Run this command to find the installed version of Ubuntu.
cat /etc/lsb-release
You will get the following info.
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=8.04
DISTRIB_CODENAME=hardy
DISTRIB_DESCRIPTION=”Ubuntu 8.04.3 LTS”
You could also use
cat /etc/issue
Which will display the version only:
Ubuntu 8.04.3 LTS n l