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If you want to check which version of Ubuntu is installed on a machine run the following command:
lsb_release -a
On Debian the command is:
cat /etc/debian_version
The redhat equivalent, which would work on redhat derivatives as well, is:
cat /etc/redhat-release
From Niall:
em, lsb_release -a will work on Debian Etch, Centos 4+ and RHEL 4+
Make life easier for yourself and just learn the one!!