Brace Expansions and How To Use Them

How brace expansions work

Have you needed to make strings on the terminal? Enter brace expressions.

Brace expressions let you generate arbitrary strings; they can be sequential, non-sequential, or a combination! At the core of the expansion is the pattern, which generates strings. Optionally, there can either be a preamble before the pattern and a postscript after the pattern. Think of both the preamble and the postscript as strings that surround the pattern that will be generated. The format can be expressed as <preamble>{ <pattern> }<postscript>.

In the following paragraphs, I will explain the patterns, preamble/postscript, and nesting for brace expansions.

Patterns

Patterns are the power behind brace expansions that generate strings. We place the patterns between curly braces like {...}

There are two patterns that you use for brace expansion:

1. Comma separated list of data
2. Sequence of expression (range)

The first pattern you can use is a list of strings. If your pattern is {banana,apple,orange} it will output banana apple orange.

Notice that without the optional preamble and postscript, the result is just a space separated list of the strings.

The second pattern you can use is a range with a start and an end. If your pattern is {1..5} it will output 1 2 3 4 5. Ranges can be numeric (1..3), alphabetic (a..c), or a combination.

We can also specify a steps value for the range. For example, {0..10..2} will produce only the even numbers from 0 to 10: 0 2 4 6 8 10.

Adding the preamble and the postscript

The preamble is like the prefix and the postscript is like the suffix around our pattern.

To add a preamble, just add a string to the left of the pattern. Likewise, add a string to the right of the pattern for the postscript.

Preambles and postscripts are always optional in brace expansion. If we wanted, we can omit either or both. When we use a brace expansion like {tasty,yum}, there is neither a preamble nor a postscript. We can also use empty strings if our pattern is a list. This will basically only concatenate the preamble and the postscript.

Nesting

When you combine brace expansions, you use them as either a preamble or postscript for another brace expansion. Its a Cartesian product!

A good example comes from The Bash Hackers Wiki. It first generates the uppercase letters then the lowercase letters.

$ echo {{A..Z},{a..z}}

Here's another example of nesting. I only have one set of cats that I can pet - but many, many dogs to pet!

$ echo pet-{{cat,kitten},{1..5}{dog,puppy}}
pet-cat pet-kitten pet-1dog pet-1puppy pet-2dog pet-2puppy pet-3dog pet-3puppy pet-4dog pet-4puppy pet-5dog pet-5puppy

Summary

In short, brace expansions generate a series of strings.

If you are confused on what strings a brace expansion will make, I recommend echo'ing it out.

$ echo {a,b,c}
a b c
$ echo {0..5}
0 1 2 3 4 5
$ echo {0..5}{a,b,c}
0a 0b 0c 1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c

The output from brace expansions produces strings that can be invaluable on the terminal. You can use them for command arguments, parameters, or output.

Note: Brace expansions are a feature that was introduced in Bash version 3.0. They are one of the 7 kinds of expansion that the Bash shell performs and are actually the very first expansion that the shell does in the order of expansions! Unfortunately, brace expansion is not portable since it isn't POSIX compliant.

1. Rename files

You can rename files with brace expansions. See mv file.{pdf,old} which translates to mv file.pdf file.old.

Below we rename a CSV file to an Excel file.

$ ls
spreadsheet.csv
$ mv spreadsheet.{csv,xlsx}
$ ls
spreadsheet.xlsx

2. Copy files

Brace expansions make it easy to make backups without having to provide the same path again for a second argument.

What if you have a super long file like audit.log.2026-01-01.gz and you wanted to make a backup copy? You could do cp audit.log.2026-01-01.gz audit.log.2026-01-01.gz.bkp but that's a lot of typing.

We can use brace expansions to literally run the same command but easier with: cp audit.log.2026-01-01.gz{,.bkp}

$ ls
audit.log.2026-01-01.gz
$ cp audit.log.2026-01-01.gz{,.bkp}
$ ls
audit.log.2026-01-01.gz
audit.log.2026-01-01.gz.bkp

What's the {,bkp}? Notice that there is no character between the opening brace and the first comma. That left part is empty. It translates to the path being just the preamble in the above.

3. Restore a backup file

It is also useful using the {bkp,} pattern. Here, its the right side of the comma that is empty.

$ ls
dnf.log.bkp
$ mv dnf.log{.bkp,}
$ ls
dnf.log 

4. Create bulk directories

You can create deeply nested directory trees with just one command.

Here are some examples:

• Generate a calendar for the months of January and February
  • mkdir -p calendar/{January/{1..31},February/{1..28}}
• Make the directory folder structure of a Eleventy project
  • mkdir -p eleventy-sample/{dist,scripts,style,es,asset,site{_include,_layout,_data}}

Make sure to use the -p command with mkdir if you are making directories where the subdirectories (or path) before them don't exist yet.

5. Loop over files

What if you want to go over a bunch of log files from log01.txt to log20.txt? You may do something funky with output from ls, grep, or seq. However, it can be simpler with brace expansion. The following pattern is useful when your files follow a sequential pattern.

for file in log{01..20}.txt ; do
  processLog(file)
done

You can use brace expansions to replace the command seq. for i in {1..10} is the same as for i in $(seq 10). In general, I would recommend using seq in scripts and brace expansion at the terminal prompt. This is because seq will provide exit codes (success/failure).

Note: Bash v4.0 introduced the zero-padding of expanded numeric values.

6. Bulk download files

Sometimes you need to download files from a website, this makes it easy!

You can use curl or wget to download files. The commands will see the expansions as separate URLs so you can download a bunch all at once!

wget http://blog.com/posts/{0..10}.html
wget http://blog.com/{index.html,styles.css,script.js}

7. Make an inventory of hosts

You can generate a hosts file with a list of your managed servers and hosts with brace expansions. Lets say you are given a couple parameters where a server is defined like <region>-<instance>-<role>:

• Region: north,south,west
• Instance: 1..20
• Role: app, db, cache, dev

Your brace expansion would be: {north,south,west}-{01..20}-{app,db,cache}

Now you can create your hosts file with one line: printf "%s\n" {north,south,west}-{01..20}-{app,db,cache,dev} > hosts.txt.

For an extra challenge, you can build inventory lists off of available naming schemes.

You can also use this same technique to generate usernames or email accounts.

References:

• Bash manpage
• GNU Bash Manual
• Mastering Eleventy Folder Structures: From Default Setups to Real-World Best Practices
• Linux Journal: My Favorite bash Tips and Tricks
• The BashHackers Wiki: Brace expansion

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