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APT-preferences overview

APT (the Advanced Package Tool) is configured by several resources, including

... in addition to the APT-preferences discussed below. APT-preferences can be used for various tasks; unfortunately only pinning is currently discussed below. For more detailed, authoritative information regarding

APT-preferences configuration

Like the rest of the APT configuration resources, APT-preferences is configured by writing information to files in one of two ways:

  1. The newer, more easily extensible, preferred way: writing multiple files to a single directory. For APT-preferences, that directory is /etc/apt/preferences.d/

  2. The old, deprecated way: writing all APT-preferences information to a single file, /etc/apt/preferences

Unlike the other APT configuration resources, APT-preferences is closely tied to APT-sources, so we will need to discuss that here also. APT-sources are similarly configured in one of two ways:

  1. The newer, more easily extensible, preferred way: writing multiple files to /etc/apt/sources.list.d/

  2. The old, deprecated way: writing all sources/repositories information to /etc/apt/sources.list

However, pinning also utilizes the concept of target release, which is typically set using APT-conf ... which is (you guessed) similarly configured in one of two ways:

  1. The newer, more easily extensible, preferred way: writing multiple files to /etc/apt/apt.conf.d/

  2. The old, deprecated way: writing all target-release information to /etc/apt/conf

Finally, the reader should understand that all of the following can be specified or overridden using various commandline, TUI, or GUI tools such as apt, apt-get and aptitude.

Pinning

<!> When pinning, you must ensure compatibility of packages by yourself since Debian does not guarantee it. Note that pinning is completely optional, and Debian does not encourage pinning without thorough consideration.

/!\ Seriously, don't do this. Doing this will break Debian and leave you with a system that doesn't work and can't be fixed. Use Backports instead!

Pinning allows you to pull particular packages from one version (e.g., stable, testing, unstable) without running your entire system from that version. Pinning is usually (though not always) used to pull one or more packages from a later version, in order to access more up-to-date versions of packages: unstable is considered later than testing which is later than stable. However, packages have dependencies which can conflict, so pinning (i.e., not pulling all your packages from one version) can cause problems. Accordingly, before you consider pinning, check to see if the version of the package that you want has been backported to the version from which you are already pulling packages.

/etc/apt/sources.list

#### testing  #########
deb http://deb.debian.org/debian testing main contrib non-free

#### unstable #########
deb http://deb.debian.org/debian unstable 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.

/etc/apt/preferences

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' 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.

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 install enlightenment/unstable
 # apt -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' will ask you before proceeding.

Notes from ZugSchlus

Disclaimer

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

-->Here is what one user thinks it should be.

ToDo: This needs to be verified

Examples of /etc/apt/preferences file

Example 1

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:

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.

Example 2

Objective: On an unstable system, pull dpatch from experimental.

A possible (and not completely correct) solution:

Example 3

From Mihamina rakotomandimby

Suppose you have a personnal repository where you have a personnal version of Postfix. You want Apt to prefer you version over official ones.

You may set preferences by Origin

Package: *
Pin: origin www.rktmb.org
Pin-Priority: 610

Package: *
Pin: origin ftp.fr.debian.org
Pin-Priority: 600

Debugging

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. The number on the right of the version number displays the actual pin priority being placed on the package.

Notes from Ryan B.

If you're wondering what the options are for release file preferences, based on this link: http://www.debian.org/doc/manuals/repository-howto/repository-howto

we find:

Archive: archive

Component: component

Origin: Your Company

Label: Your Company Debian repository

Architecture: architecture

Thus, the line: "Pin: release a=testing" would find archive values in the release file named "testing".

Notes from GeorgiosZarkadas

I found that using a higher than 500 value to pin testing/unstable resulted in apt upgrade wanting to upgrade all packages with a newer version to testing/unstable even while I had stable pinned with a higher value than testing/unstable.

After, querying the outcome of apt-cache policy <some-package> for a number of the packages that apt upgrade considered that they should be upgraded, I concluded that if there are multiple versions with candidates pinned to 500 or higher, then pinning is considered after the version number. Thus the highest version with the highest pin weight among the candidates of that (and only that) version is selected.

For example (having stable pinned to 700, testing to 650 and unstable to 600):

root@freedom:/etc/apt# apt-cache policy deborphan
deborphan:
  Εγκατεστημένα: 1.7.28.3+squeeze1
  Υποψήφιο:      1.7.28.5
  Πίνακας Έκδοσης:
     1.7.28.5 0
        650 http://ftp.gr.debian.org/debian/ wheezy/main amd64 Packages
        600 http://ftp.gr.debian.org/debian/ sid/main amd64 Packages
 *** 1.7.28.3+squeeze1 0
        500 http://ftp.gr.debian.org/debian/ squeeze-proposed-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.7.28.3 0
        500 cdrom://[Debian GNU/Linux 6.0.1a _Squeeze_ - Official amd64 DVD Binary-1 20110322-16:05]/ squeeze/main amd64 Packages
        700 http://ftp.gr.debian.org/debian/ squeeze/main amd64 Packages

Since my aim for using multiple sources and pinning was to be able to occasionally install a package from testing or unstable and not upgrade all my packages, pinning testing / unstable to values equal or greater than 500 is clearly an insuitable method for the stated cause.

However, I found that using a value less than 500 for testing/unstable has the desired effect. For example (removed stable pin and set testing to 450 and unstable to 400):

root@freedom:/etc/apt# apt-cache policy deborphan
deborphan:
  Εγκατεστημένα: 1.7.28.3+squeeze1
  Υποψήφιο:      1.7.28.3+squeeze1
  Πίνακας Έκδοσης:
     1.7.28.5 0
        450 http://ftp.gr.debian.org/debian/ wheezy/main amd64 Packages
        400 http://ftp.gr.debian.org/debian/ sid/main amd64 Packages
 *** 1.7.28.3+squeeze1 0
        500 http://ftp.gr.debian.org/debian/ squeeze-proposed-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.7.28.3 0
        500 cdrom://[Debian GNU/Linux 6.0.1a _Squeeze_ - Official amd64 DVD Binary-1 20110322-16:05]/ squeeze/main amd64 Packages
        500 http://ftp.gr.debian.org/debian/ squeeze/main amd64 Packages

Notes from Dmitriy Matrosov

I think, that Georgios Zarkadas's conclusion (from previous notes) about "pinning considered by apt after version number":

I concluded that if there are multiple versions with candidates pinned to 500 or higher, then pinning is considered '''after''' the version number.

is wrong. Let's look once again at example he posted:

root@freedom:/etc/apt# apt-cache policy deborphan
deborphan:
  Εγκατεστημένα: 1.7.28.3+squeeze1
  Υποψήφιο:      1.7.28.5
  Πίνακας Έκδοσης:
     1.7.28.5 0
        650 http://ftp.gr.debian.org/debian/ wheezy/main amd64 Packages
        600 http://ftp.gr.debian.org/debian/ sid/main amd64 Packages
 *** 1.7.28.3+squeeze1 0
        500 http://ftp.gr.debian.org/debian/ squeeze-proposed-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.7.28.3 0
        500 cdrom://[Debian GNU/Linux 6.0.1a _Squeeze_ - Official amd64 DVD Binary-1 20110322-16:05]/ squeeze/main amd64 Packages
        700 http://ftp.gr.debian.org/debian/ squeeze/main amd64 Packages

First, apt tries to upgrade from stable (squeeze/main) since stable has highest priority (700), but it sees, that installed package is newer:

sgf@shilvana:~$ dpkg --compare-versions 1.7.28.3 lt '1.7.28.3+squeeze1' ; echo $?
0

and priority 700 is not enough to downgrade. So, it looks further for candidate version. And now apt tries to upgrade from testing (wheezy/main) and sees, that version from testing is newer:

sgf@shilvana:~$ dpkg --compare-versions '1.7.28.3+squeeze1' lt 1.7.28.5 ; echo $?
0

And hence, this version (from testing) become a candidate.

So, in order to upgrade from stable (as Georgios Zarkadas wants), he must have this package at version <= than one found in stable.

References


CategoryPackageManagement | CategoryRedundant: merge with AptConf -> AptConfiguration