Introduction to curly brace expansion in bash

From UnixCasts.com (formerly Learning the UNIX command line on OS X)

Hey there!

It’s time for our 6th installment of UNIX command line tips. In the previous newsletter, we talked about the .ssh/config file in Do you manage a lot of servers with ssh?, and how it allows you to quickly login to a server. This week we’re going to look at a technique that the bash shell provides, and how that can be used as a shortcut for creating certain files and directory structures.

Using curly brace expansion in bash

Did you know that the bash shell in UNIX offers a shortcut for specifying sometimes complex directory structures and filename combinations?

Well, it can, and that functionality is generally referred to as “curly brace expansion”, or just simply “brace expansion”. To illustrate this further, let’s walk through a few examples of it’s usage.

First, let’s say you wanted to create the following files using the touch command. Here’s the longhand version of how you could accomplish that:

chip@iMac ~/code/foo
☺ touch 1.txt 2.txt 3.txt 1.out 2.out 3.out
chip@iMac ~/code/foo
☺ ls -l
total 0
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 1.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 1.txt
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 2.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 2.txt
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 3.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:45 3.txt

That works fine, but if you had to create a lot of files it might become far too cumbersome.

There’s an easier way.

Let’s try it again, but this time using the brace expansion technique:

chip@iMac ~/code/foo
☺ touch {1,2,3}{.out,.txt}

And here are the files that the command created:

chip@iMac ~/code/foo
☺ ls -l
total 0
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 1.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 1.txt
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 2.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 2.txt
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 3.out
-rw-r--r--  1 chip  staff    0 Aug 11 13:38 3.txt

This produces the same result as before, but in a much easier fashion.

We can also use brace expansion for removing files as well, so let’s clean up our mess:

chip@iMac ~/code/foo
☺ rm {1,2,3}*

Another usage of this technique can be applied when creating directories. Here’s what I used recently for a Sinatra project that I was working on recently:

chip@iMac ~/code/foo
☺ mkdir -p config public/{images,javascripts,stylesheets} views db

And let’s take a look at the result:

chip@iMac ~/code/foo
☺ ls -lR
total 0
drwxr-xr-x  2 chip  staff   68 Aug 11 13:13 config
drwxr-xr-x  2 chip  staff   68 Aug 11 13:13 db
drwxr-xr-x  5 chip  staff  170 Aug 11 13:13 public
drwxr-xr-x  2 chip  staff   68 Aug 11 13:13 views

./config:

./db:

./public:
total 0
drwxr-xr-x  2 chip  staff  68 Aug 11 13:13 images
drwxr-xr-x  2 chip  staff  68 Aug 11 13:13 javascripts
drwxr-xr-x  2 chip  staff  68 Aug 11 13:13 stylesheets

./public/images:

./public/javascripts:

./public/stylesheets:

./views:

So that’s the basic idea of the brace expansion technique in bash. I encourage you to think about other combinations that might come in handy in your own work flow.

For more UNIX tips, please check out UnixCasts.com.