8.1 What the Shell Does

As we've said, the shell is just another program. It's responsible for interpreting the commands you type; there are four or five commonly used shells, and several other variants (1.8) kicking around.

Interpreting your commands might seem simple enough, but a lot of things happen between the time you press RETURN and the time the computer actually does what you want. The process of interpretation is very complex: the shell has to break the command into words, expand aliases (10.2), history operators (11.1), and shell and environment variables (6.8, 6.1). It also sets up standard input and output streams (13.1) and performs a lot of other tasks. Indeed, if a command looks right but doesn't work right, the cause is probably either:

• File permissions are set incorrectly.

• You don't understand how the shell is processing your command line.

I'd say that file permission problems are more common, but it's a close call. File permission problems are usually easy to understand, once you know what to look for, but the rules by which a shell interprets your command line are another thing altogether. Lest I scare you, we'll try to go slow with this material. Although it's difficult, understanding how the shell parses your commands is important to becoming a power user.

In this chapter, we'll look at how a UNIX shell interprets commands. The standard shells (the C shell, Bourne shell, and Korn shell) have similar interpretation rules. The C shell can be tricky at times, mostly because its behavior isn't as well defined as the others. However, there's nothing "magical" about these rules. Tomorrow morning, you may grab some new shell from the Net (52.9), and find out that it has a new and different way of interpreting commands. For better or worse, that's what UNIX is all about.

As part of this discussion, we'll cover quoting, which is the mechanism by which you can turn off the special meanings that the shell assigns to some characters. Quoting is an integral part of command-line processing; it allows you to control what the shell will do to your commands.