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:
The data for these matches are taken from the Release files that APT downloads during an update. The available keys are:
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.
Referes to the sub-component of the archive, main, contrib etc. Component may be omitted if there are no components for this archive.
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.
This specifies who is providing this archive. In the case of Debian the string will read Debian. Other providers may use their own string.
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.
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
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.
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:
Pin: version 0.4.0
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.
Pin: release v=2.1
If the Pin-Priority field is omitted then the priority defaults to 989 for both cases.
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.