Bash variable existence check

• If the variable is not defined, you can stop executing the Bash script with the following syntax:

${varName?Error varName is not defined}
${varName:?Error varName is not defined or is empty}

• This is useful for a sanity checking
• The script will stop executing if the variable is not defined.

Example

• Create a shell script called varcheck.sh:

#!/bin/bash
# varcheck.sh: Variable sanity check with :? 
path=${1:?Error command line argument not passed}
 
echo "Backup path is $path."
echo "I'm done if \$path is set."

Run it as follows:

chmod +x varcheck.sh
./varcheck.sh /home

Sample outputs:

Backup path is /home.
I'm done if $path is set.

Run the script without any arguments:

./varcheck.sh

Sample outputs:

./varcheck.sh: line 3: 1: Error command line argument not passed