Shell Scripting: If Variable Is Not Defined, Set Default Variable

Posted on in Categories , , last updated July 27, 2009

If var is defined AND NOT EMPTY, use var, otherwise set a default variable under Bash. For e.g. my script needs a parameter for output variable. It can be text or html. I set it as follows in my script

output=$1 # either text or html

However, sometime user forget to pass the parameter to my shell script and my enter logic fails. So how do I set default value to text, if no parameter passed?

BASH, POSIX shell, and Korn (all versions) support the parameter expansion and testing. For e.g. if $1 is defined AND NOT EMPTY, use $1; otherwise, set to “text”, enter:

echo $output

OR (see my comment below):

echo $output

Try another example at a shell prompt:
$ vech=Bus
$ echo ${vech-Car}
$ echo ${vech:-Car}
$ unset vech
$ echo ${vech-Car}
$ echo ${vech:-Car}

Finally, here is a sample script:

echo "Setting output to $output..."

Now, run it as follows:
$ ./ html
$ ./ text
$ ./

You can also force to user to pass the parameter:

[ $# -eq 0 ] && { echo "Usage: $0 format" ; exit 1; }
echo "Setting output to $output..."

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+.

Share this on (or read 8 comments/add one below):

8 comment

  1. It surprises me that this works (and, at least, on bash 3, it does). I have always used the form
    ${VAR:-VALUE} in such a case, not ${VAR-VALUE}, but it seems that both work. The man pages of bash describe, however, ONLY the variant with a colon, so I wonder whether omitting the colon just exploits an undocumented feature, which might be gone with the next version of bash. Or did I miss here something in ‘man bash’?

  2. @Ronald,

    The original Bourne shell only supported above syntax and it works with all shells to keep portability. POSIX shells (KSH and BASH) offer a slight variant (as mentioned in bash man page):


    I should have mentioned both syntax..


  3. I do completely agree with what Ronald has said. I strongly recommend using : – because it is more logical. A lot of people will like it since they are looking for equivalent of If – Then – Else logic and in this case : stands for “then” and – for “else”.

Leave a Comment