“Mac OS X is so interesting because it is Unix — or, rather, one of the many Unix variants — through and through. Strip away the colorful candy shell, and any number of bearded, suspendered old-school Unix hackers would feel right at home inside the confines of Mac OS X. In fact, if you ignore the graphical user interface (GUI) entirely and limit your interaction with the operating system to an 80-by-25-character text window, you’d be hard-pressed to identify Mac OS X as anything other than a true-blue FreeBSD (Berkeley Software Distribution) release, on which OS X is based.”