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APT_PREFERENCES(5)
Contents
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apt_preferences - Preference control file for APT
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 /etc/apt/preferences.
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 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:
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 archive.
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 designating
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 [Toc] [Back] Internally APT maintains a list of all available versions for
all packages. 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
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.
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
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.
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.
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 [Toc] [Back]
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
unstable 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.
apt-cache(8) apt.conf(5)
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.
APT was written by the APT team <apt@packages.debian.org>.
12 March 2001 APT_PREFERENCES(5)
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