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APT_PREFERENCES(5)					    APT_PREFERENCES(5)

NAME
       apt_preferences - Preference control file for APT

DESCRIPTION
       The  APT	 preferences  file controls various aspects of the APT system.
       It is meant to be user editable and manipulatable  from	software.  The
       file  consists of a number of records formed like the dpkg status file,
       space seperated sections of text with at the start of  each  line  tags
       seperated by a colon. It is stored in /sw/etc/apt/preferences.

VERSIONING
       One  purpose  of	 the  preferences file is to let the user select which
       version of a package will be installed. This selection can be made in a
       number  of  ways	 that fall into three categories, version, release and
       origin.

       Selection by version can be done by exact match or  prefix  match.  The
       format  is  2.1.2 or 2.2* for a prefix match. Matching by prefix can be
       used to ignore the r in the Debian release versioning, like 2.1r* or to
       ignore Debian specific revisions, 1.1-*.	 When matching versions with a
       prefix the highest matching version will always be picked.

       Selection by release is more complicated and has three forms. The  pri‐
       mary  purpose  of  release  selections is to identify a set of packages
       that match a specific vendor, or release (ie Debian 2.1). The first two
       forms  are  shortcuts intended for quick command line use. If the first
       character of the specification is a digit then it is considered to be a
       release	version match, otherwise a release label match. Specifications
       which contain equals are full release data  matches  and	 are  a	 comma
       seperated  list	of  one	 letter keys followed by an equals then by the
       string. Examples:

       v=2.1*,o=Debian,c=main
       l=Debian
       a=stable

       The data for these matches are taken from the Release  files  that  APT
       downloads during an update. The available keys are:

       a= Archive
	      This  is the common name we give our archives, such as stable or
	      unstable. The special name now is used to designate the  set  of
	      packages that are currently installed.

       c= Component
	      Referes  to the sub-component of the archive, main, contrib etc.
	      Component may be omitted if there are no components for this ar‐
	      chive.

       v= Version
	      This  is	a  version  string  with the same properties as in the
	      Packages file.  It represents the release level of the  archive.
	      Typical Debian release numbers look like 2.1r2 with the r desig‐
	      nating the release of 2.1. New releases are limited to  security
	      updates and other important changes.

       o= Origin
	      This  specifies  who  is	providing this archive. In the case of
	      Debian the string will read  Debian.  Other  providers  may  use
	      their own string.

       l= Label
	      This  carries  the  encompassing	name  of the distribution. For
	      Debian proper this field reads Debian. For derived distributions
	      it should contain their proper name.

       The  final  selection method is by origin. This is simply the site name
       of the originating package files. The empty string  is  used  for  file
       URIs.

       Version	selection,  particularly  the  latter two methods, are used in
       many different parts of APT, not just the preferences file.

CANDIDATE VERSION POLICY
       Internally APT maintains a list of all available versions for all pack‐
       ages.	If   you   place   multiple   releases	 or  vendors  in  your
       sources.list(5) file then these features are available. By default  APT
       selects	the  highest version from all automatic sources. Some sources,
       such as project/experimental are marked Not Automatic - these  fall  to
       the bottom of the selection pile.

       When deciding what version to use APT assigns a priority to each avail‐
       able version of the package. It then does two things, first it  selects
       the highest priorty version that is newer than the installed version of
       the package, then it selects the highest priority version that is older
       than the installed version. Next, if the older versions have a priority
       greater than 1000 they are compared with the priority  of  the  upgrade
       set,  the  larger  becomes the selected result. Otherwise the downgrade
       versions are ignored and the highest priority of	 the  ugprade  set  is
       selected.

       It is possible to think of the priorities in strata:

       1000 and up
	      Downgradable priorities

       1000   The downgrade prevention barrier

       100 to 1000
	      Standard	priorities.  990  is the priority set by the --target-
	      release  apt-get(8) option. 989 is the start for auto priorities
	      and 500 are all the default package files.

       100    The currently installed version

       0 to 100
	      Non  automatic priorities. These are only used if the package is
	      not installed and there is no other version available.

       less than 0
	      The version is never selected.

       Giving a pin a priority greater than 1000 will allow APT	 to  downgrade
       in order to get to that version.

       Each package may be pinned to a specific version and each Packages file
       has a priority for every package inside. The highest priority  assigned
       to a package is the one that is used.

       A package pin looks like this:

       Package: apt
       Pin: version 0.4.0
       Pin-Priority: 1001

       The first line specifies the package, the second gives the Pin specifi‐
       cation and the last gives the priority of this pin. The first  word  of
       the  pin specification may be version, release or origin, the remainder
       of the field is described in the Versioning section above.

       A default pin is how the priorities of package files are set. Any  num‐
       ber  of	default pins may be specified, the first matching default will
       select the priority of the package file. Only release or origin may  be
       used in the Pin specification since they match Packages files.

       Package: *
       Pin: release v=2.1*
       Pin-Priority: 998

       If  the Pin-Priority field is omitted then the priority defaults to 989
       for both cases.

   INTERESTING EFFECTS
       Due to the downgrade prevention barrier at priority 1000 it is possible
       that  a	lower priority version will be selected if the higher priority
       would cause a downgrade. For instance, if package foo has versions 1.2,
       1.1  and	 1.0 installed, with 1.1 being the currently installed version
       and the priorities of each version being 900, 100 and  950  repectively
       the winning version will be 1.2.

       In practice this is often desired. A user may use a default pin to make
       the stable distribution the default  and	 then  use  the	 --target-dist
       option  with  apt-get(8)	 to  select  newer versions from unstable. The
       packages that have been upgraded to unstable will  continue  to	follow
       the  versions  that are available in unstable since the stable versions
       now fall below the downgrade prevention barrier.

       If this is not desired then a default pin should be used to make unsta‐
       ble have a priority less than 100.

       Users  of  3rd party add ons such as Helix GNOME can use this mechanism
       to force the usage of Helix packages, or	 force	the  usage  of	Debian
       packages	 by  setting the priority of that source sufficiently high. It
       is even possible to mass downgrade from one set of packages to  another
       by using a priority larger than 1000.

SEE ALSO
       apt-cache(8) apt.conf(5)

BUGS
       See  the APT bug page <URL:http://bugs.debian.org/apt>.	If you wish to
       report a bug in APT, please see /usr/share/doc/debian/bug-reporting.txt
       or the bug(1) command.

AUTHOR
       APT was written by the APT team <apt@packages.debian.org>.

				 12 March 2001		    APT_PREFERENCES(5)
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