Kterou versi mám nainstalovanou?

lsb_release -d

which should give you the the description including the OS name ("Ubuntu" on an Ubuntu system) and the release number.

lsb_release -c

will give you just the codename (e.g., "precise" for Ubuntu 12.04 LTS). For the release number only, use

lsb_release -r

For all lsb version details, use

lsb_release -a