Learning the bash Shell, Second Edition

Shell Variables

bash derives much of its programming functionality from shell variables. We’ve already seen the basics of variables. To recap briefly: they are named places to store data, usually in the form of character strings, and their values can be obtained by preceding their names with dollar signs ($). Certain variables, called environment variables, are conventionally named in all capital letters, and their values are made known (with the export statement) to subprocesses.

If you are a programmer, you already know that just about every major programming language uses variables in some way; in fact, an important way of characterizing differences between languages is comparing their facilities for variables.

The chief difference between bash’s variable schema and those of conventional languages is that bash’s places heavy emphasis on character strings. (Thus it has more in common with a special-purpose language like SNOBOL than a general-purpose one like Pascal.) This is also true of the Bourne shell and the C shell, but bash goes beyond them by having additional mechanisms for handling integers explicitly.

Positional Parameters

As we have already seen, you can define values for variables with statements of the form varname = value, e.g.:

$ hatter=mad
$ echo "$hatter"
mad

Some environment variables are predefined by the shell when you log in. There are other built-in variables that are vital to shell programming. We will look at a few of them now and save the others for later. ...