I find a surprising number of otherwise skilled web developers that still have trepidation around the command line. I’m no expert myself, but a few of the basics, especially pertaining to grep, can really do wonders when you’re in a pinch in the middle of a development session.
Mark Bates has written a excellent book on the subject; I’ve already read the first two chapters and like what I’ve read. It’s easy to follow along and comprehensive. You can read the entire book contents online or purchase an ebook, screencasts, or a physical copy.