APT_PREFERENCES(5) APT_PREFERENCES(5)
NAME
apt_preferences - Preference control file for APT
DESCRIPTION
The APT preferences file /etc/apt/preferences can be used to control
which versions of packages will be selected for installation.
Several versions of a package may be available for installation when
the sources.list(5) file contains references to more than one
distribution (for example, stable and testing). APT assigns a priority
to each version that is available. Subject to dependency constraints,
apt-get selects the version with the highest priority for installation.
The APT preferences file overrides the priorities that APT assigns to
package versions by default, thus giving the user control over which
one is selected for installation.
Several instances of the same version of a package may be available
when the sources.list(5) file contains references to more than one
source. In this case apt-get downloads the instance listed earliest in
the sources.list(5) file. The APT preferences file does not affect the
choice of instance, only the choice of version.
APT's Default Priority Assignments
If there is no preferences file or if there is no entry in the file
that applies to a particular version then the priority assigned to that
version is the priority of the distribution to which that version
belongs. It is possible to single out a distribution, "the target
release", which receives a higher priority than other distributions do
by default. The target release can be set on the apt-get command line
or in the APT configuration file /etc/apt/apt.conf. For example,
apt-get install -t testing some-package
APT::Default-Release "stable";
If the target release has been specified then APT uses the following
algorithm to set the priorities of the versions of a package. Assign:
priority 100
to the version that is already installed (if any).
priority 500
to the versions that are not installed and do not belong to the
target release.
priority 990
to the versions that are not installed and belong to the target
release.
If the target release has not been specified then APT simply assigns
priority 100 to all installed package versions and priority 500 to all
uninstalled package versions.
APT then applies the following rules, listed in order of precedence, to
determine which version of a package to install.
o Never downgrade unless the priority of an available version exceeds
1000. ("Downgrading" is installing a less recent version of a
package in place of a more recent version. Note that none of APT's
default priorities exceeds 1000; such high priorities can only be
set in the preferences file. Note also that downgrading a package
can be risky.)
o Install the highest priority version.
o If two or more versions have the same priority, install the most
recent one (that is, the one with the higher version number).
o If two or more versions have the same priority and version number
but either the packages differ in some of their metadata or the
--reinstall option is given, install the uninstalled one.
In a typical situation, the installed version of a package (priority
100) is not as recent as one of the versions available from the sources
listed in the sources.list(5) file (priority 500 or 990). Then the
package will be upgraded when apt-get install some-package or apt-get
upgrade is executed.
More rarely, the installed version of a package is more recent than any
of the other available versions. The package will not be downgraded
when apt-get install some-package or apt-get upgrade is executed.
Sometimes the installed version of a package is more recent than the
version belonging to the target release, but not as recent as a version
belonging to some other distribution. Such a package will indeed be
upgraded when apt-get install some-package or apt-get upgrade is
executed, because at least one of the available versions has a higher
priority than the installed version.
The Effect of APT Preferences
The APT preferences file allows the system administrator to control the
assignment of priorities. The file consists of one or more multi-line
records separated by blank lines. Records can have one of two forms, a
specific form and a general form.
o The specific form assigns a priority (a "Pin-Priority") to a
specified package and specified version or version range. For
example, the following record assigns a high priority to all
versions of the perl package whose version number begins with "5.8".
Package: perl
Pin: version 5.8*
Pin-Priority: 1001
o The general form assigns a priority to all of the package versions
in a given distribution (that is, to all the versions of packages
that are listed in a certain Release file) or to all of the package
versions coming from a particular Internet site, as identified by
the site's fully qualified domain name.
This general-form entry in the APT preferences file applies only to
groups of packages. For example, the following record assigns a high
priority to all package versions available from the local site.
Package: *
Pin: origin ""
Pin-Priority: 999
A note of caution: the keyword used here is "origin". This should not
be confused with the Origin of a distribution as specified in a Release
file. What follows the "Origin:" tag in a Release file is not an
Internet address but an author or vendor name, such as "Debian" or
"Ximian".
The following record assigns a low priority to all package versions
belonging to any distribution whose Archive name is "unstable".
Package: *
Pin: release a=unstable
Pin-Priority: 50
The following record assigns a high priority to all package versions
belonging to any release whose Archive name is "stable" and whose
release Version number is "3.0".
Package: *
Pin: release a=stable, v=3.0
Pin-Priority: 500
How APT Interprets Priorities
Priorities (P) assigned in the APT preferences file must be positive or
negative integers. They are interpreted as follows (roughly speaking):
P > 1000
causes a version to be installed even if this constitutes a
downgrade of the package
990 < P <=1000
causes a version to be installed even if it does not come from the
target release, unless the installed version is more recent
500 < P <=990
causes a version to be installed unless there is a version available
belonging to the target release or the installed version is more
recent
100 < P <=500
causes a version to be installed unless there is a version available
belonging to some other distribution or the installed version is
more recent
0 < P <=100
causes a version to be installed only if there is no installed
version of the package
P < 0
prevents the version from being installed
If any specific-form records match an available package version then
the first such record determines the priority of the package version.
Failing that, if any general-form records match an available package
version then the first such record determines the priority of the
package version.
For example, suppose the APT preferences file contains the three
records presented earlier:
Package: perl
Pin: version 5.8*
Pin-Priority: 1001
Package: *
Pin: origin ""
Pin-Priority: 999
Package: *
Pin: release unstable
Pin-Priority: 50
Then:
o The most recent available version of the perl package will be
installed, so long as that version's version number begins with
"5.8". If any 5.8* version of perl is available and the installed
version is 5.9*, then perl will be downgraded.
o A version of any package other than perl that is available from the
local system has priority over other versions, even versions
belonging to the target release.
o A version of a package whose origin is not the local system but some
other site listed in sources.list(5) and which belongs to an
unstable distribution is only installed if it is selected for
installation and no version of the package is already installed.
Determination of Package Version and Distribution Properties
The locations listed in the sources.list(5) file should provide
Packages and Release files to describe the packages available at that
location.
The Packages file is normally found in the directory
.../dists/dist-name/component/arch: for example,
.../dists/stable/main/binary-i386/Packages. It consists of a series of
multi-line records, one for each package available in that directory.
Only two lines in each record are relevant for setting APT priorities:
the Package: line
gives the package name
the Version: line
gives the version number for the named package
The Release file is normally found in the directory
.../dists/dist-name: for example, .../dists/stable/Release, or
.../dists/woody/Release. It consists of a single multi-line record
which applies to all of the packages in the directory tree below its
parent. Unlike the Packages file, nearly all of the lines in a Release
file are relevant for setting APT priorities:
the Archive: line
names the archive to which all the packages in the directory tree
belong. For example, the line "Archive: stable" specifies that all
of the packages in the directory tree below the parent of the
Release file are in a stable archive. Specifying this value in the
APT preferences file would require the line:
Pin: release a=stable
the Version: line
names the release version. For example, the packages in the tree
might belong to Debian GNU/Linux release version 3.0. Note that
there is normally no version number for the testing and unstable
distributions because they have not been released yet. Specifying
this in the APT preferences file would require one of the following
lines.
Pin: release v=3.0
Pin: release a=stable, v=3.0
Pin: release 3.0
the Component: line
names the licensing component associated with the packages in the
directory tree of the Release file. For example, the line
"Component: main" specifies that all the packages in the directory
tree are from the main component, which entails that they are
licensed under terms listed in the Debian Free Software Guidelines.
Specifying this component in the APT preferences file would require
the line:
Pin: release c=main
the Origin: line
names the originator of the packages in the directory tree of the
Release file. Most commonly, this is Debian. Specifying this origin
in the APT preferences file would require the line:
Pin: release o=Debian
the Label: line
names the label of the packages in the directory tree of the Release
file. Most commonly, this is Debian. Specifying this label in the
APT preferences file would require the line:
Pin: release l=Debian
All of the Packages and Release files retrieved from locations listed
in the sources.list(5) file are stored in the directory
/var/lib/apt/lists, or in the file named by the variable
Dir::State::Lists in the apt.conf file. For example, the file
debian.lcs.mit.edu_debian_dists_unstable_contrib_binary-i386_Release
contains the Release file retrieved from the site debian.lcs.mit.edu
for binary-i386 architecture files from the contrib component of the
unstable distribution.
Optional Lines in an APT Preferences Record
Each record in the APT preferences file can optionally begin with one
or more lines beginning with the word Explanation:. This provides a
place for comments.
The Pin-Priority: line in each APT preferences record is optional. If
omitted, APT assigs a priority of 1 less than the last value specified
on a line beginning with Pin-Priority: release ....
EXAMPLES
Tracking Stable
The following APT preferences file will cause APT to assign a priority
higher than the default (500) to all package versions belonging to a
stable distribution and a prohibitively low priority to package
versions belonging to other Debian distributions.
Explanation: Uninstall or do not install any Debian-originated
Explanation: package versions other than those in the stable distro
Package: *
Pin: release a=stable
Pin-Priority: 900
Package: *
Pin: release o=Debian
Pin-Priority: -10
With a suitable sources.list(5) file and the above preferences file,
any of the following commands will cause APT to upgrade to the latest
stable version(s).
apt-get install package-name
apt-get upgrade
apt-get dist-upgrade
The following command will cause APT to upgrade the specified package
to the latest version from the testing distribution; the package will
not be upgraded again unless this command is given again.
apt-get install package/testing
Tracking Testing or Unstable
The following APT preferences file will cause APT to assign a high
priority to package versions from the testing distribution, a lower
priority to package versions from the unstable distribution, and a
prohibitively low priority to package versions from other Debian
distributions.
Package: *
Pin: release a=testing
Pin-Priority: 900
Package: *
Pin: release a=unstable
Pin-Priority: 800
Package: *
Pin: release o=Debian
Pin-Priority: -10
With a suitable sources.list(5) file and the above preferences file,
any of the following commands will cause APT to upgrade to the latest
testing version(s).
apt-get install package-name
apt-get upgrade
apt-get dist-upgrade
The following command will cause APT to upgrade the specified package
to the latest version from the unstable distribution. Thereafter,
apt-get upgrade will upgrade the package to the most recent testing
version if that is more recent than the installed version, otherwise,
to the most recent unstable version if that is more recent than the
installed version.
apt-get install package/unstable
SEE ALSO
apt-get(8) apt-cache(8) apt.conf(5) sources.list(5)
BUGS
[1]APT bug page. If you wish to report a bug in APT, please see
/usr/share/doc/debian/bug-reporting.txt or the reportbug(1) command.
AUTHOR
APT team
Author.
REFERENCES
1. APT bug page
http://bugs.debian.org/src:apt
Linux 29 February 2004 APT_PREFERENCES(5)
