Wildcard Parsing in Linux Shells
I have just discovered a quirk in linux shells.
When you issue a command with a wildcard, bash will replace the wildcard with a list of matching files. So when you type mogrify -resize 640x *.jpg
on a folder containing beach.jpg, pc.jpg and college.jpg, the shell will rewrite that as
mogrify -resize 640x beach.jpg college.jpg pc.jpg
and mogrify is given the options -resize 640x beach.jpg college.jpg pc.jpg (filenames in alphabetical order)
Now let us consider a folder with the following files: -r foo.bar foofolder barfolder
where foofolder and barfolder are folders. Issuing a rm *, might, at first glance just delete the files and not the folders. But alas this is expanded to
rm -r foo.bar foofolder barfolder
and rm recieves these options -r foo.bar foofolder barfolder
In fact, only the -r file will survive.
OK, you say, but who has files beginning with hyphens? Well, it could happen. And some commands, like tar, dont need a hyphen at the beginning of their arguments.
martin@xenon:~/tests$ ls
folder -r
martin@xenon:~/tests$ rm *
martin@xenon:~/tests$ ls
-r
I remember reading about how you can use this quirk to your advantage: for example, stick a file with name ‘-i’ in a directory full of irreplaceable files, and rm will run in interactive mode and prompt you to confirm all deletions. Not something I’d rely on, though, I have to say…
Comment by David — 6 October 2007 @ 2:44 pm