By default almost every *nix-based operating system (OS) stores the distro / version info in a file or variable.

For bsd systems you can find out the version by typing:
uname -a

On linux systems, you can try the following command
cat /etc/issue

If that file doesn't exist or doesn't give the right info, try this
ls /etc/ | grep release
Usually a file will appear with a name like
redhat-release
debian-release
etc.
To read this file simply type
cat /etc/redhat-release or whatever the file is called, and the version info will appear