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Expansion
Each time we type a command and press the enter key, bash performs several processes upon the text before it carries out our 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, we enter something and it is expanded into something else before the shell acts upon it. To demonstrate what we mean by this, let's take a look at the echo command. echo is a shell builtin that per- forms a very simple task. It prints out its text arguments on standard output:
[me@linuxbox ~]$ echo this is a test
this is a test
[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
[me@linuxbox ~]$ echo *
Desktop Documents ls-output.txt Music Pictures Public Templates Videos
So what just happened? Why didn't echo print “*”? As we 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 current working directory) before the echo command is executed. When the enter key is
pressed, the shell automatically expands any qualifying characters on the command line before the command is carried out, so the echo command never saw the “*”, only its ex- panded result. Knowing this, we can see that echo behaved as expected.