I agree with acid_kewpie: if you are happy with your shell scripting ability and want to move on to more advanced programming, then go to advanced scripting, then compiled programming, in that order.
My personal choice would be Perl, then C.
However, choosing a lang is like choosing an editor; can easily end up in a flame war
Just try a few of those suggestions and see how you go.
for Perl, here's a couple of very good links
I also heavily recommend the Perl CookBook if you can beg/buy/borrow/steal