if [ -n "$1" ]

The following solution is incorrect if [ -z "$var" ]; then echo "var is unset"; else echo "var is set to '$var'"; fi This is because it doesn't distinguish between a variable that is unset and a variable that is set to the empty string. That is to say, if var='', then the above solution will incorrectly output that var is unset. But this distinction is essential in situations where the user has to specify an extension, or additional list of properties, and that not specifying them defaults to a non-empty value, whereas specifying the empty string should make the script use an empty extension or list of additional properties. A correct solution is to use if [ -z ${var+x} ]; then echo "var is unset"; else echo "var is set to '$var'"; fi