这是什么意思壳当我们把一个命令美元符号和括号内：$（命令） [英] What does it mean in shell when we put a command inside dollar sign and parentheses: $(command)

问题描述

我只是想了解外壳code的下面一行。它是用来获取当前的工作目录。据我所知， $（变量）名称返回变量名中的数值，但什么是 $（命令）应该返回？是否执行命令后返回值？在这种情况下，我们可以使用`来执行命令。

I just want to understand following line of code in shell. It is used to get the current working directory. I am aware that $(variable) name return the value inside the variable name, but what is $(command) supposed to return? Does it return the value after executing the command? In that case, we can use ` to execute the command.

CWD="$(cd "$(dirname $0)"; pwd)"

相同的输出可以从下面的行code的采取也可在不同版本的外壳

Same output can be taken from the following line of code also in different version of shell

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

我无法理解 $（的意思CD .. 和 $（目录名。

任何人可以帮我找出这个命令得到执行的如何？

Could anybody help me to figure out how this command get executed?

推荐答案

的用法 $ 如 $ {HOME} 给HOME的值。的用法 $ 如 $（回声富）表示运行无论是在括号内的一个子shell并返回，作为价值。在我的例子，你会得到富，因为回声会写富到标准输出

Usage of the $ like ${HOME} gives the value of HOME. Usage of the $ like $(echo foo) means run whatever is inside the parentheses in a subshell and return that as the value. In my example, you would get foo since echo will write foo to standard out

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