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This question is a sequel of how does grep work?

It seems that the shell does the expansion of *.* to all applicable/available files and provide that to utilities like grep.

Why does the shell need to do that? Why not leave pathname expansion to the utility itself?

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migrated from stackoverflow.com Dec 1 '11 at 0:52

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Why do hammer manufacturers screw the head to the handle? Why not leave that to the craftsman? –  delnan Aug 21 '11 at 16:57
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This is especially fun if you have a file named -rf and you use rm... –  Kerrek SB Aug 21 '11 at 16:58
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One thing that you should be aware of if you're coming from a Windows environment: the correct wildcard for "all non-hidden files in the current directory" is *, not *.*. It's very common in Unix/Linux to have files with no dots in their names. The system makes far less use of file extensions to decide what kind of file something is. For example, you'll commonly see "README" rather than "README.TXT". –  Keith Thompson Aug 21 '11 at 18:05
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and in Windows the shell does not do the expansion the utility does. So allowing rename *.txt *.bak –  Mark Aug 26 '11 at 12:38
    
@Mark: Thanks. Windows is really a beautiful piece of code :D –  hari Aug 26 '11 at 19:13

5 Answers 5

up vote 4 down vote accepted

Well one reason is that every utility would have to implement that functionality,. If the shell does it, it's only implemented once.

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Because if the shell didn't provided this feature, every single small utility would have to implement it.

This also ensures that the expansion works exactly the same way for every command.

BTW the expansion does not occur only in command arguments, it can occur anywhere in a shell script; for example:

for file in *.c; do something; done
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Not quite anywhere, though: in case $foo in *) command;; esac the * is syntactically a pattern, not a glob character for pathname expansion. Your shell man page or POSIX spec has all the gory details. –  Jens Aug 26 '11 at 14:03

If the shell didn't provide that functionality, then every utility would need to implement their own wildcard functionality (and probably most wouldn't bother doing so). As it is, each utility only has to process individual file names, which is much simpler.

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Because life would be nightmarishly horrible if it didn't.

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Ok, I admit this isn't terribly constructive. –  Keith Thompson Aug 21 '11 at 21:53

It's the Unix philosophy of "Have one tool do one thing well". Why would anybody want that code repetition in every utility? What's the point of having each and every utility after main() call expand_pathnames()? What's the point of putting that burden not only on C programs, but on all compiled languages and script languages as well? There is no point in all of that. Which is why Microsoft and the cmd.exe abomination got it wrong from day one :-)

BTW, if you want to turn off file name globbing, you can do that with

$ set -f
$ echo *
*
$ set +f
$ echo *
readme foo.o bar.txt
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Yes, I got the point. Thanks! –  hari Aug 22 '11 at 16:32

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