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<!-- -*- mode: sgml; mode: fold -*- -->
<!doctype refentry PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
<!ENTITY % aptent SYSTEM "apt.ent">
%aptent;
]>
<refentry>
&apt-docinfo;
<refmeta>
<refentrytitle>apt_preferences</>
<manvolnum>5</>
</refmeta>
<!-- Man page title -->
<refnamediv>
<refname>apt_preferences</>
<refpurpose>Preference control file for APT</>
</refnamediv>
<RefSect1><Title>Description</>
<para>
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 <filename>/etc/apt/preferences</>.
</RefSect1>
<RefSect1><Title>Versioning</>
<para>
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.
<para>
Selection by version can be done by exact match or prefix match. The format
is <literal/2.1.2/ or <literal/2.2*/ for a prefix match. Matching by prefix
can be used to ignore the <literal/r/ in the Debian release versioning, like
<literal/2.1r*/ or to ignore Debian specific revisions, <literal/1.1-*/.
When matching versions with a prefix the highest matching version will
always be picked.
<para>
Selection by release is more complicated and has three forms. The primary
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:
<informalexample><programlisting>
v=2.1*,o=Debian,c=main
l=Debian
a=stable
</programlisting></informalexample>
<para>
The data for these matches are taken from the <filename/Release/ files
that APT downloads during an <literal/update/. The available keys are:
<VariableList>
<VarListEntry><term>a= Archive</term>
<ListItem><Para>
This is the common name we give our archives, such as <literal/stable/ or
<literal/unstable/. The special name <literal/now/ is used to designate
the set of packages that are currently installed.
</VarListEntry>
<VarListEntry><term>c= Component</term>
<ListItem><Para>
Referes to the sub-component of the archive, <literal/main/,
<literal/contrib/ etc. Component may be omitted if there are no
components for this archive.
</VarListEntry>
<VarListEntry><term>v= Version</term>
<ListItem><Para>
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 <literal/2.1r2/ with the r designating the release of
2.1. New releases are limited to security updates and other important
changes.
</VarListEntry>
<VarListEntry><term>o= Origin</term>
<ListItem><Para>
This specifies who is providing this archive. In the case of Debian the
string will read <literal/Debian/. Other providers may use their own
string.
</VarListEntry>
<VarListEntry><term><term>l= Label</term>
<ListItem><Para>
This carries the encompassing name of the distribution. For Debian proper
this field reads <literal/Debian/. For derived distributions it should
contain their proper name.
</VarListEntry>
</VariableList>
<para>
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.
<para>
Version selection, particularly the latter two methods, are used in many
different parts of APT, not just the preferences file.
</RefSect1>
<RefSect1><Title>Candidate Version Policy</>
<para>
Internally APT maintains a list of all available versions for all packages.
If you place multiple releases or vendors in your &sources-list; file then
these features are available. By default APT selects the highest version
from all automatic sources. Some sources, such as
<filename>project/experimental</> are marked Not Automatic - these fall
to the bottom of the selection pile.
<para>
When deciding what version to use APT assigns a priority to each available
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.
<para>
It is possible to think of the priorities in strata:
<VariableList>
<VarListEntry><term>above 1000</term>
<ListItem><Para>
Downgradable priorities
</VarListEntry>
<VarListEntry><term>1000</term>
<ListItem><Para>
The downgrade prevention barrier
</VarListEntry>
<VarListEntry><term>101 to 999</term>
<ListItem><Para>
Standard priorities. 990 is the priority set by the
<option/--target-release / &apt-get; option. 989 is the start for auto
priorities and 500 are all the default package files.
</VarListEntry>
<VarListEntry><term>100</term>
<ListItem><Para>
The currently installed version
</VarListEntry>
<VarListEntry><term>0 to 99</term>
<ListItem><Para>
Non automatic priorities. These are only used if the package
is not installed and there is no other version available.
</VarListEntry>
<VarListEntry><term>less than 0</term>
<ListItem><Para>
The version is never selected.
</VarListEntry>
</VariableList>
<para>
Giving a pin a priority greater than 1000 will allow APT to downgrade
in order to get to that version.
<para>
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.
<para>
A package pin looks like this:
<informalexample><programlisting>
Package: apt
Pin: version 0.4.0
Pin-Priority: 1001
</programlisting></informalexample>
The first line specifies the package, the second gives the Pin specification
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.
<para>
A default pin is how the priorities of package files are set. Any number
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.
<informalexample><programlisting>
Package: *
Pin: release v=2.1*
Pin-Priority: 998
</programlisting></informalexample>
<para>
If the Pin-Priority field is omitted then the priority defaults to 989 for
both cases.
<RefSect2><title>Interesting Effects</>
<para>
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
<literal/1.2/, <literal/1.1/ and <literal/1.0/ installed, with
<literal/1.1/ being the currently installed version and the priorities of
each version being 900, 100 and 950 repectively the winning version will be
<literal/1.2/.
<para>
In practice this is often desired. A user may use a default pin to
make the stable distribution the default and then use the
<option/--target-dist/ option with &apt-get; 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.
<para>
If this is not desired then a default pin should be used to make unstable
have a priority less than 100.
<para>
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.
</RefSect2>
</RefSect1>
<RefSect1><Title>See Also</>
<para>
&apt-cache; &apt-conf;
</RefSect1>
&manbugs;
&manauthor;
</refentry>
|
