 |  |
| |
Here's another example in which command-line parsing is important. Consider this shell alias for counting the number of words in all files:
wc Section 16.6
% alias words "wc -w *" csh, tcsh $ alias words="wc -w *" ksh, bash
Right away, we can see one effect of command-line parsing. The shell sees the quotation marks and knows not to expand wildcards inside them. Therefore, words is aliased to wc -w *; the * isn't evaluated when you create the alias. (If wildcards were processed before quotes, this wouldn't work.)
Now, think about what happens when you execute the alias. You type:
% words
The shell starts working through its steps 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
|  | |
| 27.6. Controlling Shell Command Searches |  | 27.8. eval: When You Need Another Chance |
Copyright © 2003 O'Reilly & Associates. All rights reserved.