From Bash to Z Shell
If your usage of Mac OS X is the slightest bit “technical” then you should do yourself the favor of getting familiar with the extremely powerful shell environment.
Even if you already know a dozen scripting languages far more expressive than the shell, there are still contexts in which the shell is just better suited for the job, and understanding the POSIX environment is knowledge that is very valuable even outside the shell.
I’ve already noticed dramatic differences. I’m using shell loops at the command-line now to process many files at once; I actually understand shell quoting and when to use which types of quotes and escapes to get the desired effect; I can easily strip off a file extension or get a directory name from a full path when I need one; I make constant use of the command history now whether I’m searching for a past command, correcting a typo, or just pulling a single argument out of a previous command for reuse in a new command; and I’ve written a few shell functions to provide shortcuts to my common tasks.