在 Bash 中，双方括号 [[]] 比单方括号 [] 更可取吗? [英] Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash?

问题描述

一位同事最近在一次代码审查中声称，在像

A co-worker claimed recently in a code review that the [[ ]] construct is to be preferred over [ ] in constructs like

if [ "`id -nu`" = "$someuser" ] ; then 
     echo "I love you madly, $someuser"
fi

他无法提供理由.有吗?

He couldn't provide a rationale. Is there one?

推荐答案

[[ 有更少的惊喜，使用起来通常更安全.但它不是可移植的——POSIX 没有指定它的作用，只有一些 shell 支持它(除了 bash，我听说 ksh 也支持它).例如，你可以做

[[ has fewer surprises and is generally safer to use. But it is not portable - POSIX doesn't specify what it does and only some shells support it (beside bash, I heard ksh supports it too). For example, you can do

[[ -e $b ]]

测试文件是否存在.但是对于 [，您必须引用 $b，因为它拆分了参数并扩展了诸如 "a*"(其中 [[ 从字面上理解).这也与 [ 如何成为外部程序并像其他程序一样正常接收其参数有关(尽管它也可以是内置程序，但它仍然没有这种特殊处理).

to test whether a file exists. But with [, you have to quote $b, because it splits the argument and expands things like "a*" (where [[ takes it literally). That has also to do with how [ can be an external program and receives its argument just normally like every other program (although it can also be a builtin, but then it still has not this special handling).

[[ 也有一些其他不错的特性，比如与 =~ 匹配的正则表达式以及类似 C 语言的运算符.这是一个很好的页面:测试之间有什么区别，[ 和 <代码>[[ ? 和 Bash 测试

[[ also has some other nice features, like regular expression matching with =~ along with operators like they are known in C-like languages. Here is a good page about it: What is the difference between test, [ and [[ ? and Bash Tests

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