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..."

• Tweet
• 
• 

You should follow me on twitter here or grab rss feed to keep track of new changes.

Featured Articles:

• 30 Handy Bash Shell Aliases For Linux / Unix / Mac OS X
• Top 30 Nmap Command Examples For Sys/Network Admins
• 25 PHP Security Best Practices For Sys Admins
• 20 Linux System Monitoring Tools Every SysAdmin Should Know
• 20 Linux Server Hardening Security Tips
• Linux: 20 Iptables Examples For New SysAdmins
• Top 20 OpenSSH Server Best Security Practices
• Top 20 Nginx WebServer Best Security Practices
• 20 Examples: Make Sure Unix / Linux Configuration Files Are Free From Syntax Errors
• 15 Greatest Open Source Terminal Applications Of 2012
• My 10 UNIX Command Line Mistakes
• Top 10 Open Source Web-Based Project Management Software
• Top 5 Email Client For Linux, Mac OS X, and Windows Users
• The Novice Guide To Buying A Linux Laptop