-z在Bash中意味着什么？

Question

我正在看下面的代码:

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

(3个点是不相关的使用细节.)
也许我在谷歌搜索错误,但我找不到该-z选项的解释.

Answer 1

-z string 如果字符串为null(空字符串),则为True

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.

Answer 3

test -z如果参数为空,则返回true(请参阅man sh或man test)

Answer 4

-z string如果,字符串的长度是,则表达式为真zero.