What Tacit said!!!
I've gotten my sum-total understanding (as far as it goes) of BSD UNIX (and several versions of embedded Linux) from my time spent "tinkering under the hood" of the various releases of Mac OS X over the past several years....and while I haven't installed the developer tools, I have done a tremendous amount of digging around in and modifying the underlying UNIX code that provides the Mac OS X experience for the rest of us.
Having a smoothly functional UNIX installed base to explore using both online and paper-based textual resources for additional info beyond the installed "man" files has been very effective at developing a working understanding of the language "in-context".
