带或不带美元的双括号 [英] Double parenthesis with and without dollar

问题描述

bash中的$(...)与(...)相同吗? $((...))和((...))一样吗? ${...}和{...}一样吗?一般来说，美元符号代表什么?谢谢.

Is $(...) the same as (...) in bash? Also is $((...)) the same as ((...))? Also is ${...} the same as {...}? More generally what does the dollar sign stand for? Thank you.

推荐答案

• $(...)表示在子外壳的paren中执行命令并返回其stdout.示例:

  • $(...) means execute the command in the parens in a subshell and return its stdout. Example:

    $ echo "The current date is $(date)"
    The current date is Mon Jul  6 14:27:59 PDT 2015
    

  • (...)表示在子外壳中运行括号中列出的命令.示例:

  • (...) means run the commands listed in the parens in a subshell. Example:

    $ a=1; (a=2; echo "inside: a=$a"); echo "outside: a=$a"
    inside: a=2
    outside: a=1
    

  • $((...))表示执行算术并返回计算结果.示例:

  • $((...)) means perform arithmetic and return the result of the calculation. Example:

    $ a=$((2+3)); echo "a=$a"
    a=5
    

  • ((...))表示执行算术运算，可能会更改Shell变量的值，但不返回其结果.示例:

  • ((...)) means perform arithmetic, possibly changing the values of shell variables, but don't return its result. Example:

    $ ((a=2+3)); echo "a=$a"
    a=5
    

  • ${...}表示返回括号中命名的shell变量的值.示例:

  • ${...} means return the value of the shell variable named in the braces. Example:

    $ echo ${SHELL}
    /bin/bash
    

  • {...}表示将大括号中的命令作为一组执行.示例:

  • {...} means execute the commands in the braces as a group. Example:

    $ false || { echo "We failed"; exit 1; }
    We failed
    

  一般来说，美元符号代表什么?

  More generally what does the dollar sign stand for?

  这意味着在给定上下文中的含义.

  It means whatever it means in the given context.

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