bash Bash中单方括号和双方括号的区别
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Difference between single and double square brackets in Bash
提问by rkmax
I'm reading bash examples about ifbut some examples are written with single square brackets:
我正在阅读有关 bash 的示例，if但有些示例是用单个方括号编写的：
if [ -f $param ]
then
#...
fi
others with double square brackets:
其他带双方括号的：
if [[ $? -ne 0 ]]
then
start looking for errors in yourlog
fi
What is the difference?
有什么不同？
回答by cmh
Single []are posix shell compliant condition tests.
单个[]是符合 posix shell 的条件测试。
Double [[]]are an extension to the standard []and are supported by bash and other shells (e.g. zsh, ksh). They support extra operations (as well as the standard posix operations). For example: ||instead of -oand regex matching with =~. A fuller list of differences can be found in the bash manual section on conditional constructs.
Double[[]]是标准的扩展，[]并且被 bash 和其他 shell（例如 zsh、ksh）支持。它们支持额外的操作（以及标准的 posix 操作）。例如：||代替-o和正则表达式匹配=~。有关条件结构的bash 手册部分提供了更完整的差异列表。
Use []whenever you want your script to be portable across shells. Use [[]]if you want conditional expressions not supported by []and don't need to be portable.
使用[]时，你想要你的脚本是跨弹便携。使用[[]]，如果你想不被支持的条件表达式[]，不需要是便携式的。
回答by sampson-chen
Inside single brackets for condition test (i.e. [ ... ]), some operators such as single =is supported by all shells, whereas use of operator ==is not supported by some of the older shells.
在条件测试的单括号内（即 [ ... ]），=所有 shell 都支持诸如 single 之类的某些运算符，而==某些较旧的 shell 不支持使用运算符。
Inside double brackets for condition test (i.e. [[ ... ]]), there is no difference between using =or ==in old or new shells.
在条件测试的双括号内（即 [[ ... ]]），使用=或==在旧壳或新壳中没有区别。
Edit: I should also note that: In bash, always use double brackets [[ ... ]] if possible, because it is safer than single brackets. I'll illustrate why with the following example:
编辑：我还应该注意：在 bash 中，如果可能，请始终使用双括号 [[ ... ]]，因为它比单括号更安全。我将通过以下示例说明原因：
if [ $var == "hello" ]; then
if $var happens to be null / empty, then this is what the script sees:
如果 $var 恰好为 null / 空，那么这就是脚本所看到的：
if [ == "hello" ]; then
which will break your script. The solution is to either use double brackets, or always remember to put quotes around your variables ("$var"). Double brackets is better defensive coding practice.
这会破坏你的脚本。解决方案是使用双括号，或者始终记住在变量 ( "$var")周围加上引号。双括号是更好的防御性编码实践。
回答by Gilles Quenot
[[is a bash keyword similar to (but more powerful than) the [command.
[[是一个 bash 关键字，类似于（但比[命令更强大）。
See
看
http://mywiki.wooledge.org/BashFAQ/031and http://mywiki.wooledge.org/BashGuide/TestsAndConditionals
http://mywiki.wooledge.org/BashFAQ/031和http://mywiki.wooledge.org/BashGuide/TestsAndConditionals
Unless you're writing for POSIX sh, we recommend [[.
除非您为 POSIX sh 编写代码，否则我们推荐[[.
回答by asf107
you can use the double square brackets for light regex matching, e.g. :
您可以使用双方括号进行轻度正则表达式匹配，例如：
if [[ $1 =~ "foo.*bar" ]] ; then
if [[ $1 =~ "foo.*bar" ]] ; then
(as long as the version of bash you are using supports this syntax)
（只要您使用的 bash 版本支持此语法）
回答by user5500105
Bash manualsays:
Bash手册说：
When used with [[, the ‘<' and ‘>' operators sort lexicographically using the current locale. The test command uses ASCII ordering.
当与 [[ 一起使用时，'<' 和 '>' 运算符使用当前语言环境按字典顺序排序。测试命令使用 ASCII 排序。
(The test command is identical to [ ] )
（测试命令与 [ ] 相同）
