Great introduction to the Unix/OS X $PATH variable (and the command line) from A List Apart.
The command line is inherently spooky to many people—it’s the arcane technology wielded by “hackers” and “computer wizards” in popular culture. In reality, though, it isn’t that cool. It’s a set of ridiculously simple tools created by Bell (now AT&T) employees to accomplish mostly simple tasks in the 1970s. It’s about as “space-age” as your microwave oven.
It’s also extremely useful—like going from building a house by hand to using power tools. And through a few concepts and metaphors, we can shine a light on the darkest corners of this command line.