I think that the heart of any OS is found on the command line. Yes, you need to learn vi(m) and maybe emacs, but I think that can be done by using them to do other tasks.

I developed my skills through shell scripting. A great book on the topic is Unix Shell Scripting by Stephen G. Kochan and Patrick H. Wood. The concepts are usable in just about any environment, plus they'll learn the main tools found in the shell.

Shut up foo! (new window)