If you only need the version number the easiest way is to execute the following command
You should get a result like this:
There you have it, the server is running Debian 7.6!
Another command, that also gives a bit more information is
You should get a result like:
To find the Kernel version use the ‘uname‘ command. Using it without parameters will only print out ‘Linux’ and if you didn’t know that you were running Linux you are in trouble 
prints all Kernel information
prints your kernel version which is useful for installing things like the kernel headers, for example:
