Well, actually it can do everything a programming language can do. As others have said, it is not the best choice for certain things, notably low level stuff like device drivers, but it can certainly de everything shell scripting can and much much more.

Nowadays I no longer write shell scripts, preferring Perl's better precision, control and error checking, but without some knowledge of the shell tools, it is probably hard to write the Perl version.

I would second suggestions to learn the basics of C and SQL as the most generally useful. Shell scripting in bash or similar a close third. I have gotten away with not knowning sed and awk at all, but I wouldn't like to completely be without find and grep from the comandline, although I don't use them in scripts anymore.

Regards,
Helgi Briem