Answer by Dennis Williamson for How to determine if a bash variable is empty?

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.

See my answer here and BashFAQ/031 for more information about the differences between double and single square brackets.

You can test to see if a variable is specifically unset (as distinct from an empty string):

if [[ -z ${variable+x} ]]

where the "x" is arbitrary.

If you want to know whether a variable is null but not unset:

if [[ -z $variable && ${variable+x} ]]