7. Declaring relationships between packages¶
7.1. Syntax of relationship fields¶
These fields all have a uniform syntax. They are a list of package names separated by commas.
control fields of the package, which declare dependencies on other
packages, the package names listed may also include lists of alternative
package names, separated by vertical bar (pipe) symbols
|. In such a
case, that part of the dependency can be satisfied by any one of the
alternative packages. 1
All of the fields may restrict their applicability to particular versions of each named package. This is done in parentheses after each individual package name; the parentheses should contain a relation from the list below followed by a version number, in the format described in Version.
The relations allowed are
strictly earlier, earlier or equal, exactly equal, later or equal and
strictly later, respectively. The exception is the Provides field, for
= is allowed. 2
Whitespace may appear at any point in the version specification subject
to the rules in Syntax of control files, and must appear
where it’s necessary to disambiguate; it is not otherwise significant.
All of the relationship fields can only be folded in source package
control files. For consistency and in case of future changes to
it is recommended that a single space be used after a version
relationship and before a version number; it is also conventional to put
a single space after each comma, on either side of each vertical bar,
and before each open parenthesis. When opening a continuation line in a
relationship field, it is conventional to do so after a comma and before
the space following that comma.
For example, a list of dependencies might appear as:
Package: mutt Version: 1.3.17-1 Depends: libc6 (>= 2.2.1), default-mta | mail-transport-agent
Relationships may be restricted to a certain set of architectures. This is indicated in brackets after each individual package name and the optional version specification. The brackets enclose a non-empty list of Debian architecture names in the format described in Architecture specification strings, separated by whitespace. Exclamation marks may be prepended to each of the names. (It is not permitted for some names to be prepended with exclamation marks while others aren’t.)
For build relationship fields (
Build-Conflicts-Arch), if the current
Debian host architecture is not in this list and there are no
exclamation marks in the list, or it is in the list with a prepended
exclamation mark, the package name and the associated version
specification are ignored completely for the purposes of defining the
Source: glibc Build-Depends-Indep: texinfo Build-Depends: kernel-headers-2.2.10 [!hurd-i386], hurd-dev [hurd-i386], gnumach-dev [hurd-i386]
kernel-headers-2.2.10 on all architectures other than
hurd-i386 and requires
gnumach-dev only on
hurd-i386. Here is another example showing multiple architectures
separated by spaces:
Build-Depends: libluajit5.1-dev [i386 amd64 kfreebsd-i386 armel armhf powerpc mips], liblua5.1-dev [hurd-i386 ia64 kfreebsd-amd64 s390x sparc],
For binary relationship fields and the
Built-Using field, the
architecture restriction syntax is only supported in the source package
debian/control. When the corresponding binary package
control file is generated, the relationship will either be omitted or
included without the architecture restriction based on the architecture
of the binary package. This means that architecture restrictions must
not be used in binary relationship fields for architecture-independent
Depends: foo [i386], bar [amd64]
Depends: foo when the package is built on the
Depends: bar when the package is built on the
amd64 architecture, and omitted entirely in binary packages built
on all other architectures.
If the architecture-restricted dependency is part of a set of
|, that alternative is ignored completely on
architectures that do not match the restriction. For example:
Build-Depends: foo [!i386] | bar [!amd64]
is equivalent to
bar on the
i386 architecture, to
amd64 architecture, and to
foo | bar on all other
Relationships may also be restricted to a certain set of architectures using architecture wildcards in the format described in Architecture wildcards. The syntax for declaring such restrictions is the same as declaring restrictions using a certain set of architectures without architecture wildcards. For example:
Build-Depends: foo [linux-any], bar [any-i386], baz [!linux-any]
is equivalent to
foo on architectures using the Linux kernel and any
bar on architectures using any kernel and an i386 cpu, and
baz on any architecture using a kernel other than Linux.
Note that the binary package relationship fields such as
appear in one of the binary package sections of the control file,
whereas the build-time relationships such as
Build-Depends appear in
the source package section of the control file (which is the first
7.2. Binary Dependencies -
Packages can declare in their control file that they have certain relationships to other packages - for example, that they may not be installed at the same time as certain other packages, and/or that they depend on the presence of others.
This is done using the
Conflicts control fields.
Breaks is described in Packages which break other packages - Breaks, and
Conflicts is described in Conflicting binary packages - Conflicts. The
rest are described below.
These seven fields are used to declare a dependency relationship by one
package on another. Except for
Breaks, they appear
in the depending (binary) package’s control file. (
in the recommending package’s control file, and
Breaks appears in
the version of depended-on package which causes the named package to
Depends field takes effect only when a package is to be
configured. It does not prevent a package being on the system in an
unconfigured state while its dependencies are unsatisfied, and it is
possible to replace a package whose dependencies are satisfied and which
is properly installed with a different version whose dependencies are
not and cannot be satisfied; when this is done the depending package
will be left unconfigured (since attempts to configure it will give
errors) and will not function properly. If it is necessary, a
Pre-Depends field can be used, which has a partial effect even when
a package is being unpacked, as explained in detail below. (The other
three dependency fields,
are only used by the various front-ends to
dpkg such as
Depends only places requirements on the order in which
packages are configured, packages in an installation run are usually all
unpacked first and all configured later. 3
If there is a circular dependency among packages being installed or
removed, installation or removal order honoring the dependency order is
impossible, requiring the dependency loop be broken at some point and
the dependency requirements violated for at least one package. Packages
involved in circular dependencies may not be able to rely on their
dependencies being configured before they themselves are configured,
depending on which side of the break of the circular dependency loop
they happen to be on. If one of the packages in the loop has no
postinst script, then the cycle will be broken at that package; this
ensures that all
postinst scripts are run with their dependencies
properly configured if this is possible. Otherwise the breaking point is
arbitrary. Packages should therefore avoid circular dependencies where
possible, particularly if they have
The meaning of the five dependency fields is as follows:
This declares an absolute dependency. A package will not be configured unless all of the packages listed in its
Dependsfield have been correctly configured (unless there is a circular dependency as described above).
Dependsfield should be used if the depended-on package is required for the depending package to provide a significant amount of functionality.
Dependsfield should also be used if the
prermscripts require the depended-on package to be unpacked or configured in order to run. In the case of
postinst configure, the depended-on packages will be unpacked and configured first. (If both packages are involved in a dependency loop, this might not work as expected; see the explanation a few paragraphs back.) In the case of
postinstactions, the package dependencies will normally be at least unpacked, but they may be only “Half-Installed” if a previous upgrade of the dependency failed.
Dependsfield should be used if the depended-on package is needed by the
postrmscript to fully clean up after the package removal. There is no guarantee that package dependencies will be available when
postrmis run, but the depended-on package is more likely to be available if the package declares a dependency (particularly in the case of
postrm remove). The
postrmscript must gracefully skip actions that require a dependency if that dependency isn’t available.
This declares a strong, but not absolute, dependency.
Recommendsfield should list packages that would be found together with this one in all but unusual installations.
This is used to declare that one package may be more useful with one or more others. Using this field tells the packaging system and the user that the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.
This field is similar to Suggests but works in the opposite direction. It is used to declare that a package can enhance the functionality of another package.
This field is like
Depends, except that it also forces
dpkgto complete installation of the packages named before even starting the installation of the package which declares the pre-dependency, as follows:
When a package declaring a pre-dependency is about to be unpacked the pre-dependency can be satisfied if the depended-on package is either fully configured, or even if the depended-on package(s) are only in the “Unpacked” or the “Half-Configured” state, provided that they have been configured correctly at some point in the past (and not removed or partially removed since). In this case, both the previously-configured and currently “Unpacked” or “Half-Configured” versions must satisfy any version clause in the
When the package declaring a pre-dependency is about to be configured, the pre-dependency will be treated as a normal
Depends. It will be considered satisfied only if the depended-on package has been correctly configured. However, unlike with
Pre-Dependsdoes not permit circular dependencies to be broken. If a circular dependency is encountered while attempting to honor
Pre-Depends, the installation will be aborted.
Pre-Dependsare also required if the
preinstscript depends on the named package. It is best to avoid this situation if possible.
Pre-Dependsshould be used sparingly, preferably only by packages whose premature upgrade or installation would hamper the ability of the system to continue with any upgrade that might be in progress.
You should not specify a
Pre-Dependsentry for a package before this has been discussed on the
debian-develmailing list and a consensus about doing that has been reached. See Dependencies.
When selecting which level of dependency to use you should consider how
important the depended-on package is to the functionality of the one
declaring the dependency. Some packages are composed of components of
varying degrees of importance. Such a package should list using
Depends the package(s) which are required by the more important
components. The other components’ requirements may be mentioned as
Suggestions or Recommendations, as appropriate to the components’
7.3. Packages which break other packages -
When one binary package declares that it breaks another,
refuse to allow the package which declares
Breaks to be unpacked
unless the broken package is deconfigured first, and it will refuse to
allow the broken package to be reconfigured.
A package will not be regarded as causing breakage merely because its configuration files are still installed; it must be at least “Half-Installed”.
A special exception is made for packages which declare that they break their own package name or a virtual package which they provide (see below): this does not count as a real breakage.
Breaks entry will have an “earlier than” version clause;
Breaks is introduced in the version of an (implicit or
explicit) dependency which violates an assumption or reveals a bug in
earlier versions of the broken package, or which takes over a file from
earlier versions of the package named in
Breaks. This use of
Breaks will inform higher-level package management tools that the
broken package must be upgraded before the new one.
If the breaking package also overwrites some files from the older
package, it should use
Replaces to ensure this goes smoothly. See
Overwriting files and replacing packages - Replaces for a full discussion of taking over
files from other packages, including how to use
Breaks in those
Many of the cases where
Breaks should be used were previously
Breaks did not yet exist. Many
Conflicts fields should now be
Conflicting binary packages - Conflicts for more information about the
7.4. Conflicting binary packages -
When one binary package declares a conflict with another using a
dpkg will refuse to allow them to be unpacked
on the system at the same time. This is a stronger restriction than
Breaks, which prevents the broken package from being configured
while the breaking package is in the “Unpacked” state but allows both
packages to be unpacked at the same time.
If one package is to be unpacked, the other must be removed first. If
the package being unpacked is marked as replacing (see
Overwriting files and replacing packages - Replaces, but note that
normally be used in this case) the one on the system, or the one on the
system is marked as deselected, or both packages are marked
dpkg will automatically remove the package which
is causing the conflict. Otherwise, it will halt the installation of the
new package with an error. This mechanism is specifically designed to
produce an error when the installed package is
Essential, but the
new package is not.
A package will not cause a conflict merely because its configuration files are still installed; it must be at least “Half-Installed”.
A special exception is made for packages which declare a conflict with their own package name, or with a virtual package which they provide (see below): this does not prevent their installation, and allows a package to conflict with others providing a replacement for it. You use this feature when you want the package in question to be the only package providing some feature.
Breaks should be used instead of
Conflicts imposes a stronger restriction on the ordering of package
installation or upgrade and can make it more difficult for the package
manager to find a correct solution to an upgrade or installation
Breaks should be used
when moving a file from one package to another (see Overwriting files and replacing packages - Replaces),
when splitting a package (a special case of the previous one), or
when the breaking package exposes a bug in or interacts badly with particular versions of the broken package.
Conflicts should be used
when two packages provide the same file and will continue to do so,
in conjunction with
Provideswhen only one package providing a given virtual facility may be unpacked at a time (see Virtual packages - Provides),
in other cases where one must prevent simultaneous installation of two packages for reasons that are ongoing (not fixed in a later version of one of the packages) or that must prevent both packages from being unpacked at the same time, not just configured.
Be aware that adding
Conflicts is normally not the best solution
when two packages provide the same files. Depending on the reason for
that conflict, using alternatives or renaming the files is often a
better approach. See, for example, Binaries.
Conflicts should be used unless two packages
cannot be installed at the same time or installing them both causes one
of them to be broken or unusable. Having similar functionality or
performing the same tasks as another package is not sufficient reason to
Conflicts with that package.
Conflicts entry may have an “earlier than” version clause if the
reason for the conflict is corrected in a later version of one of the
packages. However, normally the presence of an “earlier than” version
clause is a sign that
Breaks should have been used instead. An
“earlier than” version clause in
upgrading or installing the package which declares such a conflict until
the upgrade or removal of the conflicted-with package has been
completed, which is a strong restriction.
7.5. Virtual packages -
As well as the names of actual (“concrete”) packages, the package
Build-Conflicts-Arch may mention “virtual packages”.
A virtual package is one which appears in the
field of another package. The effect is as if the package(s) which
provide a particular virtual package name had been listed by name
everywhere the virtual package name appears. (See also
If there are both concrete and virtual packages of the same name, then the dependency may be satisfied (or the conflict caused) by either the concrete package with the name in question or any other concrete package which provides the virtual package with the name in question. This is so that, for example, supposing we have
Package: foo Depends: bar
and someone else releases an enhanced version of the
they can say:
Package: bar-plus Provides: bar
bar-plus package will now also satisfy the dependency for
Provides field may contain version numbers, and such a version number
will be considered when considering a dependency on or conflict with the
virtual package name. For example, given the following packages:
Package: foo Depends: bar (>= 1.0) Package: bar Version: 0.9 Package: bar-plus Provides: bar (= 1.0)
bar-plus package will satisfy the dependency for the
package with the virtual package name, as above. If the
field does not specify a version number, it will not satisfy versioned
dependencies or violate versioned
Breaks. For example,
given the following packages:
Package: foo Depends: bar (>= 1.0) Package: bar Version: 0.9 Package: bar-plus Provides: bar (= 1.0) Package: bar-clone Provides: bar
bar-plus package will satisfy the dependency for the
package, but the
bar-clone package will not.
To specify which of a set of real packages should be the default to satisfy a particular dependency on a virtual package, list the real package as an alternative before the virtual one.
If the virtual package represents a facility that can only be provided
by one real package at a time, such as the mail-transport-agent virtual
package that requires installation of a binary that would conflict with
all other providers of that virtual package (see
Mail transport, delivery and user agents), all packages providing
that virtual package should also declare a conflict with it using
Conflicts. This will ensure that at most one provider of that
virtual package is unpacked or installed at a time.
7.6. Overwriting files and replacing packages -
Packages can declare in their control file that they should overwrite
files in certain other packages, or completely replace other packages.
Replaces control field has these two distinct purposes.
7.6.1. Overwriting files in other packages¶
It is usually an error for a package to contain files which are on the
system in another package. However, if the overwriting package declares
Replaces the one containing the file being overwritten, then
dpkg will replace the file from the old package with that from the
new. The file will no longer be listed as “owned” by the old package and
will be taken over by the new package. Normally,
Breaks should be
used in conjunction with
For example, if a package foo is split into foo and foo-data starting at version 1.2-3, foo-data would have the fields
Replaces: foo (<< 1.2-3) Breaks: foo (<< 1.2-3)
in its control file. The new version of the package foo would normally have the field
Depends: foo-data (>= 1.2-3)
Recommends or even
Suggests if the files moved into
foo-data are not required for normal operation).
If a package is completely replaced in this way, so that
not know of any files it still contains, it is considered to have
“disappeared”. It will be marked as not wanted on the system (selected
for removal) and “Not-Installed”. Any
conffiles details noted for
the package will be ignored, as they will have been taken over by the
overwriting package. The package’s
postrm script will be run with a
special argument to allow the package to do any final cleanup required.
See Summary of ways maintainer scripts are called. 5
For this usage of
Replaces, virtual packages (see
Virtual packages - Provides) are not considered when looking at a
Replaces field. The packages declared as being replaced must be
mentioned by their real names.
This usage of
Replaces only takes effect when both packages are at
least partially on the system at once. It is not relevant if the
packages conflict unless the conflict has been overridden.
7.6.2. Replacing whole packages, forcing their removal¶
Replaces allows the packaging system to resolve which
package should be removed when there is a conflict (see
Conflicting binary packages - Conflicts). This usage only takes effect when
the two packages do conflict, so that the two usages of this field do
not interfere with each other.
In this situation, the package declared as being replaced can be a virtual package, so for example, all mail transport agents (MTAs) would have the following fields in their control files:
Provides: mail-transport-agent Conflicts: mail-transport-agent Replaces: mail-transport-agent
ensuring that only one MTA can be unpacked at any one time. See Virtual packages - Provides for more information about this example.
7.7. Relationships between source and binary packages -
Source packages that require certain binary packages to be installed or absent at the time of building the package may declare relationships to those binary packages.
This is done using the
Build-Conflicts-Arch control fields.
Build-dependencies on “build-essential” binary packages can be omitted. Please see Package relationships for more information.
The dependencies and conflicts they define must be satisfied (as defined
earlier for binary packages) in order to invoke the targets in
debian/rules, as follows:
Build-Conflictsfields must be satisfied when this target is invoked.
Build-Conflicts-Archfields must be satisfied when these targets are invoked.
Build-Conflicts-Indepfields must be satisfied when these targets are invoked.
Build-Conflicts-Archfields must be satisfied when these targets are invoked.
7.8. Additional source packages used to build the binary -
Some binary packages incorporate parts of other packages when built but do not have to depend on those packages. Examples include linking with static libraries or incorporating source code from another package during the build. In this case, the source packages of those other packages are part of the complete source (the binary package is not reproducible without them).
When the license of either the incorporated parts or the incorporating
binary package requires that the full source code of the incorporating
binary package be made available, the
Built-Using field must list
the corresponding source package for any affected binary package
incorporated during the build, 6 including an “exactly equal” (“=”)
version relation on the version that was used to build that version of
the incorporating binary package. 7
This causes the Debian archive to retain the versions of the source packages that were actually incorporated. In particular, if the versions of the incorporated parts are updated but the incorporating binary package is not rebuilt, the older versions of the incorporated parts will remain in the archive in order to satisfy the license.
A package using the source code from the gcc-4.6-source binary package built from the gcc-4.6 source package would have this field in its control file:
Built-Using: gcc-4.6 (= 4.6.0-11)
A package including binaries from grub2 and loadlin would have this field in its control file:
Built-Using: grub2 (= 1.99-9), loadlin (= 1.6e-1)
This field should be used only when there are license or DFSG requirements to retain the referenced source packages. It should not be added solely as a way to locate packages that need to be rebuilt against newer versions of their build dependencies.
Build-Depends-Archpermit the use of alternative dependencies, these are not normally used by the Debian autobuilders. To avoid inconsistency between repeated builds of a package, the autobuilders will default to selecting the first alternative, after reducing any architecture-specific restrictions for the build architecture in question. While this may limit the usefulness of alternatives in a single release, they can still be used to provide flexibility in building the same package across multiple distributions or releases, where a particular dependency is met by differently named packages.
>were previously allowed, but they were confusingly defined to mean earlier/later or equal rather than strictly earlier/later.
dpkgstill supports them with a warning, but they are no longer allowed by Debian Policy.
This approach makes dependency resolution easier. If two packages A and B are being upgraded, the installed package A depends on exactly the installed package B, and the new package A depends on exactly the new package B (a common situation when upgrading shared libraries and their corresponding development packages), satisfying the dependencies at every stage of the upgrade would be impossible. This relaxed restriction means that both new packages can be unpacked together and then configured in their dependency order.
To see why
Breaksis normally needed in addition to
Replaces, consider the case of a file in the package foo being taken over by the package foo-data.
Replaceswill allow foo-data to be installed and take over that file. However, without
Breaks, nothing requires foo to be upgraded to a newer version that knows it does not include that file and instead depends on foo-data. Nothing would prevent the new foo-data package from being installed and then removed, removing the file that it took over from foo. After that operation, the package manager would think the system was in a consistent state, but the foo package would be missing one of its files.
Replaces is a one way relationship. You have to install the replacing package after the replaced package.
Build-Dependsin the source package is not adequate since it (rightfully) does not document the exact version used in the build.
The archive software might reject packages that refer to non-existent sources.