[Chapter 8] 8.9 Wildcards Inside of Aliases
Author: Jerry Peek, Tim O'Reilly & Mike Loukides
Date: 1998-08-04T21:34:23Z

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| Chapter 8 How the Shell Interprets What You Type |

## 8.9 Wildcards Inside of Aliases

Here's another example in which command-line parsing is important. Consider this alias for counting the number of words in all files:

    wc
    % alias words "wc -w *"

Right away, we can see one effect of command-line parsing. The shell sees the quotation marks, and knows not to expand wildcards inside the quotation marks. Therefore, `words` is aliased to `wc -w *`; the `*` isn't evaluated when you create the alias. (If wildcards were processed before quotes, this won't work.)

Now, think about what happens when you execute the alias. You type:

>     % words

The shell starts working through its steps (8.5), and eventually performs alias substitution. When this happens, it converts your command into:

>     wc -w *

Now, watch carefully. The shell continues working through the process of interpretation (redirection, variable substitution, command substitution), and eventually gets to filename expansion. At this point, the shell sees the `*` on the command line, expands it, and substitutes the files in the current directory. Seems simple enough. But think: you didn't type this `*`; the shell put it there when it expanded the wildcard. What would have happened if the shell expanded wildcards before substituting aliases? The `*` would never have been expanded; by the time the shell put it on the command line, the wildcard expansion stage would be over, and you'd just count the words in a file named `*` (which probably doesn't exist).

To me, the amazing thing is that all this works - and works well! The workings of the command line are intricate and complex, but the shell almost always does what you want - and without a lot of thought.

- ML

8.8 A Directory for Commands You Shouldn't Run |  | 8.10 eval: When You Need Another Chance