Not sure how far you can use Ruby for operating system admin tasks, and PHP is only useful for web applications. Python is a nice (OO) scripting language with a lot of library support for operating system tasks, and I think Perl offers a lot of system functionality but is also a lot harder to learn.
But you should start with Linux shell scripting, firstly because that's what it's for, and secondly because you will probably be able to find lots of sample scripts on the web to do common administrative tasks.
You should also be familiar with the differences between bash (which has all the bells and whistles, but is Linux specific) and the Korn shell (universal to just about all *nix OS's, a subset of bash).
Second scripting language? A toss-up between Perl and Python. "Python" is a toss-up between Python 2 (currently the most prevalent) and Python 3.
Ruby is a good language; it's popular with "Ruby-on-Rails" web applications. Similarly, PHP is most commonly used in PHP web apps.
I agree with the comments above - learn shell scripting first.
chris webster wrote:... and PHP is only useful for web applications
PHP works quite well from the command line, and you can do admin work with it. However I don't know of any admins who grab it as their tool of choice.
Paul Santa Maria wrote:... bash (which has all the bells and whistles, but is Linux specific)
GNU Bash is the shell used by default in Linux systems, but it is GNU software, not Linux specific software. It is also the default shell on a number of other operating systems, including Macintosh OS. For what it is worth, I don't remember an OS that I have used in the last 10 years that I have not used Bash on (including Windows 95, NT, 2000, XP; AIX, HPUX, VOS, FTX, ...)
Personally I use awk for a lot of my scripting, simply because when I first started using Unix systems, awk was always installed, but Perl was not guaranteed. Plus it is very C like. However if I were to start over, I would learn shell scripting first, then learn Perl.