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
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!