bash: $[<算术表达式>] vs. $((<算术表达式>)) [英] bash: $[&lt;arithmetic-expression&gt;] vs. $((&lt;arithmetic-expression&gt;))

问题描述

我刚刚偶然发现了 bash 语法:

I have just stumbled upon the bash syntax:

foo=42
bar=$[foo+1] # evaluates an arithmetic expression

当我为此在 Google 上搜索时，我发现了 http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_05:

When I Googled for this, I found http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_05:

3.4.6.算术展开

算术扩展允许对算术表达式求值并替换结果.算术展开的格式为:

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:

$(( EXPRESSION )) 

...

在可能的情况下，Bash 用户应该尝试使用带方括号的语法:

Wherever possible, Bash users should try to use the syntax with square brackets:

$[ EXPRESSION ] 

不过，这只会计算EXPRESSION的结果，不做测试...

However, this will only calculate the result of EXPRESSION, and do no tests...

在我的 bash 手册页中，我只能找到 $(( EXPRESSION )) 形式，例如:

In my bash man page I can only find the $(( EXPRESSION )) form such as:

foo=42
bar=$((foo+1)) # evaluates an arithmetic expression

那么哪些测试不是用 $[...] 执行的，而是用 $((...)) 执行的，或者是 $[...] 只是 $((...)) 的旧版本?

So what tests are not performed with $[...] that do with $((...)), or is the $[...] just a legacy version of $((...))?

推荐答案

bash v3.2.48 的联机帮助页说:

The manpage for bash v3.2.48 says:

[...] 算术展开的格式为:

[...] The format for arithmetic expansion is:

     $((expression))

旧格式 $[expression] 已弃用，并将在即将推出的版本中删除bash.

The old format $[expression] is deprecated and will be removed in upcoming versions of bash.

所以 $[...] 是不应该再使用的旧语法.

So $[...] is old syntax that should not be used anymore.

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