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APT HOWTO
Chapter 3 - Managing packages

3.1 Updating the list of available packages

The packaging system uses a private database to keep track of which packages are installed, which are not installed and which are available for installation. The apt-get program uses this database to find out how to install packages requested by the user and to find out which additional packages are needed in order for a selected package to work properly.

To update this list, you would use the command apt-get update. This command looks for the package lists in the archives found in /etc/apt/sources.list; see The /etc/apt/sources.list file, Section 2.1 for more information about this file.

It's a good idea to run this command regularly to keep yourself and your system informed about possible package updates, particularly security updates.

3.2 Installing packages

Finally, the process you've all been waiting for! With your sources.list ready and your list of available packages up to date, all you have to do is run apt-get to get your desired package installed. For example, you can run:

APT will search it's database for the most recent version of this package and will retrieve it from the corresponding archive as specified in sources.list. In the event that this package depends on another -- as is the case here -- APT will check the dependencies and install the needed packages. See this example:

The package nautilus depends on the shared libraries cited, therefore APT will get them from the archive. If you had specified the names of these libraries on the apt-get command line, APT would not have asked if you wanted to continue; it would automatically accept that you wanted to install all of those packages.

This means that APT only asks for confirmation when it needs to install packages which weren't specified on the command line.

The following options to apt-get may be useful:

Multiple packages may be selected for installation in one line. Files downloaded from the network are placed in the directory /var/cache/apt/archives for later installation.

You can specify packages to be removed on the same command line, as well. Just put a '-' immediately after the name of the package to be removed, like this:

See section Removing packages, Section 3.3 for more details on package removal.

If you somehow damage an installed package, or simply want the files of a package to be reinstalled with the newest version that is available, you can use the --reinstall option like so:

3.3 Removing packages

If you no longer want to use a package, you can remove it from your system using APT. To do this just type: apt-get remove package. For example:

As you can see in the above example, APT also takes care of removing packages which depend on the package you have asked to remove. There is no way to remove a package using APT without also removing those packages that depend on it.

Running apt-get as above will cause the packages to be removed but their configuration files, if any, will remain intact on the system. For a complete removal of the package, run:

Note the '*' after the names. This indicates that the configuration files for each of these packages will also be removed.

Just as in the case of the install method, you can use a symbol with remove to invert the meaning for a particular package. In the case of removing, if you add a '+' right after the package name, the package will be installed instead of being removed.

Note that apt-get lists the extra packages which will be installed (that is, the packages whose installation is needed for the proper functioning of the package whose installation has been requested), those which will be removed, and those which will be installed (including the extra packages again).

3.4 Upgrading packages

Package upgrades are a great success of the APT system. They can be achieved with a single command: apt-get upgrade. You can use this command to upgrade packages within the same distribution, as well as to upgrade to a new distribution, although for the latter the command apt-get dist-upgrade is preferred; see section Upgrading to a new release, Section 3.5 for more details.

It's useful to run this command with the -u option. This option causes APT to show the complete list of packages which will be upgraded. Without it, you'll be upgrading blindly. APT will download the latest versions of each package and will install them in the proper order. It's important to always run apt-get update before you try this. See section Updating the list of available packages, Section 3.1. Look at this example:

The process is very simple. Note that in the first few lines, apt-get says that some packages were kept back. This means that there are new versions of these packages which will not be installed for some reason. Possible reasons are broken dependencies (a package on which it depends doesn't have a version available for download) or new dependencies (the package has come to depend on new packages since the last version).

There's no clean solution for this first case. For the second case, it's sufficient to run apt-get install for the specific package in question, as this will download the dependencies. An even cleaner solution is to use dist-upgrade. See section Upgrading to a new release, Section 3.5.

3.5 Upgrading to a new release

This feature of APT allows you to upgrade an entire Debian system at once, either through the Internet or from a new CD (purchased or downloaded as an ISO image).

It is also used when changes are made to the relationships between installed packages. With apt-get upgrade, these packages would be kept untouched (kept back).

For example, suppose that you're using revision 0 of the stable version of Debian and you buy a CD with revision 3. You can use APT to upgrade your system from this new CD. To do this, use apt-cdrom (see section Adding a CD-ROM to the sources.list file, Section 2.4) to add the CD to your /etc/apt/sources.list and run apt-get dist-upgrade.

It's important to note that APT always looks for the most recent versions of packages. Therefore, if your /etc/apt/sources.list were to list an archive that had a more recent version of a package than the version on the CD, APT would download the package from there.

In the example shown in section Upgrading packages, Section 3.4, we saw that some packages were kept back. We'll solve this problem now with the dist-upgrade method:

Note now that the packages will be upgraded, and new packages will also be installed (the new dependencies of the packages). Note too that lilo is still being kept back. It probably has a more serious problem than a new dependency. We can find out by running:

As noted in the above, lilo has a new conflict with the package debconf-tiny, which means it couldn't be installed (or upgraded) without removing debconf-tiny.

To know what's keeping or removing a package you may use:

This way, it's easy to notice that the python1.5-dev package cannot be installed because of an unsatisfied dependency: python1.5.

3.6 Removing unused package files: apt-get clean and autoclean

When you install a package APT retrieves the needed files from the hosts listed in /etc/apt/sources.list, stores them in a local repository (/var/cache/apt/archives/), and then proceeds with installation, see Installing packages, Section 3.2.

In time the local repository can grow and occupy a lot of disk space. Fortunately, APT provides tools for managing its local repository: apt-get's clean and autoclean methods.

apt-get clean removes everything except lock files from /var/cache/apt/archives/ and /var/cache/apt/archives/partial/. Thus, if you need to reinstall a package APT should retrieve it again.

apt-get autoclean removes only package files that can no longer be downloaded.

The following example show how apt-get autoclean works:

In /var/cache/apt/archives there are two files for the package logrotate and one for the package gpm.

apt-show-versions shows that logrotate_3.5.9-8_i386.deb provides the up to date version of logrotate, so logrotate_3.5.9-7_i386.deb is useless. Also gpm_1.19.6-11_i386.deb is useless because a more recent version of the package can be retrieved.

Finally, apt-get autoclean removes only the old files. See How to upgrade packages from specific versions of Debian, Section 3.9 for more information on apt-show-versions.

3.7 Using APT with dselect

dselect is a program that helps users select Debian packages for installation. It's considered somewhat complicated and rather boring, but with practice you can get the hang of its console-based ncurses interface.

One feature of dselect is that it knows how to make use of the capacity Debian packages have for "recommending" and "suggesting" other packages for installation. To use the program, run `dselect' as root. Choose 'apt' as your access method. This isn't truly necessary, but if you're not using a CD ROM and you want to download packages from the Internet, it's the best way to use dselect.

To gain a better understanding of dselect's usage, read the dselect documentation found on the Debian page http://www.debian.org/doc/ddp.

After making your selections with dselect, use:

as in the example below:

Compare with what we see when running apt-get dist-upgrade on the same system:

Note that many of the packages from above are being installed because other packages "suggested" or "recommended" them. Others are being installed or removed (in the case of lbxproxy, for example) per the choices we made while navigating through dselect's package listing. Dselect can be a powerful tool when used in conjunction with APT.

3.8 How to keep a mixed system

People are sometimes interested in using one of the Debian versions as its main system distribution and one or more packages from another branch.

To set up what is your main version of Debian you should edit the /etc/apt/apt.conf (it does not usually exist, create it if you don't have one) to contain the following line:

Where version is the version of Debian you want to use as the main distribution. The versions you can use are stable, testing and unstable. To install packages from another version, then, you must use APT in the following way:

For that to work, though, you need at least one APT source line in your /etc/apt/sources.list for the distribution you want the package from, and the package must exist on that source.

You can also request a specific version of a package using the following sintax:

For example, the line below will install version 2.2.4-1 of the nautilus package.:

IMPORTANT: the `unstable' version of Debian is the version to which the newest versions of Debian packages are uploaded first. This distribution sees all of the changes that packages go through, both small ones and more drastic ones which affect many packages or the whole system. For this reason, this version of the distribution should not be used by inexperienced users or by those who need proven stability.

The `testing' distribution is not necessarily better than `unstable', because it does not receive security updates quickly. For servers and other production systems stable should always be used.

3.9 How to upgrade packages from specific versions of Debian

apt-show-versions provides a safe way for users of mixed distributions to upgrade their systems without getting more of the less-stable distribution than they had in mind. For instance, it is possible to upgrade just your unstable packages by running after having installed the apt-show-versions package:

3.10 How to keep specific versions of packages installed (complex)

You may have occasion to modify something in a package and don't have time or don't want to port those changes to a new version of the program. Or, for instance, you may have just upgraded your Debian distribution to 3.0, but want to continue with the version of a certain package from Debian 2.2. You can "pin" the version you have installed so that it will not be upgraded.

Using this resource is simple. You just need to edit the file /etc/apt/preferences.

The format is simple:

Each entry must be separated from any other entries by a blank line. For example, to keep package sylpheed that I have modified to use "reply-to-list" at version 0.4.99, I add:

Note that I used an * (asterisk). This is a "wildcard"; it say that I want that this "pin" to be valid for all versions beginning with 0.4.99. This is because Debian versions its packages with a "Debian revision" and I don't want to avoid the installation of these revisions. So, for instance, versions 0.4.99-1 and 0.4.99-10 will be installed as soon as they are made available. Note that if you modified the package you won't want to do things this way.

The pin priority helps determine whether a package matching the "Packages:" and "Pin:" lines will be installed, with higher priorities making it more likely that a matching package will be installed. You can read apt_preferences(7) for a thorough discussion of priorities, but a few examples should give the basic idea. The following describes the effect of setting the priority field to different values in the sylpheed example above.

1001

Sylpheed version 0.4.99 will never be replaced by apt. If available, apt will install version 0.4.99 even if it would replace an installed package with a higher version. Only packages of priority greater than 1000 will ever downgrade an existing package.

1000

The effect is the same as priority 1001, except that apt will refuse to downgrade an installed version to 0.4.99

990

Version 0.4.99 will be replaced only by a higher version available from a release designated as preferred using the "APT::Default-Release" variable (see How to keep a mixed system, Section 3.8, above).

500

Any version higher than 0.4.99 of sylpheed which is available from any release will take preference over version 0.4.99, but 0.4.99 will still be preferred to a lower version.

100

Higher versions of sylpheed available from any release will take preference over version 0.4.99, as will any installed higher version of slypheed; so 0.4.99 will be installed only if no version is installed already. This is the priority of installed packages.

-1

Negative priorities are allowed as well, and prevent 0.4.99 from ever being installed.

A pin can be specified on a package's version, release or origin.

Pinning on a version, as we have seen, supports literal version numbers as well as wildcards to specify several versions at one time.

Option release depends on the Release file from an APT repository or from a CD. This option may be of no use at all if you're using package repositories that don't provide this file. You may see the contents of the Release files that you have on /var/lib/apt/lists/. The parameters for a release are: a (archive), c (components), v (version), o (origin) and l (label).

An example:

In this example, we chose version 2.2* of Debian (which can be 2.2r2, 2.2r3 -- this accommodates "point releases" that typically include security fixes and other very important updates), the stable repository, section main (as opposed to contrib or non-free) and origin and label Debian. Origin (o=) defines who produced that Release file, the label (l=) defines the name of the distribution: Debian for Debian itself and Progeny for Progeny, for example. A sample Release file:

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APT HOWTO