Mac OS X in a Nutshell

Chapter 21. Shells and Shell Programming

As discussed in Chapter 19, a shell is a command-line interpreter program. It receives your typed-in commands when using the Terminal (or a similar program) and decides what to do with it. You can also use a shell in batch mode, treating it like an interpreted programming language and feeding it an entire scriptful of statements to execute, possibly including logic and flow control statements.

This chapter covers the shells included with Mac OS X, along with tcsh, its default shell program.

Introduction to the Shell

The shell is the user interface to Unix, and by the same token, several shells are available in Unix. Mac OS X provides you with more than one shell to choose from. Each shell has different features, but all of them affect how commands will be interpreted and provide tools to create your Unix environment.

Let’s suppose that the Unix operating system is a car. When you drive, you issue a variety of “commands”: you turn the steering wheel, press the accelerator, or press the brake. But how does the car translate your commands into the action you want? The car’s drive mechanism, which can be thought of as the car’s user interface, is responsible. Cars can be equipped with front-wheel drive, rear-wheel drive, four-wheel drive, and sometimes combinations of these.

The shell is simply a program that allows the system to understand your commands. (That’s why the shell is often called a command interpreter .) For many users, the shell ...