为什么在[ x“$VAR"中使用'x'?= x“值"]? [英] Why 'x' used in [ x"$VAR" = x"VALUE" ]?

问题描述

我可以在 unix shell 脚本中看到以下使用的构造

I can see in unix shell scripts that following construction used

[ x"$VAR" = x"VALUE" ] 

代替

[ "$VAR" = "VALUE" ] 

为什么?

推荐答案

因为如果其中一个变量为空，shell 并不总是完全表现良好.

Because shells were not always entirely well-behaved if one of the variables was empty.

考虑如果 $VAR 为空/null 并且 $VALUE 是foo"，你的两个结构扩展为:

Consider if $VAR was empty/null and $VALUE is "foo", your two constructs expand to:

[ x = xfoo ]

和

[ = foo ]

后者会因为是非法构造而在某些shell中导致错误，而前者在任何情况下都是有效的.这在最新版本的 bash(甚至可能是旧版本的 bash)中不是问题，但这就是它的历史来源-除了偶尔会遇到遇到您情况的人感到困惑之外，几乎没有理由不 这样做是为了与更广泛的 shell 兼容.

The latter would cause an error in some shells due to being an illegal construct, while the former is valid in any case. This is not a problem in recent version of bash (perhaps even old versions of bash) but that's where it comes from historically - and asides from the occasional moment of puzzlement from people in your situation, there's little reason not to do it for compatibility with a wider range of shells.

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