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

by on July 24, 2009 · 6 comments· LAST UPDATED July 27, 2009

in , ,

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:

output=${1-text}
echo $output

OR (see my comment below):

output=${1:-text}
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:

#!/bin/bash
output=${1-text}
echo "Setting output to $output..."

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

You can also force to user to pass the parameter:

#!/bin/bash
output=${1-text}
[ $# -eq 0 ] && { echo "Usage: $0 format" ; exit 1; }
echo "Setting output to $output..."
TwitterFacebookGoogle+PDF versionFound an error/typo on this page? Help us!

{ 6 comments… read them below or add one }

1 Chris July 25, 2009 at 1:15 pm

Wow, this is very helpful. Thanks!

Reply

2 Rahul Sonar July 26, 2009 at 3:45 pm

thanks buddy, very useful post..

Reply

3 Susinthiran July 27, 2009 at 6:59 am

I didn’t know that we could easily use “output=${1-text}” to test & set a variable in shell..
Thx

B.R
Susinthiran

Reply

4 Ronald Fischer July 27, 2009 at 7:11 am

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’?

Reply

5 nixCraft July 27, 2009 at 7:59 am

@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):

${1:-text}

I should have mentioned both syntax..

HTH

Reply

6 Shantanu Oak July 27, 2009 at 12:32 pm

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”.

Reply

Leave a Comment

Tagged as: , , , , , , , , , , , ,

Previous Faq:

Next Faq: