哪个`[`不能比较空字符串? [英] Which `[` can not compare empty strings?

问题描述

在一些古老的 Solaris 的 Shell 脚本中，我发现了以下符号来检查变量是否为空:

In some ancient Shell scripts for Solaris I found the following notation to check if a variable is empty:

[ x"$var" = x ]

在当前的 Linux 系统上，我会像这样编写

On a current Linux system I would write it just as

[ "$var" = "" ]

或

[ -z "$var" ]

哪个版本的 [ 实际上需要第一个符号?

Which version of [ actually requires the first notation?

推荐答案

None.the alias from [] to test 出现在 test 的选项 -z 之后.因此，如果你可以使用[ x"$var" = x]，你也可以写[ -z "$var" ].

None.The alias from [] to test appears after the option -z of test. Therefore, if you can use [ x"$var" = x], you can also write [ -z "$var" ].

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