什么是单，双方括号中的Bash之间的区别？ [英] What is the difference between single and double square brackets in Bash?

问题描述

我读的bash举例如果，但一些例子单方括号写的：

I'm reading bash examples about if but some examples are written with single square brackets:

if [ -f $param ]
then
  #...
fi

别人用双括号：

if [[ $? -ne 0 ]]
then
    start looking for errors in yourlog
fi

有什么区别呢？

推荐答案

单 [] 是POSIX兼容的外壳条件测试。

Single [] are posix shell compliant condition tests.

双击 [[]] 是一个可扩展标准 [] 和bash和其他shell都支持shell（例如zsh，ksh）中。它们支持额外的操作（以及标准的POSIX操作）。例如： || 而不是 -o 和 =〜。差异更完整的列表可以在条件结构中的 bash的手册部分找到

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 -o and regex matching with =~. A fuller list of differences can be found in the bash manual section on conditional constructs.

只要你想你的脚本是可跨炮弹。使用 [[]] 如果你想有条件的前pressions不受支持[] ，不需要可移植。

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.

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