Pin-Priority: -10
a=stable defines the name of the selected distribution. o=Debian limits the scope to packages whose origin is “Debian”.
Let's now assume that you have a server with several local programs depending on the version 5.10 of Perl and that you want to ensure that upgrades will not install another version of it. You could use this entry:
Package: perl
Pin: version 5.10*
Pin-Priority: 1001
The reference documentation for this configuration file is available in the manual page apt_preferences(5), which you can display with man apt_preferences.
TIP Comments in /etc/apt/preferences
There is no official syntax to put comments in the /etc/apt/preferences file, but some textual descriptions can be provided by putting one or more “Explanation” fields at the start of each entry:
Explanation: The package xserver-xorg-video-intel provided
Explanation: in experimental can be used safely
Package: xserver-xorg-video-intel
Pin: release a=experimental
Pin-Priority: 500
6.2.6. Working with Several Distributions
aptitude being such a marvelous tool, it is tempting to pick packages coming from other distributions. For example, after having installed a Stable system, you might want to try out a software available in Testing or Unstable without diverging too much from the system's initial state.
Even if you will occasionnaly encounter problems while mixing packages from different distributions, aptitude manages such coexistence very well and limits risks very effectively. The best way to proceed is to list all distributions used in /etc/apt/sources.list (some people always put the three distributions, but remember that Unstable is reserved for experienced users) and to define your reference distribution with the APT::Default-Release parameter (see Section 6.2.3, “System Upgrade”).
Let's suppose that Stable is your reference distribution but that Testing and Unstable are also listed in your sources.list file. In this case, you can use aptitude install package/testing to install a package from Testing. If the installation fails due to some unsatisfiable dependencies, let it solve those dependencies within Testing by adding the -t testing parameter. The same obviously applies to Unstable.
In this situation, upgrades (safe-upgrade and dist-upgrade) are done within Stable except for packages already upgraded to an other distribution: those will follow updates available in the other distributions. We'll explain this behavior with the help of the default priorities set by APT below. Do not hesitate to use apt-cache policy (see sidebar) to verify the given priorities.
Everything centers around the fact that APT only considers packages of higher or equal version than the installed one (assuming that /etc/apt/preferencs has not been used to force priorities higher than 1000 for some packages).
TIP apt-cache policy
To gain a better understanding of the mechanism of priority, do not hesitate to execute apt-cache policy to display the default priority associated with each package source. You can also use apt-cache policy package to display the priorities of all available versions of a given package.
Let's assume that you have installed version 1 of a first package from Stable and that version 2 and 3 are available respectively in Testing and Unstable. The installed version has a priority of 100 but the version available in Stable (the very same) has a priority of 990 (because it is part of the target release). Packages in Testing and Unstable have a priority of 500 (the default priority of a non-installed version). The winner is thus version 1 with a priority of 990. The package “stays in Stable”.
Let's take the example of another package whose version 2 has been installed from Testing. Version 1 is available in Stable and version 3 in Unstable. Version 1 (of priority 990 — thus lower than 1000) is discarded because it is lower than the installed version. This only leaves version 2 and 3, both of priority 500. Faced with this alternative, APT selects the newest version, the one from Unstable.If you don't want a package installed from Testing to migrate to Unstable, you have to assign a priority lower than 500 (490 for example) to packages coming from Unstable. You can modify /etc/apt/preferences to this effect:
Package: *
Pin: release a=unstable
Pin-Priority: 490
6.3. The apt-cache Command
The apt-cache command can display much of the information stored in APT's internal database. This information is a sort of cache since it is gathered from the different sources listed in the sources.list file. This happens during the aptitude update operation.
VOCABULARY Cache
A cache is a temporary storage system used to speed up frequent data access when the usual access method is expensive (performance-wise). This concept can be applied in numerous situations and at different scales, from the core of microprocessors up to high-end storage systems.
In the case of APT, the reference Packages files are those located on Debian mirrors. That said, it would be very ineffective to go through the network for every search that we might want to do in the database of available packages. That is why APT stores a copy of those files (in /var/lib/apt/lists/) and searches are done within those local files. Similarly, /var/cache/apt/archives/ contains a cache of already downloaded packages to avoid downloading them again if you need to reinstall them after a removal.
The apt-cache command can do keyword-based package searches with apt-cache search keyword. It can also display the headers of the package's available versions with apt-cache show package. This command provides the package's description, its dependencies, the name of its maintainer, etc. Note that aptitude search and aptitude show work in the same way.
Some features are more rarely used. For instance, apt-cache policy displays the priorities of package sources as well as those of individual packages. Another example is apt-cache dumpavail which displays the headers of all available versions of all packages. apt-cache pkgnames displays the list of all the packages which appear at least once in the cache.
6.4. Frontends: aptitude, synaptic
APT is a C++ program whose code mainly resides in the libapt-pkg shared library. Using a shared library facilitates the creation of user interfaces (front-ends), since the code contained in the library can easily be reused. Historically, apt-get was only designed as a test front-end for libapt-pkg but its success tends to obscure this fact.
6.4.1. aptitude
aptitude is an interactive program that can be used in semi-graphical mode on the console. You can browse the list of installed and available packages, look up all the available information, and select packages to install or remove. The program is designed specifically to be used by administrators, so that its default behaviors are much more intelligent than apt-get's, and its interface much easier to understand.
Figure 6.1. The aptitude package manager