The package management system on Ubuntu makes it extremely simple to install and remove packages, but sometimes it's important to figure out what version of a particular package that you actually have installed.

The apt-get utilities are simply a front-end to the debian dpkg utility, which actually does the real work. You can use this utility to figure out what version is installed.

dpkg -s <packagename>

Here's an example, where I was trying to figure out what version of Ruby I had installed on my system:

geek@ubuntuServ:~$ dpkg -s ruby1.8

Package: ruby1.8
Status: install ok installed
Priority: optional
Section: interpreters
Installed-Size: 272
Maintainer: Ubuntu Core Developers <ubuntu-devel@lists.ubuntu.com>
Architecture: i386
Version: 1.8.4-5ubuntu1.2
Depends: libc6 (>= 2.4-1), libruby1.8 (>= 1.8.4)
Suggests: ruby1.8-examples, rdoc1.8, ri1.8
Description: Interpreter of object-oriented scripting language Ruby 1.8
Ruby is the interpreted scripting language for quick and easy
object-oriented programming. It has many features to process text
files and to do system management tasks (as in perl). It is simple,
straight-forward, and extensible.
.
This package provides version 1.8 series of Ruby.
.
On Debian, Ruby 1.8 is provided as separate packages. You can get
full Ruby 1.8 distribution by installing following packages.
.
ruby1.8 ruby1.8-dev ri1.8 rdoc1.8 irb1.8 ruby1.8-elisp
ruby1.8-examples libdbm-ruby1.8 libgdbm-ruby1.8 libtcltk-ruby1.8
libopenssl-ruby1.8 libreadline-ruby1.8
Original-Maintainer: akira yamada <akira@debian.org>

You can see that I've got version 1.8.4 installed.