Lists the contents of /usr/bin, providing each file's size in kilobytes, sorted by size. More advanced tcsh programming, including looping and if/ else processing, is beyond the scope of this book, but if you'd like to read further, the tcsh manual page (and the Internet) contain a wealth of further information on shell programming. The default shell in OS X is called tcsh, and is used whenever you launch Terminal (unless you've specifically told the system to use a different shell) we'll look at tcsh later in the chapter. Modern shells also provide a great deal of assistance in managing the command line-to help make using other command-line programs more efficient. The shell is your agent, relaying your commands to the kernel and other programs, showing you their responses, and providing a programming environment in which you can execute commands and scripts. The most important program in a command-line interface is the command shell. Terminal is simply a tool that acts as an intermediary between you and the command-line environment it doesn't do much of anything itself (somewhat like the steering wheel and pedals of a car, which don't actually move the car down the road, but enable you to direct the engine and wheels). However, it's important to realize that working at the command line isn't really working in Terminal so much as through it. You can see what's going on, and even poke around a bit, if you like. You can think of Terminal's command-line interface as a way to look 'under the hood' of Mac OS X.

Getting Started with Terminal and a Shell

Basic mac shell script example