(Or: Unix is not just a four-letter word.)
Although OS X is built on Unix, Apple has done an amazing job of shielding users from the command line; in fact, it's possible to completely ignore the Unix layers running "under the hood," if desired. Nevertheless, every time OS X boots up, it starts out as a command-line system, loading the familiar Aqua interface only after the Unix-based Darwin environment—which is always in the background, managing hardware and software—is ready. This means the command line is the ultimate power tool in OS X. It provides a "back door" to many advanced and hidden capabilities built into OS X.
In addition, OS X's Unix compatibility allows you to use a host of powerful (and often free) Unix programs. The best known is probably the Apache web server, but OS X also runs hundreds of other Unix-based programs such as the vi and emacs editors, the ipfw firewall, and the perl programming language (all built into the standard OS X installation; adding more programs can be surprisingly easy). Although OS X's Unix subsystem can be a bit intimidating to new users, Darwin brings a wealth of new capabilities to OS X. I'll spend this chapter talking about many of these features, and showing you ways to take advantage of them.
As I mentioned in the book's acknowledgments, the bulk of this chapter's content was contributed by Chris Pepper. You can find Chris online at http://www.reppep.com/~pepper/.