为什么 $((40-35)) 变成了 5？

Question

背后隐藏着什么奇怪的shell语法，它是$((40-35))如何计算值的？

例子：

$> echo $((40-35))
5

Answer 1

从bash手册页：

  ((expression))
          The expression is evaluated according  to  the  rules  described
          below  under ARITHMETIC EVALUATION.  If the value of the expres-
          sion is non-zero, the return status is 0; otherwise  the  return
          status is 1.  This is exactly equivalent to let "expression".

并进一步：

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

          $((expression))

任何放入双括号的东西都会成为一个bash可以求值的数学表达式。