Bash: Brace expansion

#bash #linux #programming

Somewhere in my storage I keep PDF files with random content just for fun, for about six years now.

The funny thing was, I was revising its owner/group/user permissions and some of them were with a different permission than the usual.

A simple chmod command like the following would do the trick:

chmod 600 file01.pdf file05.pdf file09.pdf

But, as a developer, some times we just have that itch to spend 30 minutes trying an easier way of doing something that usually takes 20 seconds. Just for the sake of future proof your repo.

To make things challenging I didn't want to update needlessly the files that would match the pattern file0*.pdf. What would I use on this situation?

Since I new exactly which files I needed to update, using find regex felt a bit of overkill for this and quite verbose.

What came in mind to me were two things:

Brace expansion
Filename expansion

Both of them would work in a similar way but I chose to use brace expansion because I was already using it for some brute force scripting in a cybersecurity project that I was having fun with.

The final command looks like something that resembles this:

chmod 600 file0{5,9}.pdf

# which will expland to: chmod 600 file05.pdf file09.pdf

Problem solved. All done!

The cool part about brace expansion is that you have in hands something very powerful to work with patterns, and progressions.

For instance, if you need to loop through data with leading zeroes from 0000
to 9999:

# loop example
for f in {0000..9999}; echo $f; done

If you need to follow a pattern, showing only every n-th element you can use {0..100..2}, in this case it will expand to every even number from 0 to 100, the last portion is how big the step is in this pattern.

Aside from numbers you can also create expansions from letters: