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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 override 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 do 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 question the preferences, so wrong settings can lead to
       uninstallable packages or wrong decisions while upgrading packages.
       Even more problems will arise if multiple distribution releases are
       mixed without a good understanding of the following paragraphs.
       Packages included in a specific release aren't tested in (and therefore
       don'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, unless that file matches a pattern in the
       Dir::Ignore-Files-Silently configuration list - in which 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 do not belong to the target release.

       priority 990
	   to the versions that belong to the target release.
       The highest of those priorities whose description matches the version
       is assigned to the version.

       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.

       ·   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.)

       ·   Install the highest priority version.

       ·   If two or more versions have the same priority, install the most
	   recent one (that is, the one with the higher version number).

       ·   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.

       ·   The specific form assigns a priority (a "Pin-Priority") to one or
	   more specified packages with a 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
	   "5.20". Multiple packages can be separated by spaces.

	       Package: perl
	       Pin: version 5.20*
	       Pin-Priority: 1001

       ·   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 "buster".

	       Package: *
	       Pin: release n=buster
	       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 "9".

	       Package: *
	       Pin: release a=stable, v=9
	       Pin-Priority: 500

       The effect of the comma operator is similar to an "and" in logic: All
       conditions must be satisfied for the pin to match. There is one
       exception: For any type of condition (such as two "a" conditions), only
       the last such condition is checked.

   Regular expressions and glob(7) syntax
       APT also supports pinning by glob(7) expressions, and regular
       expressions surrounded by slashes. For example, the following example
       assigns the priority 500 to all packages from experimental where the
       name starts with gnome (as a glob(7)-like expression) or contains the
       word kde (as a POSIX extended regular expression surrounded by
       slashes).

	   Package: gnome* /kde/
	   Pin: release a=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 xenial.

	   Package: *
	   Pin: release n=xenial*
	   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(7) 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

       P = 0
	   has undefined behaviour, do not use it.

       The first specific-form record matching an available package version
       determines the priority of the package version. Failing that, the
       priority of the package is defined as the maximum of all priorities
       defined by generic-form records matching the version. Records defined
       using patterns in the Pin field other than "*" are treated like
       specific-form records.

       For example, suppose the APT preferences file contains the three
       records presented earlier:

	   Package: perl
	   Pin: version 5.20*
	   Pin-Priority: 1001

	   Package: *
	   Pin: origin ""
	   Pin-Priority: 999

	   Package: *
	   Pin: release unstable
	   Pin-Priority: 50

       Then:

       ·   The most recent available version of the perl package will be
	   installed, so long as that version's version number begins with
	   "5.20". If any 5.20* version of perl is available and the installed
	   version is 5.24*, then perl will be downgraded.

       ·   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.

       ·   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/stretch/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: buster" specifies that all
	   of the packages in the directory tree below the parent of the
	   Release file belong to a version named buster. Specifying this
	   value in the APT preferences file would require the line:

	       Pin: release n=buster

       the Version: line
	   names the release version. For example, the packages in the tree
	   might belong to Debian release version 9. 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=9
	       Pin: release a=stable, v=9
	       Pin: release 9

       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 buster or sid
	   Package: *
	   Pin: release n=buster
	   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 buster.

	   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 buster 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

APT 1.6~alpha6			15 August 2015		    APT_PREFERENCES(5)
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