How to determine if a bash variable is empty?

Question

What is the best way to determine if a variable in bash contains ""?

I have heard it recommended that I do if [ "x$variable" = "x" ]

Is that the correct way? (there must be something more straightforward)

Answer 1

This will return true if a variable is unset.

if [ -z "$VAR" ];

Answer 2

If you're interested in distinguishing the cases of set-empty versus unset status, look at the -u option for bash:

$ set -u
$ echo $BAR
bash: BAR: unbound variable
$ [ -z "$BAR" ] && echo true
bash: BAR: unbound variable
$ BAR=""
$ echo $BAR

$ [ -z "$BAR" ] && echo true
true

Answer 3

-z is a the best way.

Another options I've used is to set a variable, but it can be overridden by another variable eg

export PORT=${MY_PORT:-5432}

If the $MY_PORT variable is empty, then PORT gets set to 5432, otherwise PORT is set to the value of MY_PORT. Note the syntax include the colon and dash.

Answer 4

An alternate I've seen to [ -z "$foo" ] is the following, however I'm not sure why people use this method, anyone know?

[ "x${foo}" = "x" ]

Anyway if you're disallowing unset variables (either by set -u or set -o nounset), then you'll run into trouble with both of those methods. There's a simple fix to this:

[ -z "${foo:-}" ]

Note: this will leave your variable undef.

Answer 5

the entire if-then and -z are unnecessary.

[ "$foo" ] && echo "foo is not empty"
[ "$foo" ] || echo "foo is indeed empty"

Answer 6

This is true exactly when $FOO is set and empty:

[ "${FOO+x}" = x ] && [ -z "$FOO" ]

Answer 7

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 zsh and 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} ]]

Answer 8

Personally prefer more clear way to check :

if [ "${VARIABLE}" == "" ]; then
  echo VARIABLE is empty
else
  echo VARIABLE is not empty
fi

Answer 9

The following is an example of how you can distinguish unset from set to the empty string, and it works in any POSIX-compatible shell:

[ -n "${VAR}" ] || echo "VAR is unset or set to the empty string"
[ -n "${VAR+set}" ] || echo "VAR is unset"
[ -z "${VAR+set}" ] || [ -n "${VAR}" ] || echo "VAR is set to the empty string"
[ -z "${VAR}" ] || echo "VAR is set to a non-empty string"

The ${VAR+foo} construct expands to the empty string if VAR is unset or to foo if VAR is set to anything (including the empty string).

The reason why [ x"${VAR}" = x ] is often recommended is because some implementations of the [ command (also known as test) are buggy. If VAR is set to something like -n, then some implementations will do the wrong thing when given [ "${VAR}" = "" ] (because the first argument to [ is erroneously interpreted as an operator, not a string).