How to Use Brace Expansion in Bash Scripting [3 Cases]

Brace expansion is a feature in Bash that permits to generate a range of strings of various types. These strings can be used to generate files or directories, search path names, or in many other useful contexts. In this article, you’ll get a complete idea about brace expansion and its usage.

3 Different Cases of Brace Expansion in Bash Scripting

The list or lists of brace expansion may differ based on the context of expansion. For instance, expanding a range doesn’t require including all the items within the range while expanding a simple list does. However, the basic syntax of brace expansion remains the same. The list that needs to be expanded must be put with curly “{}” braces.

Case 1: Brace Expansion of a List to Create Directories in Bash

Brace expansion of a simple list without any pattern requires explicitly indicating all the items with the curly brace. To create multiple directories using brace expansion, follow the steps:

1. At first, launch an Ubuntu Terminal.
2. Open a file named bexpansion.sh using:

   nano bexpansion.sh

   

3. Copy the following scripts and paste them into the file.

   #!/bin/bash
   mkdir {Ubuntu,Redhat,Fedora}

   EXPLANATION

   #!/bin/bash specifies that the script should be interpreted and run using the Bash shell. It utilizes the mkdir command to create three directories named “Ubuntu”, “Redhat”, and “Fedora” in the current directory. The script employs brace expansion, to generate these directory names within curly braces, separated by commas.

4. Use the following two commands to make the file executable:

   chmod u+x bexpansion.sh

   

5. Run the script by the following command:

   ./bexpansion.sh

   As you can see, the program created three different directories by expanding the list within curly braces.

Case 2: Brace Expansion to Create a Sequence of Files

Brace expansion is super useful when you use it for expanding a range. To expand a range the start and the end of the range should be specified within the curly braces separated by two dots. The syntax should be- “{start . . end}”. The below Bash script will make files for each day of a particular month using brace expansion of a range:

#!/bin/bash

mkdir January
cd January
touch {1..31}
ls

As you see, the program creates empty files for each day of the month “January” by expanding the range {1..31}.

Apart from expanding a range with start and end values only, you can generate strings or numbers with increments or decrements as well. For example, let’s create a sequence from 1 to 15 with an increment of 2.

echo {1..15..2}
# Output: 1 3 5 7 9 11 13 15

Moreover, you can generate a sequence with a negative increment as follows.

echo {15..1..-1}

Finally, brace expansion has its nested use. You can combine multiple brace expansions to generate complex strings. For example:

echo {a,b,c{1,2,3},d}

Here, “{1,2,3}” is nested within the outer curly braces. While expanding, these nested curly braces also expand as you can see c1, c2 and c3 in the output.

The below table contains possible brace expansion associated with range.

Not All Ranges Can Be Expanded!

The below list contains some probable ranges that may end up with errors. Because these ranges are not valid or brace expansion doesn’t work that way.

Case 3: Brace Expansion With Preamble and Postscript in Bash

Sometimes brace expansions are used with preamble (prefix) and postscript (suffix). These help users to employ brace expansion efficiently and creatively to achieve certain tasks.

A. Expansion With Only Preamble

Preamble or prefix means certain text that is added before brace expansion. This structure is quite helpful when the common prefix is too long. A basic example of a preamble in brace expansion can be the following.

"I love scripting in "{Ubuntu, Fedora, Kali}

This script creates the preamble for globbing.

#!/bin/bash

mkdir /usr/local/bin{documents,code,backup,temp}
echo “Folders created successfully”

Once executed the program will create directories in the folder that is specified by the preamble “/usr/local/bin”. You can check the contents of the preamble using ls command.It shows that directories are created successfully in the location specified by the preamble.

B. Expansion With Postscript in Bash

Like a preamble, postscript, or suffix to brace expansion enhances its functionality, especially when you want to include longer text after the expansion. Let’s begin by examining a straightforward example of brace expansion with a postscript.

echo {readme,bug,update}".txt has been processed."

Here, postscript “.txt has been processed” has been added with each of the items of brace expansion.

Let’s see how this can be useful for creating multiple text file.

#!/bin/bash

rm {1..31}.txt
echo "Successfully deleted the files"

Once executed the script deletes files ranging from “1.txt” to “31.txt” using “.txt” as a postscript after brace expansion.

C. Expansion With Preamble and Postscript

One can use preamble and postscript to perform more complex and exceptional tasks. It can be file creation, globbing, or any other task. Just see the basic example to have an idea of how it works.

echo "I know "{Anita,Mohammed,Kelly}" as an adroit programmer."
# Output:I know Anita as an adroit programmer. I know Mohammed as an adroit programmer. I know Kelly as an adroit programmer.

Here the text “I know ” and “ as an adroit programmer” are used as preamble and postscript respectively.

You can use preamble and postscript to create multiple files using brace expansion. Think about the following code.

touch file{1..31}.txt

This creates files ranging from file1.txt to file31.txt where the text file and .txt files are used as preamble and postscript respectively.

Conclusion

In conclusion, brace expansion is indeed a useful feature for generating strings efficiently and quickly. When you are familiar with the syntax and different ranges, it is easy to apply brace expansion in various contexts. I believe from now you can apply all the techniques related to brace expansion.

People Also Ask

What is brace expansion for loop?

You can use brace expansion to create an iterable or sequence to iterate for the for loop. This makes the loop structure short and simple. To be specific for i in {1..5} makes the loop to iterate from 1 to 5.

Bash brace expansion not working?

There may be multiple reasons behind brace expansion not working. Errors may lie in the syntax of brace expansion and the range you provide to expand.

What is the difference between brace expansion and globbing?

The main difference is that brace expansion creates strings or sequences of strings while globbing matches patterns.

What are the alternatives of brace expansion for generating sequence?

To generate a sequence just like brace expansion, you can use the seq command. It is multi-functional and allows you to provide arguments.

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