bash: $[<arithmetic-expression>] 与 $((<arithmetic-expression>))
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bash: $[<arithmetic-expression>] vs. $((<arithmetic-expression>))
提问by Chen Levy
I have just stumbled upon the bash syntax:
我刚刚偶然发现了 bash 语法：
foo=42
bar=$[foo+1] # evaluates an arithmetic expression
When I Googled for this, I found http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_05:
当我用谷歌搜索时，我发现 http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_05：
3.4.6. Arithmetic expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$(( EXPRESSION ))...
Wherever possible, Bash users should try to use the syntax with square brackets:
$[ EXPRESSION ]However, this will only calculate the result of EXPRESSION, and do no tests...
3.4.6. 算术展开
算术扩展允许计算算术表达式并替换结果。算术展开的格式为：
$(( EXPRESSION ))...
在可能的情况下，Bash 用户应该尝试使用带方括号的语法：
$[ EXPRESSION ]但是，这只会计算 EXPRESSION 的结果，而不会进行测试...
In my bash man page I can only find the $(( EXPRESSION ))form such as:
在我的 bash 手册页中，我只能找到如下$(( EXPRESSION ))形式：
foo=42
bar=$((foo+1)) # evaluates an arithmetic expression
So what tests are not performed with $[...]that do with $((...)), or is the $[...]just a legacy version of $((...))?
那么，哪些测试不使用$[...]与$((...)), 或者$[...]只是旧版本$((...))？
回答by sth
The manpage for bash v3.2.48 says:
bash v3.2.48 的联机帮助页说：
[...] The format for arithmetic expansion is:
$((expression))The old format $[expression] is deprecatedand will be removed in upcoming versions of bash.
[...] 算术扩展的格式是：
$((expression))在老格式$ [表达]已被弃用，并且将在bash的即将到来的版本中删除。
So $[...]is old syntax that should not be used anymore.
所以$[...]是旧语法，不应该再使用。
回答by Brandon Rhodes
@sth is entirely correct. And in case you are curious about why a more verbose syntax is now in favor, check out this old email from the mailing list.
@sth 是完全正确的。如果您对为什么现在支持更详细的语法感到好奇，请查看邮件列表中的这封旧电子邮件。
http://lists.gnu.org/archive/html/bug-bash/2012-04/msg00033.html
http://lists.gnu.org/archive/html/bug-bash/2012-04/msg00033.html
“In early proposals, a form $[expression] was used. It was functionally equivalent to the "$(())" of the current text, but objections were lodged that the 1988 KornShell had already implemented "$(())" and there was no compelling reason to invent yet another syntax. Furthermore, the "$[]" syntax had a minor incompatibility involving the patterns in case statements.”
“在早期的提案中，使用了 $[expression] 形式。它在功能上等同于当前文本的“$(())”，但有人提出反对意见，即 1988 年的 KornShell 已经实现了“$(())”，并且没有令人信服的理由去发明另一种语法。此外，“$[]”语法与 case 语句中的模式存在轻微的不兼容。”
I am not sure that I like the rationale “but someone has already done this more verbosely,” but there you have it—maybe the case-statement problem was more compelling than I am imagining from this obscure mention?
我不确定我是否喜欢“但有人已经更详细地完成了这件事”的理由，但是你明白了——也许案例陈述问题比我从这个晦涩的提及中想象的更引人注目？
