为什么没有 $ 的变量扩展在表达式中起作用？

Question

#!/bin/bash

VALUE=10

if [[ VALUE -eq 10 ]]
then
    echo "Yes"
fi

令我惊讶的是，这输出“是”。我原以为它需要[[ $VALUE -eq 10 ]]. 我已经扫描了 的CONDITIONAL EXPRESSIONS部分man bash，但没有找到任何可以解释这种行为的内容。

Answer 1

[[是 bash 保留字，因此应用了特殊的扩展规则，例如算术扩展，而不像[. 还使用算术二元运算符-eq。因此，shell 查找整数表达式，如果在第一项中找到文本，它会尝试将其扩展为参数。它被称为算术展开并且出现在man bash.

RESERVED WORDS
       Reserved words are words that have a special meaning to the shell.  
       The following words are recognized as reserved 
       …
       [[ ]]

[[ expression ]]
       Return  a  status  of 0 or 1 depending on the evaluation of 
       the conditional expression expression.  Expressions are 
       composed of the primaries described below under CONDITIONAL 
       EXPRESSIONS.  Word splitting and pathname 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.  

Arithmetic Expansion
       …
       The evaluation is performed according to the rules listed below 
       under ARITHMETIC EVALUATION.

ARITHMETIC EVALUATION
       …
       Within an expression, shell variables may also be referenced 
       by name without using the parameter expansion syntax.

例如：

[[ hdjakshdka -eq fkshdfwuefy ]]

将始终返回 true

但是这个会返回错误

$ [[ 1235hsdkjfh -eq 81749hfjsdkhf ]]
-bash: [[: 1235hsdkjfh: value too great for base (error token is "1235hsdkjfh")

也可以使用递归：

$ VALUE=VALUE ; [[ VALUE -eq 12 ]]
-bash: [[: VALUE: expression recursion level exceeded (error token is "VALUE")