What does -z mean in Bash? [duplicate]

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.

Huberthuberto

#6666041 – Movie

#229044 – Movie

Answer 1

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

See https://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions

Janiuszck

I found another one excellent and detailed explanation - #3602015 – Complacent

Also, -n is the opposite of -z. if [ -n "${1}" ] passes if the string is not null and not empty. – Malcommalcontent

Is -n redundant with the default behavior of if [ $2 ] or are there some differences? – Wallace

Answer 2

-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.

Borodin

Answer 3

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

Changchangaris

Answer 4

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

Maratha

What if it is the string undefined (if that is a thing) or is a number? – Supervisory

@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. – Mellar