I'm looking at the following code:
if [ -z $2 ]; then
echo "usage: ...
(The 3 dots are irrelevant usage details.)
Maybe I'm googling it wrong, but I couldn't find an explanation for the -z option.
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Noich
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1http://stackoverflow.com/questions/6665541/checking-for-null-string-in-bash – devnull Aug 07 '13 at 07:15
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1http://stackoverflow.com/questions/228544/how-to-tell-if-a-string-is-not-defined-in-a-bash-shell-script – devnull Aug 07 '13 at 07:17
4 Answers
693
-z string: True if the string is null (an empty string)
See https://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions
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2I found another one excellent and detailed explanation - http://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash – valentt May 11 '17 at 13:16
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110Also, `-n` is the opposite of `-z`. `if [ -n "${1}" ]` passes if the string is not null and not empty. – Ryan Mar 26 '18 at 23:48
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15Is `-n` redundant with the default behavior of `if [ $2 ]` or are there some differences? – bbarker Jan 18 '19 at 18:32
75
-z
string is null, that is, has zero length
String='' # Zero-length ("null") string variable.
if [ -z "$String" ]
then
echo "\$String is null."
else
echo "\$String is NOT null."
fi # $String is null.
33
The expression -z string is true if the length of string is zero.
Peter Mortensen
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mohit
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3What if it is the string undefined (if that is a thing) or is a number? – Peter Mortensen Oct 05 '21 at 14:30
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1@PeterMortensen if you use `if [[ -z "${YOUR_ENV_VAR}" ]]` it will get interpolated to `if [[ -z "" ]]` at runtime when that env var is empty, thus resulting in a logical true, which is what you want. – peterrus Sep 28 '22 at 14:14