Ubuntu uses the Debian package system. There are multiple applications for managing the packages on your system including Apt, aptitude, dpkg, and Synaptic. Each of these tools interacts with the deb format package files.

Synaptic is a great tool for managing packages in a GUI. But what about the command line? I run an older Dell PowerEdge 400SC as a server literally in a closet in my house. It’s got power and a network cable so I manage it over SSH from my laptop or my Ubuntu desktop. For whatever reason, I never could figure out the right way to list packages already installed. I found two methods to list the packages.

dpkg-query

This tool lists the installed packages as well as some other information. It accepts a pattern for finding matching packages. In addition, it can show package status and all the files included in the package.

dpkg-query actions
-l, --list package-name-pattern...
	List packages matching given pattern.
-s, --status package-name...
	Report status of specified package.
-L, --listfiles package-name...
	List files installed to your system from package-name.
-S, --search filename-search-pattern...
	Search for a filename from installed packages.
-p, --print-avail package-name...
	Display details about package-name, as found in
	/var/lib/dpkg/available. Users of APT-based frontends
	should use apt-cache show package-name instead.

The following is an example for the ruby package:

$ dpkg-query --list ruby1.8
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name    Version            Description
+++-=======-==================-======================================================
ii  ruby1.8 1.8.7.174-1ubuntu1 Interpreter of object-oriented scripting language Ruby

apt-show-versions

Another useful tool for listing both the installed packages and information on out-of-date packages is apt-show-versions. It is not part of the base Ubuntu install but can easily be added:

$ sudo apt-get install apt-show-versions

The tool has many useful query options. It can show specific packages, match by a regex pattern, and list upgradeable packages:

Apt-Show-Versions v.0.16 (c) Christoph Martin

Usage:
 apt-show-versions         shows available versions of installed packages.

Options:
 -stf|--status-file=<file>  Use <file> as the dpkg status file instead
                            of /var/lib/dpkg/status
 -ld|list-dir=<directory>   Use <directory> as path to apt's list files instead
                            of /var/state/apt/lists/ or /var/lib/apt/lists/
 -p|--package=<package>     Print versions for <package>.
 -r|--regex                 Read package with -p as regex
 -R|--regex-all             Like --regex, but also show not installed packages.
 -u|--upgradeable           Print only upgradeable packages
 -a|--allversions           Print all available versions.
 -b|--brief                 Short output.
 -nh|--nohold               Don't treat holded packages.
 -i|--initialize            Initialize or update package cache only (as root).
 -v|--verbose               Verbose messages.
 -h|--help                  Print this help.

Examples:

$ apt-show-versions ruby1.8
ruby1.8/karmic-updates uptodate 1.8.7.174-1ubuntu1

$ apt-show-versions -a ruby1.8
ruby1.8 1.8.7.174-1ubuntu1 install ok installed
ruby1.8 1.8.7.174-1        karmic         us.archive.ubuntu.com
ruby1.8 1.8.7.174-1ubuntu1 karmic-updates us.archive.ubuntu.com
No stable version
ruby1.8/karmic-updates uptodate 1.8.7.174-1ubuntu1