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The Debian GNU/Linux FAQ
Chapter 7 - Basics of the Debian package management system


This chapter touches on some lower level internals of Debian package management. If you're interested mainly in usage of the relevant tools, skip to chapters The Debian package management tools, Chapter 8 and/or Keeping your Debian system up-to-date, Chapter 9.


7.1 What is a Debian package?

Packages generally contain all of the files necessary to implement a set of related commands or features. There are two types of Debian packages:

Installation of software by the package system uses "dependencies" which are carefully designed by the package maintainers. These dependencies are documented in the control file associated with each package. For example, the package containing the GNU C compiler (gcc) "depends" on the package binutils which includes the linker and assembler. If a user attempts to install gcc without having first installed binutils, the package management system (dpkg) will send an error message that it also needs binutils, and stop installing gcc. (However, this facility can be overridden by the insistent user, see dpkg(8).) See more in What is meant by saying that a package Depends, Recommends, Suggests, Conflicts, Replaces, Breaks or Provides another package?, Section 7.9 below.

Debian's packaging tools can be used to:


7.2 What is the format of a Debian binary package?

A Debian "package", or a Debian archive file, contains the executable files, libraries, and documentation associated with a particular suite of program or set of related programs. Normally, a Debian archive file has a filename that ends in .deb.

The internals of this Debian binary packages format are described in the deb(5) manual page. This internal format is subject to change (between major releases of Debian GNU/Linux), therefore please always use dpkg-deb(1) if you need to do lowlevel manipulations on .deb files.


7.3 Why are Debian package file names so long?

The Debian binary package file names conform to the following convention: <foo>_<VersionNumber>-<DebianRevisionNumber>_<DebianArchitecture>.deb

Note that foo is supposed to be the package name. As a check, one can learn the package name associated with a particular Debian archive file (.deb file) in one of these ways:

The VVV component is the version number specified by the upstream developer. There are no standards in place here, so the version number may have formats as different as "19990513" and "1.3.8pre1".

The RRR component is the Debian revision number, and is specified by the Debian developer (or an individual user if he chooses to build the package himself). This number corresponds to the revision level of the Debian package, thus, a new revision level usually signifies changes in the Debian Makefile (debian/rules), the Debian control file (debian/control), the installation or removal scripts (debian/p*), or in the configuration files used with the package.

The AAA component identifies the processor for which the package was built. This is commonly i386, which refers to chips compatible to Intel's 386 or later versions. For other possibilities review Debian's FTP directory structure at What are all those directories at the Debian FTP archives?, Section 6.7. For details, see the description of "Debian architecture" in the manual page dpkg-architecture(1).


7.4 What is a Debian control file?

Specifics regarding the contents of a Debian control file are provided in the Debian Policy Manual, section 5, see What other documentation exists on and for a Debian system?, Section 12.1.

Briefly, a sample control file is shown below for the Debian package hello:

     Package: hello
     Priority: optional
     Section: devel
     Installed-Size: 45
     Maintainer: Adam Heath <doogie@debian.org>
     Architecture: i386
     Version: 1.3-16
     Depends: libc6 (>= 2.1)
     Description: The classic greeting, and a good example
      The GNU hello program produces a familiar, friendly greeting.  It
      allows nonprogrammers to use a classic computer science tool which
      would otherwise be unavailable to them.
      .
      Seriously, though: this is an example of how to do a Debian package.
      It is the Debian version of the GNU Project's `hello world' program
      (which is itself an example for the GNU Project).

The Package field gives the package name. This is the name by which the package can be manipulated by the package tools, and usually similar to but not necessarily the same as the first component string in the Debian archive file name.

The Version field gives both the upstream developer's version number and (in the last component) the revision level of the Debian package of this program as explained in Why are Debian package file names so long?, Section 7.3.

The Architecture field specifies the chip for which this particular binary was compiled.

The Depends field gives a list of packages that have to be installed in order to install this package successfully.

The Installed-Size indicates how much disk space the installed package will consume. This is intended to be used by installation front-ends in order to show whether there is enough disk space available to install the program.

The Section line gives the "section" where this Debian package is stored at the Debian FTP sites.

The Priority indicates how important is this package for installation, so that semi-intelligent software like dselect or aptitude can sort the package into a category of e.g. packages optionally installed. See What is an Essential, Required, Important, Standard, Optional, or Extra package?, Section 7.7.

The Maintainer field gives the e-mail address of the person who is currently responsible for maintaining this package.

The Description field gives a brief summary of the package's features.

For more information about all possible fields a package can have, please see the Debian Policy Manual, section 5, "Control files and their fields", see What other documentation exists on and for a Debian system?, Section 12.1.


7.5 What is a Debian conffile?

Conffiles is a list of configuration files (usually placed in /etc) that the package management system will not overwrite when the package is upgraded. This ensures that local values for the contents of these files will be preserved, and is a critical feature enabling the in-place upgrade of packages on a running system.

To determine exactly which files are preserved during an upgrade, run:

     dpkg --status package

And look under "Conffiles:".


7.6 What is a Debian preinst, postinst, prerm, and postrm script?

These files are executable scripts which are automatically run before or after a package is installed. Along with a file named control, all of these files are part of the "control" section of a Debian archive file.

The individual files are:

preinst

This script executes before that package will be unpacked from its Debian archive (".deb") file. Many 'preinst' scripts stop services for packages which are being upgraded until their installation or upgrade is completed (following the successful execution of the 'postinst' script).

postinst

This script typically completes any required configuration of the package foo once foo has been unpacked from its Debian archive (".deb") file. Often, 'postinst' scripts ask the user for input, and/or warn the user that if he accepts default values, he should remember to go back and re-configure that package as the situation warrants. Many 'postinst' scripts then execute any commands necessary to start or restart a service once a new package has been installed or upgraded.

prerm

This script typically stops any daemons which are associated with a package. It is executed before the removal of files associated with the package.

postrm

This script typically modifies links or other files associated with foo, and/or removes files created by the package. (Also see What is a Virtual Package?, Section 7.8.)

Currently all of the control files can be found in directory /var/lib/dpkg/info. The files relevant to package foo begin with the name "foo" and have file extensions of "preinst", "postinst", etc., as appropriate. The file foo.list in that directory lists all of the files that were installed with the package foo. (Note that the location of these files is a dpkg internal; you should not rely on it.)


7.7 What is an Essential, Required, Important, Standard, Optional, or Extra package?

Each Debian package is assigned a priority by the distribution maintainers, as an aid to the package management system. The priorities are:

If you do a default Debian installation all the packages of priority Standard or higher will be installed in your system. If you select pre-defined tasks you will get lower priority packages too.

Additionally, some packages are marked as Essential since they are absolutely necessary for the proper functioning of the system. The package management tools will refuse to remove these.


7.8 What is a Virtual Package?

A virtual package is a generic name that applies to any one of a group of packages, all of which provide similar basic functionality. For example, both the tin and trn programs are news readers, and should therefore satisfy any dependency of a program that required a news reader on a system, in order to work or to be useful. They are therefore both said to provide the "virtual package" called news-reader.

Similarly, smail and sendmail both provide the functionality of a mail transport agent. They are therefore said to provide the virtual package, "mail transport agent". If either one is installed, then any program depending on the installation of a mail-transport-agent will be satisfied by the existence of this virtual package.

Debian provides a mechanism so that, if more than one package which provide the same virtual package is installed on a system, then system administrators can set one as the preferred package. The relevant command is update-alternatives, and is described further in Some users like mawk, others like gawk; some like vim, others like elvis; some like trn, others like tin; how does Debian support diversity?, Section 11.10.


7.9 What is meant by saying that a package Depends, Recommends, Suggests, Conflicts, Replaces, Breaks or Provides another package?

The Debian package system has a range of package "dependencies" which are designed to indicate (in a single flag) the level at which Program A can operate independently of the existence of Program B on a given system:

More detailed information on the use of each these terms can be found in the Debian Policy manual, section 7.2, "Binary Dependencies", see What other documentation exists on and for a Debian system?, Section 12.1.


7.10 What is meant by Pre-Depends?

"Pre-Depends" is a special dependency. In the case of most packages, dpkg will unpack its archive file (i.e., its .deb file) independently of whether or not the files on which it depends exist on the system. Simplistically, unpacking means that dpkg will extract the files from the archive file that were meant to be installed on your file system, and put them in place. If those packages depend on the existence of some other packages on your system, dpkg will refuse to complete the installation (by executing its "configure" action) until the other packages are installed.

However, for some packages, dpkg will refuse even to unpack them until certain dependencies are resolved. Such packages are said to "Pre-depend" on the presence of some other packages. The Debian project provided this mechanism to support the safe upgrading of systems from a.out format to ELF format, where the order in which packages were unpacked was critical. There are other large upgrade situations where this method is useful, e.g. the packages with the required priority and their LibC dependency.

As before, more detailed information about this can be found in the Policy manual.


7.11 What is meant by unknown, install, remove, purge and hold in the package status?

These "want" flags tell what the user wanted to do with a package (as indicated either by the user's actions in the "Select" section of dselect, or by the user's direct invocations of dpkg).

Their meanings are:


7.12 How do I put a package on hold?

There are three ways of holding back packages, with dpkg, aptitude or with dselect.

With dpkg, you have to export the list of package selections, with:

     dpkg --get-selections \* > selections.txt

Then edit the resulting file selections.txt, change the line containing the package you wish to hold, e.g. libc6, from this:

     libc6                                           install

to this:

     libc6                                           hold

Save the file, and reload it into dpkg database with:

     dpkg --set-selections < selections.txt

With aptitude, you can hold a package using

     aptitude hold package_name

and remove the hold with

     aptitude unhold package_name

With dselect, you have to enter the [S]elect screen, find the package you wish to hold in its present state, and press the `=' key (or `H'). The changes will go live immediately after you exit the [S]elect screen.


7.13 How do I install a source package?

Debian source packages can't actually be "installed", they are just unpacked in whatever directory you want to build the binary packages they produce.

Source packages are distributed on most of the same mirrors where you can obtain the binary packages. If you set up your APT's sources.list(5) to include the appropriate "deb-src" lines, you'll be able to easily download any source packages by running

     apt-get source foo

To help you in actually building the source package, Debian source package provide the so-called build-dependencies mechanism. This means that the source package maintainer keeps a list of other packages that are required to build their package. To see how this is useful, run

     apt-get build-dep foo

before building the source.


7.14 How do I build binary packages from a source package?

The preferred way to do this is by using various wrapper tools. We'll show how it's done using the devscripts tools. Install this package if you haven't done so already.

Now, first get the source package:

     apt-get source foo

and change to the source tree:

     cd foo-*

Then install needed build-dependencies (if any):

     sudo apt-get build-dep foo

Then create a dedicated version of your own build (so that you won't get confused later when Debian itself releases a new version)

     dch -l local 'Blah blah blah'

And finally build your package

     debuild -us -uc

If everything worked out fine, you should now be able to install your package by running

     sudo dpkg -i ../*.deb

If you prefer to do things manually, and don't want to use devscripts, follow this procedure:

You will need all of foo_*.dsc, foo_*.tar.gz and foo_*.diff.gz to compile the source (note: there is no .diff.gz for some packages that are native to Debian).

Once you have them (How do I install a source package?, Section 7.13), if you have the dpkg-dev package installed, the following command:

     dpkg-source -x foo_version-revision.dsc

will extract the package into a directory called foo-version.

If you want just to compile the package, you may cd into foo-version directory and issue the command

     dpkg-buildpackage -rfakeroot -b

to build the package (note that this also requires the fakeroot package), and then

     dpkg -i ../foo_version-revision_arch.deb

to install the newly-built package(s).


7.15 How do I create Debian packages myself?

For a more detailed description on this, read the New Maintainers' Guide, available in the maint-guide package, or at http://www.debian.org/doc/devel-manuals#maint-guide.


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The Debian GNU/Linux FAQ

version 5.0.2, 2 June 2013

Authors are listed at Debian FAQ Authors