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This article try to explain how to use APT Preferences. At this moment, only pinning is documented here.
Before you consider 'pinning', you might want to check if the package you want has been backported to your release.
Pinning allows you to run certain packages from one version (stable, testing, unstable) without the necessity of upgrading your entire system. However, pulling in packages from "later" distributions are prone to pull in libraries as well, which might have you end up with a system that has the disadvantages of stable (old software), the disadvantages of unstable/testing (security support not as good as stable, bugs) without the advantages of either.
At its most basic level, pinning involves two files, /etc/apt/sources.list and /etc/apt/preferences.
An additional role is played by the target release, which can be set in apt.conf (or in a /etc/apt/conf.d/... file and via the apt command line.
# official debian sites #### testing ######### deb http://http.us.debian.org/debian testing main contrib non-free deb http://non-us.debian.org/debian-non-US testing/non-US main contrib non-free
#### unstable ######### deb http://ftp.us.debian.org/debian unstable main non-free contrib deb http://non-us.debian.org/debian-non-US unstable/non-US main contrib non-free
In this example, we're pulling from testing and unstable. You could, of course, modify this to pull from stable as well.
The 'preferences' file is where the actual pinning takes place. Here's an example:
Package: * Pin: release a=testing Pin-Priority: 900
Package: * Pin: release a=unstable Pin-Priority: 800
Package defaults to any, as specified by the asterisk. Pin specifies the release (testing and unstable). Pin-Priority specifies the priority level. 'apt-get' defaults to something along the lines of "highest package version wins". The above restructures this priority so that packages in testing are given a higher priority.
You should also refer to the apt_preferences(5) manpage.
Installing from unstable
Let's assume that we're running testing and we want to try enlightenment from unstable. There are basically two methods for installing:
# apt-get install enlightenment/unstable # apt-get -t unstable install enlightenment
The first will not attempt to upgrade any packages on your system, so if specific dependencies are not met, the install will fail. The second method will attempt to install/upgrade any dependencies. Of course, given the above example, 'apt-get' will ask you before proceeding.
Notes from JoshuaRodman
Personally, I found the common configuration of a higher priority Testing pin and a lower priority Unstable pin to be problematic. At times, testing packages will depend upon other packages which are not currently in testing (perhaps representing a small glitch in testing) which causes packages to be automatically pulled in from unstable. In the period of testing prior to stabilization for the Woody release, this caused me to end up with over 100 unstable packages installed without even realizing it.
As a a result, I use a more conservative "only if I say so" approach to a mixed distribution, with a Pin file like this:
Package: * Pin: release a=testing Pin-Priority: 900
Package: * Pin: release o=Debian Pin-Priority: -10
Thus all debian packages are defaulted to priority -10, while testing receives a 900 point bonus. This invokes the behaviors:
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 [...] P < 0 : prevents the version from being installed
Note that a priority above 1000 will allow even downgrades no matter the version of the prioritary package. This means that you can use priority 1001 for a stable source if you want to downgrade to the stable versions of the packages you have installed (let's say from testing) on the system. this is not recommended unless the number of changes are minimal
Installing packages with apt-get install packagename/unstable and apt-get install -t unstable packagename will both still work, but unstable packages will only be installed by these commands.
Notes from ZugSchlus
This page has been written by ZugSchlus, who not even remotely grasps the concept of pinning. So, please take the words "probably", "needs to be verified" and similiar wordings literally, and document your findings (may they be "this page is right" or "this page is wrong", optionally "this page is wrong because") here.
Description of Package Selection Process
TODO: This needs to be verified
- Ignore packages that don't meet version criteria
Ignore packages lower versioned than current, unless their priority is > 1000
- Install highest priority remaining package
- In case of priority tie, take pinned package.
- This step should probably be skipped if the pin doesn't match anything
- In case of no pinned package, take highest version.
- In case of several packages with the same version, apt-get picks the one from the distro listed first in sources.list
Examples of /etc/apt/preferences file
Package: * Pin: release o=Debian,a=testing Pin-Priority: 900
Package: * Pin: release o=Debian,a=unstable Pin-Priority: 300
Package: * Pin: release o=Debian Pin-Priority: -1
Missing: Documentation what this preferences file does.
ZugSchlus tries to explain:
- All packages from a distribution called testing are pinned to 900
- All packages from a distribution called unstable are pinned to 300
- All other packages from Debian are pinned at -1 and thus never installed.
Problem: This pin behaves differently depending on which target release is set in other parts of apt configuration. Hence, this example cannot really be documented without adding more information. A non-pinned package being part of the target release has default priority 990, while other non-pinned packages have a default priority of 500.
Objective: On an unstable system, pull dpatch from experimental.
A possible (and not completely correct) solution:
Have both unstable and experimental in sources.list
In absence of explicit pinning, experimental will be automatically pinned to priority 1. This is because experimental's Release file contains NotAutomatic: yes.
Pin the wanted packages to a value x with 100<x<500:
Package: dpatch Pin: release o=Debian,a=experimental Pin-Priority: 450
A value > 500 will always select a package from experimental, even preventing a higher numbered unstable package from being installed, and selecting "no package at all" ahead of all available packages if the distribution not containing that package is pinned higher.
- Unfortunately, the Package field understands neither wildcards nor regular expressions. You need one pin stanza per package.
- Pinning the package to 450 will probably not automatically update to the experimental package, but it will probably track the package in experimental as long as it stays higher-versioend than the one in unstable.
apt-cache policy package gives information about the selection process. Unfortunately, it is not widely known what the output means. The following is a try to interpret:
$ apt-cache policy exim4-daemon-light exim4-daemon-light Installed: 4.50-1 Candidate: 4.50-1 Package Pin: (not found) Version Table: 4.50-4 555 500 http://mirror sid/main Packages *** 4.50-1 555 100 /var/lib/dpkg/status 4.44-2 555 500 http://mirror sarge/main Packages
The priority of each version/location is the number at the left of it. In this case, 500, 100, and 500. It is unclear what the number at the right of the version number means. Some people believe that it is just the last period that was specified for this package in /etc/apt/preferences for that package, while others said it is the actual pin priority being placed on the package.
TODO: This section was written after taking a lot of more or less educated guesses. Would somebody with real knowledge of apt please verify?
man 5 apt_preferences
- This link seems to have more detailed explanation:
Apt-Pinning for Beginners - a good reference, and heavily borrowed from.
APT HOWTO Section 3.10