How to find if a package is installed in Ubuntu

In Ubuntu, you can find out if a certain package is installed or not by using the dpkg program.

For example, to find out if package Foo is installed:

$ dpkg --get-selections Foo

Wildcards can also be used. For example, to find all the installed packages whose name begins with F:

$ dpkg --get-selections F*

Tried with: Dpkg 1.16.1.2 and Ubuntu 12.04.1 LTS