Most of the Unix based OS have same set of basic commands i.e in-built commands, and apart from that, every OS has a different way to doing the same things. Well, if you want certification in Linux, then you can go with RHEL certification for System administration.
Check details here:
http://www.redhat.com/training/courses/ex200/
Also you can go for
Sun (now Oracle) certification: http://education.oracle.com/pls/web_...ms=pillar_id:6
But these certifications are not easy, and thoroughly test your practical knowledge of the domain. So it's better to keep learning & practicing for few more months, and then appear for a certification. Good luck!