How to determine if a bash variable is empty?

This will return true if a variable is unset or set to the empty string ("").

if [ -z "${VAR}" ];

4 years ago

In Bash, when you're not concerned with portability to shells that don't support it, you should always use the double-bracket syntax:

Any of the following:

if [[ -z $variable ]]

if [[ -z "$variable" ]]

if [[ ! $variable ]]

if [[ ! "$variable" ]]

In Bash, using double square brackets, the quotes aren't necessary. You can simplify the test for a variable that does contain a value to:

if [[ $variable ]]

This syntax is compatible with ksh (at least ksh93, anyway). It does not work in pure POSIX or older Bourne shells such as sh or dash.

4 years ago