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APT HOWTO (Obsolete Documentation)
Chapter 5 - Getting information about packages.

There are some front-end programs for the APT system that make it significantly easier to get listings of packages that are available for installation or are already installed, as well as to find out what section a package is in, what its priority is, what its description is, etc.

But... our goal here is to learn how to use pure APT. So how can you find out the name of a package that you want to install?

We have a number of resources for such a task. We'll begin with apt-cache. This program is used by the APT system for maintaining its database. We'll take just a brief look at some of its more practical applications.

5.1 Discovering package names

For example, suppose that you want to reminisce about the good old days of the Atari 2600. You want to use APT to install an Atari emulator, and then download some games. You can do:

     # apt-cache search atari
     atari-fdisk-cross - Partition editor for Atari (running on non-Atari)
     circuslinux - The clowns are trying to pop balloons to score points!
     madbomber - A Kaboom! clone
     tcs - Character set translator.
     atari800 - Atari emulator for svgalib/X/curses
     stella - Atari 2600 Emulator for X windows
     xmess-x - X binaries for Multi-Emulator Super System

We find several packages related to what we're looking for, together with brief descriptions. To get more information about a specific package, I can then use:

     # apt-cache show stella  
     Package: stella
     Priority: extra
     Section: non-free/otherosfs
     Installed-Size: 830
     Maintainer: Tom Lear <tom@trap.mtview.ca.us>
     Architecture: i386
     Version: 1.1-2
     Depends: libc6 (>= 2.1), libstdc++2.10, xlib6g (>= 3.3.5-1)
     Filename: dists/potato/non-free/binary-i386/otherosfs/stella_1.1-2.deb
     Size: 483430
     MD5sum: 11b3e86a41a60fa1c4b334dd96c1d4b5
     Description: Atari 2600 Emulator for X windows
      Stella is a portable emulator of the old Atari 2600 video-game console
      written in C++.  You can play most Atari 2600 games with it.  The latest
      news, code and binaries for Stella can be found at:
      http://www4.ncsu.edu/~bwmott/2600

In this output you have many details about the package that you want (or don't want) to install, together with the full description of the package. If the package is already installed on your system and there is a newer version, you'll see information about both versions. For example:

     # apt-cache show lilo
     Package: lilo
     Priority: important
     Section: base
     Installed-Size: 271
     Maintainer: Russell Coker <russell@coker.com.au>
     Architecture: i386
     Version: 1:21.7-3
     Depends: libc6 (>= 2.2.1-2), debconf (>=0.2.26), logrotate
     Suggests: lilo-doc
     Conflicts: manpages (<<1.29-3)
     Filename: pool/main/l/lilo/lilo_21.7-3_i386.deb
     Size: 143052
     MD5sum: 63fe29b5317fe34ed8ec3ae955f8270e
     Description: LInux LOader - The Classic OS loader can load Linux and others
      This Package contains lilo (the installer) and boot-record-images to
      install Linux, OS/2, DOS and generic Boot Sectors of other OSes.
      .
      You can use Lilo to manage your Master Boot Record (with a simple text screen)
      or call Lilo from other Boot-Loaders to jump-start the Linux kernel.
     
     Package: lilo
     Status: install ok installed
     Priority: important
     Section: base
     Installed-Size: 190
     Maintainer: Vincent Renardias <vincent@debian.org>
     Version: 1:21.4.3-2
     Depends: libc6 (>= 2.1.2)
     Recommends: mbr
     Suggests: lilo-doc
     Description: LInux LOader - The Classic OS loader can load Linux and others
      This Package contains lilo (the installer) and boot-record-images to
      install Linux, OS/2, DOS and generic Boot Sectors of other OSes.
      .
      You can use Lilo to manage your Master Boot Record (with a simple text screen)
      or call Lilo from other Boot-Loaders to jump-start the Linux kernel.

Note that the first in the list is the available package and the second is the one already installed. For more general information about a package, you can use:

     # apt-cache showpkg penguin-command
     Package: penguin-command
     Versions: 
     1.4.5-1(/var/lib/apt/lists/download.sourceforge.net_debian_dists_unstable_main_binary-i386_Packages)(/var/lib/dpkg/status)
     
     Reverse Depends: 
     Dependencies: 
     1.4.5-1 - libc6 (2 2.2.1-2) libpng2 (0 (null)) libsdl-mixer1.1 (2 1.1.0) libsdl1.1 (0 (null)) zlib1g (2 1:1.1.3) 
     Provides: 
     1.4.5-1 - 
     Reverse Provides:

And to just find out what packages it depends on:

     # apt-cache depends penguin-command
     penguin-command
       Depends: libc6
       Depends: libpng2
       Depends: libsdl-mixer1.1
       Depends: libsdl1.1
       Depends: zlib1g

In summary, we have a range of weapons we can use to find out the name of a package we want.

5.2 Using dpkg to find package names

One of the ways to locate the name of a package is to know the name of an important file found within the package. For example, to find the package that provides a particular ".h" file you need for compilation you can run:

     # dpkg -S stdio.h
     libc6-dev: /usr/include/stdio.h
     libc6-dev: /usr/include/bits/stdio.h
     perl: /usr/lib/perl/5.6.0/CORE/nostdio.h

or:

     # dpkg -S /usr/include/stdio.h
     libc6-dev: /usr/include/stdio.h

To find out the names of packages installed on your system, which is useful, for example, if you plan to clean up your hard drive, you can run:

     # dpkg -l | grep mozilla
     ii  mozilla-browse 0.9.6-7        Mozilla Web Browser

The problem with this command is that it can "break" the package name. In the example above, the full name of the package is mozilla-browser. To fix this, you can use the COLUMNS environment variable this way:

     [kov]@[couve] $ COLUMNS=132 dpkg -l | grep mozilla
     ii  mozilla-browser             0.9.6-7                     Mozilla Web Browser - core and browser

or the description or part of it this way:

     # apt-cache search "Mozilla Web Browser"
     mozilla-browser - Mozilla Web Browser

5.3 How to install packages "on demand"

You're compiling a program and, all of a sudden, boom! There's an error because it needs a .h file you don't have. The program auto-apt can save you from such scenarios. It asks you to install packages if they're needed, stopping the relevant process and continuing once the package is installed.

What you do, basically, is run:

     # auto-apt run command

Where `command' is the command to be executed that may need some unavailable file. For example:

     # auto-apt run ./configure

It will then ask to install the needed packages and call apt-get automatically. If you're running X, a graphical interface will replace the default text interface.

Auto-apt keeps databases which need to be kept up-to-date in order for it to be effective. This is achieved by calling the commands auto-apt update, auto-apt updatedb and auto-apt update-local.

5.4 How to discover to which package a file belongs

If you want to install a package, and you can't find out what it is called by searching with apt-cache, but know the filename of the program itself, or some other filename that belongs to the package, then you can use apt-file to find the package name. This is done like this:

     $ apt-file search filename

It works just like dpkg -S, but will also show you uninstalled packages that contain the file. It could also be used to find what packages contain necessary include files that are missing when compiling programs, although auto-apt is a much better method of solving such issues, see How to install packages "on demand", Section 5.3.

You can also list the contents of a package, by running:

     $ apt-file list packagename

apt-file keeps a database of which files all packages contain, just like auto-apt does and it needs to be up-to-date. This is done by running:

     # apt-file update

By default, apt-file uses the same database auto-apt is using, see How to install packages "on demand", Section 5.3.

5.5 How to keep informed about the changes in the packages.

Every package installs in its documentation directory (/usr/share/doc/packagename) a file called changelog.Debian.gz which contains the list of changes made to the package since the last version. You can read these files with zless' help, for example, but it is something not so easy, after an complete system upgrade, to start searching changelogs for every upgraded package.

There's a way to automatize this task by means of a tool called apt-listchanges. To begin with one needs to install the apt-listchanges package. During the package installation, Debconf will configure it. Some questions may not be shown to you depending on the priority you set up Debconf to use. Answer to the questions as you want.

The first question asks how you want the changes to be showed by apt-listchanges. You can have them mailed to you, which is good for automatic upgrades, or you can ask them in a pager like less, so you can inspect the changes before leting the upgrade continue. If you don't want apt-listchanges running automaticaly during upgrades you can answer none.

After apt-listchanges is installed, as soon as packages are downloaded (or gotten from a CD or mounted disk) by apt it will show the lists of changes made to those packages before installing them.