ubuntu: what version is installed?

The quick way is to run “lsb_release -a”

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 9.10
Release:        9.10
Codename:       karmic

Or just read the info from /etc/lsb-release

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=9.10
DISTRIB_CODENAME=karmic
DISTRIB_DESCRIPTION="Ubuntu 9.10"

It is also possible to check the file “/etc/issue”

$ cat /etc/issue
Ubuntu 9.10 \n \l

From X check out System->about.