Determining Linux Version

Linux distributions don't tend to standardize where they store information about what release is installed. Going further, there's not even a standard way to detect what distribution is installed. If a system is running a popular distro, this information can usually be found in one of the following files:

Debian          /etc/debian_release, /etc/debian_version,
Fedora          /etc/fedora-release
Gentoo          /etc/gentoo-release
Mandrake        /etc/mandrake-release
Novell SUSE     /etc/SUSE-release
Red Hat         /etc/redhat-release, /etc/redhat_version
Slackware       /etc/slackware-release, /etc/slackware-version
Solaris/Sparc   /etc/release
Sun JDS         /etc/sun-release
Ubuntu          /etc/lsb-release
UnitedLinux     /etc/UnitedLinux-release
Yellow dog      /etc/yellowdog-release

You can find more information on this problem as well as a shell script to assist with OS detection here. An even larger master list can be found here.