Installing QGIS only

Depending on whether you want to install a stable release or an experimental release, you will need to add the appropriate repository to the /etc/apt/sources.list file.

With sudo access, edit /etc/apt/sources.list and add the following line to install the current stable release or the current release source code, respectively:

deb       http://qgis.org/debian trusty main 
deb-src    http://qgis.org.debian trusty main 

Depending on the release version of Ubuntu you are using, you will need to specify the release name as trusty, saucy, or precise. For the latest list of QGIS releases for Ubuntu versions, visit download.qgis.org.

With the appropriate repository added, you can proceed with the QGIS installation by running the following commands:

sudo apt-get update
sudo apt-get install qgis python-qgis  

To install the GRASS plugin (recommended), install the optional package by running this command:

sudo apt-get install qgis-plugin-grass