bash 带美元和不带美元的双括号

Double parenthesis with and without dollar

提问by John Robins

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.

是$(...)一样的(...)在bash？也$((...))一样((...))吗？也${...}一样{...}吗？更一般地说，美元符号代表什么？谢谢你。

回答by John1024

• $(...)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
  

• (...)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
  

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

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

• ((...))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
  

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

  $ echo ${SHELL}
  /bin/bash
  

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

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

• $(...)意味着在子shell中的parens中执行命令并返回其标准输出。例子：

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

• (...)意味着在子shell中运行括号中列出的命令。例子：

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

• $((...))表示进行算术运算并返回计算结果。例子：

  $ 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。例子：

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

• ${...}表示返回括号中命名的 shell 变量的值。例子：

  $ echo ${SHELL}
  /bin/bash
  

• {...}表示将大括号中的命令作为一个组执行。例子：

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

More generally what does the dollar sign stand for?

更一般地说，美元符号代表什么？

It means whatever it means in the given context.

它意味着在给定上下文中的任何含义。