What programming language should I learn?
It depends what you want to do.
- build quickly applications, reusing existing components and understanding a lot of the Linux ecosystem => Python
- process quickly huge amount of text files => Practical Exrtaction and Reporting Language (aka. PERL, yes you can do a lot of other things with it, but not so convincingly as with Python, IMHO)
- performance and resource critical algorithms => C++
- simple, fast and powerful websites / understanding CMS such as Drupal or Wordpress => PHP
- enterprise applications => Java (=> or .Net, but then I think that MS Windows is a better platform than CentOS even though I heard that Mono is working)
I'm personnally a Java developer and tend to do all of the above with Java.
If you are a sysadmin, I would recommend you Python: I don't know it well, but all people I know who used it love it, and again there are plenty of the software around which are based on it (also GUIs)