Shell scripting and brace expansion

Posted by Vivek Gite [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="$1" with HOSTS="$@". 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". $@ 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