The command set is different. In DOS, you learn command.com language. On a UNIX system, you usually have a choice of command shells. You could use sh/bash which is the bourne shell language. You could use csh/tcsh which uses C-language-like syntax and you can use ksh which is a class of its own. ;)
Depending on your choice of shell, different knowledge will give you benefits. However, knowing any imperative programming/scripting language will give you a head start, since all of the above works on the same principle. The grammar is only slightly different.
