Renaming Files with Braces
Brian Hicks, October 19, 2020
This has been well-documented elsewhere, but in the interests of helping someone else join the lucky ten thousand today: Bash, ZSH, and other shells will expand comma-sepated arguments surrounded by braces into positional arguments.
As an altogether contrived example, imagine you want to echo "cat car can" in the terminal. You could write echo cat car can, right? But then you'd be writing ca twice. That won't do! We must be maximally efficient!
We can use brace expansion to only write ca once: echo ca{t,r,n}. The shell will expand that to your original "cat car can." Ta-da!
Next, say you want to add "bat bar ban" to the output. Good news: the shell will expand more than one set of braces in the same word! echo {c,b}a{t,r,n} will get you "cat car can bat bar ban". Sweet!
So, back to real life... when is this actually useful? Well, I use it all the time for renaming or moving files!
mv someFile.txt{,.bak} expands to mv someFile.txt someFile.txt.bak, adding the .bak extension- conversely,
mv someFile.txt{.bak,} removes the .bak mv config/database.yml{.sample,} expands to mv config/database.yml.sample config/database.ymlcp {src,dist}/index.js works great in Makefiles too!
Some shells (ZSH at least) will even expand that if you hit tab so you can see what you're going to execute before you hit commit. Handy!