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

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..."
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{ 8 comments… add one }
  • Chris July 25, 2009, 1:15 pm

    Wow, this is very helpful. Thanks!

  • Rahul Sonar July 26, 2009, 3:45 pm

    thanks buddy, very useful post..

  • Susinthiran July 27, 2009, 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

  • Ronald Fischer July 27, 2009, 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’?

  • nixCraft July 27, 2009, 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

  • Shantanu Oak July 27, 2009, 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”.

  • Jeff June 4, 2015, 4:03 am

    Thanks for the post! Found via Google and helped me out. There is apparently a difference between using the dash and using the dash with the colon. This page explains it quite well: http://www.tldp.org/LDP/abs/html/parameter-substitution.html

    Brief intro to when they differ:
    “${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null.”

  • Sadashiv July 30, 2015, 6:40 am

    Thanks a lot, Helped me to modify my script very less with this post.

    Thanks
    Sadashiv

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