser user using HTML forms. * Filling and clicking on the form entries sends one of the following URL string with encoded parameters from the browser to the web server. + "https://www.foo.dom/cgi-bin/program.pl?VAR1=VAL1&VAR2= VAL2&VAR3=VAL3" + "https://www.foo.dom/cgi-bin/program.py?VAR1=VAL1&VAR2= VAL2&VAR3=VAL3" + "https://www.foo.dom/program.php?VAR1=VAL1&VAR2=VAL2&VAR3     =VAL3" * "%nn" in URL is replaced with a character with hexadecimal nn value. * The environment variable is set as: "QUERY_STRING="VAR1=VAL1 VAR2=VAL2 VAR3=VAL3"". * CGI program (any one of "program.*") on the web server executes itself with the environment variable "$QUERY_STRING". * stdout of CGI program is sent to the web browser and is presented as an interactive dynamic web page. For security reasons it is better not to hand craft new hacks for parsing CGI parameters. There are established modules for them in     Perl and Python. PHP comes with these functionalities. When client data storage is needed, HTTP cookies are used. When client side data processing is needed, Javascript is frequently used.     For more, see the Common Gateway Interface, The Apache Software Foundation, and JavaScript. Searching "CGI tutorial" on Google by typing encoded URL https://     www.google.com/search?hl=en&ie=UTF-8&q=CGI+tutorial directly to the browser address is a good way to see the CGI script in action on the Google server. 12.8. The source code translation     There are programs to convert source codes. Table 12.19. List of source code translation tools +---------------------------------------------------------------+ | package |popcon|size| keyword | description | |---------+------+----+---------+-------------------------------| |perl |V:707,|673 |AWK→PERL |convert source codes from AWK | | |I:989 | | |to PERL: a2p(1) |     |---------+------+----+---------+-------------------------------| |f2c |V:0, |442 |FORTRAN→C|convert source codes from | | |I:3 | | |FORTRAN 77 to C/C++: f2c(1) | |---------+------+----+---------+-------------------------------| | |V:0, | | |converter from NASM (Intel | |intel2gas|I:0 |178 |intel→gas|format) to the GNU Assembler | | | | | |(GAS) | +---------------------------------------------------------------+ 12.9. Making Debian package     If you want to make a Debian package, read followings. * Chapter 2, Debian package management to understand the basic package system * Section 2.7.13, “Porting a package to the stable system” to understand basic porting process * Section 9.11.4, “Chroot system” to understand basic chroot techniques     * debuild(1), and sbuild(1) * Section 12.5.2, “Debugging the Debian package” for recompiling for debugging * Guide for Debian Maintainers (the debmake-doc package) * Debian Developer's Reference (the developers-reference package) * Debian Policy Manual (the debian-policy package)     There are packages such as debmake, dh-make, dh-make-perl, etc., which help packaging. ---------------------------------------------------------------------     ^[7] Some tweaks may be required to get them work under the current system. Appendix A. Appendix     Here are backgrounds of this document. A.1. The Debian maze The Linux system is a very powerful computing platform for a networked computer. However, learning how to use all its     capabilities is not easy. Setting up the LPR printer queue with a non-PostScript printer was a good example of stumble points. (There are no issues anymore since newer installations use the new CUPS system.) There is a complete, detailed map called the "SOURCE CODE". This is very accurate but very hard to understand. There are also     references called HOWTO and mini-HOWTO. They are easier to understand but tend to give too much detail and lose the big picture. I sometimes have a problem finding the right section in a long HOWTO when I need a few commands to invoke. I hope this "Debian Reference (version 2.122)" (2024-04-10     23:08:27 UTC) provides a good starting direction for people in the Debian maze. A.2. Copyright history The Debian Reference was initiated by me, Osamu Aoki , as a personal system administration memo. Many contents came from the knowledge I gained from the debian-user mailing list and other Debian resources. Following a suggestion from Josip Rodin, who was very active with     the Debian Documentation Project (DDP), "Debian Reference (version 1, 2001-2007)" was created as a part of DDP documents. After 6 years, I realized that the original "Debian Reference     (version 1)" was outdated and started to rewrite many contents. New "Debian Reference (version 2)" is released in 2008. I have updated "Debian Reference (version 2)" to address new topics (Systemd, Wayland, IMAP, PipeWire, Linux kernel 5.10) and     removed outdated topics (SysV init, CVS, Subversion, SSH protocol 1, Linux kernels before 2.5). References to Jessie 8 (2015-2020) release situation or older are mostly removed. This "Debian Reference (version 2.122)" (2024-04-10 23:08:27 UTC)     covers mostly Bookworm (=stable) and Trixie (=testing) Debian releases.     The tutorial contents can trace its origin and its inspiration in followings. * "Linux User's Guide" by Larry Greenfield (December 1996) + obsoleted by "Debian Tutorial" * "Debian Tutorial" by Havoc Pennington. (11 December, 1998) + partially written by Oliver Elphick, Ole Tetlie, James     Treacy, Craig Sawyer, and Ivan E. Moore II + obsoleted by "Debian GNU/Linux: Guide to Installation and Usage" * "Debian GNU/Linux: Guide to Installation and Usage" by John Goerzen and Ossama Othman (1999) + obsoleted by "Debian Reference (version 1)"     The package and archive description can trace some of their origin and their inspiration in following.     * "Debian FAQ" (March 2002 version, when this was maintained by Josip Rodin)     The other contents can trace some of their origin and their inspiration in following. * "Debian Reference (version 1)" by Osamu Aoki (2001–2007)     + obsoleted by the newer "Debian Reference (version 2)" in 2008.     The previous "Debian Reference (version 1)" was created with many contributors. * the major contents contribution on network configuration topics by Thomas Hood * significant contents contribution on X and VCS related topics by Brian Nelson     * the help on the build scripts and many content corrections by Jens Seidel * extensive proofreading by David Sewell * many contributions by the translators, contributors, and bug reporters Many manual pages and info pages on the Debian system as well as upstream web pages and Wikipedia documents were used as the primary references to write this document. To the extent Osamu     Aoki considered within the fair use, many parts of them, especially command definitions, were used as phrase pieces after careful editorial efforts to fit them into the style and the objective of this document. The gdb debugger description was expanded using Debian wiki     contents on backtrace with consent by Ari Pollak, Loïc Minier, and Dafydd Harries. Contents of the current "Debian Reference (version 2.122)"     (2024-04-10 23:08:27 UTC) are mostly my own work except as mentioned above. These has been updated by the contributors too.     The author, Osamu Aoki, thanks all those who helped make this document possible. A.3. Document format The source of the English original document is currently written     in DocBook XML files. This Docbook XML source are converted to HTML, plain text, PostScript, and PDF. (Some formats may be skipped for distribution.)