7 – Seeing The World As The Shell Sees It

In this chapter we are going to look at some of the “magic” that occurs on the command

line when you press the enter key. While we will examine several interesting and com-

plex features of the shell, we will do it with just one new command:

● echo – Display a line of text

Expansion

Each time you type a command line and press the enter key, bash performs several pro-

cesses upon the text before it carries out your command. We have seen a couple of cases

of how a simple character sequence, for example “*”, can have a lot of meaning to the

shell. The process that makes this happen is called expansion. With expansion, you enter

something and it is expanded into something else before the shell acts upon it. To demon-

strate what we mean by this, let's take a look at the echo command. echo is a shell

builtin that performs a very simple task. It prints out its text arguments on standard out-

put:

[me@linuxbox ~]$ echo this is a test

this is a test

That's pretty straightforward. Any argument passed to echo gets displayed. Let's try an-

other example:

[me@linuxbox ~]$ echo *

Desktop Documents ls-output.txt Music Pictures Public Templates

Videos

So what just happened? Why didn't echo print “*”? As you recall from our work with

wildcards, the “*” character means match any characters in a filename, but what we didn't

see in our original discussion was how the shell does that. The simple answer is that the

shell expands the “*” into something else (in this instance, the names of the files in the