Shell scripting and brace expansion

Posted on in Categories News last updated December 18, 2005

Expansion is performed on the command line after it has been split into words. Brace expansion is a mechanism by which arbitrary strings may be generated. A sequence expression takes the form {x..y}, where x and y are either integers or single characters. Simple bash brace expansion example:

$ echo F{1,2,3,4,5}
F1 F2 F3 F4 F5

It works with almost any command:

$ mkdir -p /home/project/{sales,purchase,reports}

It is funny but some time you can stuck in shell scripting very badly and you do not understand what is going on … For example I need to expand hostnames using host1.my.com,host2.my.com,…host10.my.com then I can use brace expansion at shell prompt as follows:

$ echo host{1..5}.my.com
host1.my.com host2.my.com host3.my.com host4.my.com host5.my.com

Now try it in a shell script:

#!/bin/bash
HOSTS="$1"
for i in $HOSTS
do
ping $i
# rest of logic
done

And then executed script by typing command:

$ ./myscript host{1..5}.my.com 

It will not expand to host1.my.com, host2.my.com….. :/? It took me more than two hours, finally while chatting with my friend he told me to replace HOSTS=”$1″ with HOSTS=”[email protected]”. Bingo it worked!
According to bash man page,”A sequence expression takes the form {x..y}, where x and y are either integers or single characters. When integers are supplied, the expression expands to each number between x and y, inclusive. When characters are supplied, the expression expands to each character lexicographically between x and y, inclusive. Note that both x and y must be of the same type”. [email protected] is a special shell variable which. expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. I must admit I need to master shell shell scripting skills 😉

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

2 comment

  1. I am trying to understand a shell script demo1.sh
    this is used to count the number of files in a directory

    declare -a args=( “[email protected]” )
    echo ${#args[@]} # count the elements of the array args

    bash demo1.sh *pdf will give you the correct number of pdf files.

    Can you explain how does this work or how is it getting expanded specially in first line
    (“[email protected]”) why the bracket () is used here and in second line args[@] what does this gets expanded to?

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