Bash 操作符 : "!" vs "-z"

Question

运算符“!”有什么区别？和“-z”应用于字符串？

#Example 1
if [ ! STRING ]; then ...

#Example 2
if [ -z STRING ]; then ...

谢谢

Answer 1

首先，您使用单个括号。这意味着您使用的是 test 命令而不是 Bash 内置函数。从手册:

test EXPRESSION or [ EXPRESSION ]: this exits with the status returned by EXPRESSION

! EXPRESSION: test returns true of EXPRESSION is false

-z STRING: test returns true if the length of STRING is zero.

例子:

$ [ -z "foo" ] && echo "zero length" || echo "non-zero length"
non-zero length
$ [ ! -z "foo" ] && echo "non-zero length" || echo "zero length"
non-zero length
$ [ -z "" ] && echo "zero length" || echo "non-zero length"
zero length
$ [ ! -z "" ] && echo "non-zero length" || echo "zero length"
zero length

但是现在您想知道 [ ! STRING ] :
手册指出 [ STRING ] 等效于 [ -n STRING ] ，它测试 STRING 是否具有非零长度。因此 [ ! STRING ] 等价于 [ -z STRING ] 。

-n STRING: the length of STRING is nonzero.

STRING: equivalent to -n STRING

_{source: man test}

答案: [ ! STRING ] 等价于 [ -z STRING ]