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Dangers of wildcards in bash

By June 27, 2014 No Comments

Many web developers and some filemaker developers likely use bash while using the terminal.  You may have come across some odd issues when you use a command like “rm *”, when you’re in a folder with a lot of files or a file that starts with dash.  The challenge is that bash expands * to be a list of the files and folders rather than passing it on the command.  The issue with a lot of files is there’s a max command length and the issue with files that start with a dash is the command your trying to call might think your trying to pass it a argument and not a file.  This feature can get fun when you use something like gnu tar.

There’s a nice write-up of the feature/issue here.

	$ touch -- "--checkpoint=1"
	$ touch -- "--checkpoint-action=exec=sh shell.sh"
	$ echo "whoami" > shell.sh
	$ touch a
	$ sudo tar cf archive.tar *
	root

What happens when you run “tar cf archive.tar *” is bash turns that into “tar cf archive.tar a –checkpoint=1 –checkpoint-action=exec=sh shell.sh  shell.sh“.  GNU tar tries to be as helpful as it can process each — parameter in to something it should act on regardless of where it is in the command.

The question quickly becomes “What should I do to prevent issues like this?”.  For most commands there’s two methods around it.  The first is prefix the * with ./ (tar cf archive.tar ./*) because that will cause bash to prefix what’s passed with ./.  The second solution that works with most commands is pass before the * (tar cf archive.tar — *) as most commands take that as a hint that everything that’s after it is a filename.

Have questions? Let us know in a comment below, or contact our team directly. You can also check out our other posts on software development.

Brian Engert

Brian Engert

Brian is a DevOps Engineer for Soliant in Chicago. Brian specializes in PHP development and system administration and has been doing so for over 5 years. In Brian’s free time he likes to work on open source software and play online multiplayer games.

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