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Linux 2.6 - man page for apt_preferences (linux section 5)

APT_PREFERENCES(5)			       APT			       APT_PREFERENCES(5)

NAME
       apt_preferences - Preference control file for APT

DESCRIPTION
       The APT preferences file /etc/apt/preferences and the fragment files in the
       /etc/apt/preferences.d/ folder 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.

       Preferences are a strong power in the hands of a system administrator but they can become
       also their biggest nightmare if used without care! APT will not questioning the
       preferences so wrong settings will therefore lead to uninstallable packages or wrong
       decisions while upgrading packages. Even more problems will arise if multiply distribution
       releases are mixed without a good understanding of the following paragraphs. Packages
       included in a specific release aren't tested in and therefore doesn't always work as
       expected in older or newer releases or together with other packages from different
       releases. You have been warned.

       Note that the files in the /etc/apt/preferences.d directory are parsed in alphanumeric
       ascending order and need to obey the following naming convention: The files have either no
       or "pref" as filename extension and only contain alphanumeric, hyphen (-), underscore (_)
       and period (.) characters. Otherwise APT will print a notice that it has ignored a file if
       the file doesn't match a pattern in the Dir::Ignore-Files-Silently configuration list - in
       this case it will be silently ignored.

   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. Note that this has precedence over any general
       priority you set in the /etc/apt/preferences file described later, but not over
       specifically pinned packages. 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 1
	   to the versions coming from archives which in their Release files are marked as
	   "NotAutomatic: yes" but not as "ButAutomaticUpgrades: yes" like the debian
	   experimental archive.

       priority 100
	   to the version that is already installed (if any) and to the versions coming from
	   archives which in their Release files are marked as "NotAutomatic: yes" and
	   "ButAutomaticUpgrades: yes" like the debian backports archive since squeeze-backports.

       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, except
       versions coming from archives which in their Release files are marked as "NotAutomatic:
       yes" - these versions get the priority 1 or priority 100 if it is additionally marked as
       "ButAutomaticUpgrades: yes".

       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 one or more specified
	   packages 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". Multiple packages can be separated by spaces.

	       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" which can be used to match a
	   hostname. The following record will assign a high priority to all versions available
	   from the server identified by the hostname "ftp.de.debian.org"

	       Package: *
	       Pin: origin "ftp.de.debian.org"
	       Pin-Priority: 999

	   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
	   distribution whose Codename is "wheezy".

	       Package: *
	       Pin: release n=wheezy
	       Pin-Priority: 900

	   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

   Regular expressions and glob() syntax
       APT also supports pinning by glob() expressions and regular expressions surrounded by /.
       For example, the following example assigns the priority 500 to all packages from
       experimental where the name starts with gnome (as a glob()-like expression) or contains
       the word kde (as a POSIX extended regular expression surrounded by slashes).

	   Package: gnome* /kde/
	   Pin: release n=experimental
	   Pin-Priority: 500

       The rule for those expressions is that they can occur anywhere where a string can occur.
       Thus, the following pin assigns the priority 990 to all packages from a release starting
       with karmic.

	   Package: *
	   Pin: release n=karmic*
	   Pin-Priority: 990

       If a regular expression occurs in a Package field, the behavior is the same as if this
       regular expression were replaced with a list of all package names it matches. It is
       undecided whether this will change in the future, thus you should always list wild-card
       pins first, so later specific pins override it.

       The pattern "*" in a Package field is not considered a glob() expression in itself.

   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/squeeze/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: or Suite: line
	   names the archive to which all the packages in the directory tree belong. For example,
	   the line "Archive: stable" or "Suite: 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 Codename: line
	   names the codename to which all the packages in the directory tree belong. For
	   example, the line "Codename: wheezy" specifies that all of the packages in the
	   directory tree below the parent of the Release file belong to a version named wheezy.
	   Specifying this value in the APT preferences file would require the line:

	       Pin: release n=wheezy

       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.

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

   Tracking the evolution of a codename release
       The following APT preferences file will cause APT to assign a priority higher than the
       default (500) to all package versions belonging to a specified codename of a distribution
       and a prohibitively low priority to package versions belonging to other Debian
       distributions, codenames and archives. Note that with this APT preference APT will follow
       the migration of a release from the archive testing to stable and later oldstable. If you
       want to follow for example the progress in testing notwithstanding the codename changes
       you should use the example configurations above.

	   Explanation: Uninstall or do not install any Debian-originated package versions
	   Explanation: other than those in the distribution codenamed with wheezy or sid
	   Package: *
	   Pin: release n=wheezy
	   Pin-Priority: 900

	   Explanation: Debian unstable is always codenamed with sid
	   Package: *
	   Pin: release n=sid
	   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 version(s) in the release codenamed with
       wheezy.

	   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 sid distribution. Thereafter, apt-get upgrade will upgrade the package to
       the most recent wheezy version if that is more recent than the installed version,
       otherwise, to the most recent sid version if that is more recent than the installed
       version.

	   apt-get install package/sid

FILES
       /etc/apt/preferences
	   Version preferences file. This is where you would specify "pinning", i.e. a preference
	   to get certain packages from a separate source or from a different version of a
	   distribution. Configuration Item: Dir::Etc::Preferences.

       /etc/apt/preferences.d/
	   File fragments for the version preferences. Configuration Item:
	   Dir::Etc::PreferencesParts.

SEE ALSO
       apt-get(8) apt-cache(8) apt.conf(5) sources.list(5)

BUGS
       APT bug page[1]. 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

NOTES
	1. APT bug page
	   http://bugs.debian.org/src:apt

Linux					 16 February 2010		       APT_PREFERENCES(5)


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