What does -z mean in Bash?

Question

I'm looking at the following code:

if [ -z $2 ]; then
        echo "usage: ...

(The 3 dots are irrelevant usage details.)
Maybe I'm googling it wrong, but I couldn't find an explanation for the -z option.

http://stackoverflow.com/questions/6665541/checking-for-null-string-in-bash — devnull, Aug 07 '13 at 07:15
http://stackoverflow.com/questions/228544/how-to-tell-if-a-string-is-not-defined-in-a-bash-shell-script — devnull, Aug 07 '13 at 07:17

score 591 · Accepted Answer · answered Aug 07 '13 at 07:04

591

-z string True if the string is null (an empty string)

answered Aug 07 '13 at 07:04

Yu Hao

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I found another one excellent and detailed explanation - http://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash – valentt May 11 '17 at 13:16
90

Also, `-n` is the opposite of `-z`. `if [ -n "${1}" ]` passes if the string is not null and not empty. – Ryan Mar 26 '18 at 23:48
13

Is `-n` redundant with the default behavior of `if [ $2 ]` or are there some differences? – bbarker Jan 18 '19 at 18:32

score 70 · Answer 2 · answered Aug 07 '13 at 07:03

-z

string is null, that is, has zero length

String=''   # Zero-length ("null") string variable.

if [ -z "$String" ]
then
  echo "\$String is null."
else
  echo "\$String is NOT null."
fi     # $String is null.

knittl · Answer 3 · 2020-01-01T12:11:47.740

34

test -z returns true if the parameter is empty (see man sh or man test).

edited Jan 01 '20 at 12:11

answered Aug 07 '13 at 07:02

knittl

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score 26 · Answer 4 · edited Jan 01 '20 at 00:55

26

The expression -z string is true if the length of string is zero.

edited Jan 01 '20 at 00:55

Peter Mortensen

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answered Aug 07 '13 at 07:02

mohit

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What if it is the string undefined (if that is a thing) or is a number? – Peter Mortensen Oct 05 '21 at 14:30

What does -z mean in Bash?

4 Answers4

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