Possible Duplicate:
bash: double or single bracket, parentheses, curly braces

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查看archlinux中的rc.dcron脚本：

虽然我能找出大多数语法，但这到底是做什么的：

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我看到它也写了：

在参考文献中，我发现if语句中使用了[]，但这里没有看到。任何帮助都非常感谢。谢谢！

左括号([)是test命令的别名，它执行所有测试，并返回0表示真或其他值表示假。"if"只对测试命令的返回值作出反应。右括号告诉测试表达式的结束位置。双括号([)是一个内置的bash，可以替换外部的测试调用。

单括号模拟了一个旧的Unix实用程序/usr/bin/test。他们按你的要求做，但不是你想要的。双括号是改进的，这是bash特有的。

http://tldp.org/ldp/abs/html/testconstructs.html_dblbrackets

There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false).

With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command.

No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution.

Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

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根据本手册：

Brackets to return a binary result of expression: [[ ]]

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression. Word splitting and filename expansion are not performed on the words between the [[' and]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution,
process substitution, and quote removal are performed.

The && and || commands do not execute expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

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