bash 当我们将命令放在美元符号和括号内时，shell 中的含义是什么：$(command)
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What does it mean in shell when we put a command inside dollar sign and parentheses: $(command)
提问by KItis
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 $(command)supposed to return? Does it return the value after executing the command? In that case, we can use `to execute the command.
我只想了解 shell 中的以下代码行。它用于获取当前工作目录。我知道$(variable)name 返回变量 name 中的值，但是$(command)应该返回什么？执行命令后是否返回值？在这种情况下，我们可以使用`来执行命令。
CWD="$(cd "$(dirname DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
)"; pwd)"
Same output can be taken from the following line of code also in different version of shell
在不同版本的 shell 中也可以从以下代码行中获取相同的输出
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
I am unable to understand the meaning of $(cd..and $(dirname.
我无法理解的含义$(cd..和$(dirname。
Could anybody help me to figure out how this command get executed?
有人能帮我弄清楚这个命令是如何执行的吗？
回答by Eric Urban
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 foosince echowill write footo standard out
$类似的用法${HOME}给出了 HOME 的值。使用$like$(echo foo)意味着在子shell 中运行括号内的任何内容并将其作为值返回。在我的例子中，你会得到foo因为echo将写入foo标准输出
回答by devnull
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"Could anybody help me to figure out how this command get executed?
##代码##有人能帮我弄清楚这个命令是如何执行的吗？
Let's look at different parts of the command. BASH_SOURCEis a bash array variable containing source filenames. So "${BASH_SOURCE[0]}"would return you the name of the script file.
让我们看看命令的不同部分。BASH_SOURCE是一个包含源文件名的 bash 数组变量。所以"${BASH_SOURCE[0]}"会返回脚本文件的名称。
dirnameis a utility provided by GNU coreutils that remove the last componentfrom the filename. Thus if you execute your script by saying bash foo, "$( dirname "${BASH_SOURCE[0]}" )"would return .. If you said bash ../foo, it'd return ..; for bash /some/path/fooit'd return /some/path.
dirname是 GNU coreutils 提供的实用程序，用于从文件名中删除最后一个组件。因此，如果您通过说bash foo,"$( dirname "${BASH_SOURCE[0]}" )"来执行您的脚本，将返回.。如果你说bash ../foo，它会回来..；因为bash /some/path/foo它会回来/some/path。
Finally, the entire command "$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"gets the absolute directory containing the script being invoked.
最后，整个命令"$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"获取包含正在调用的脚本的绝对目录。
$(...)allows command substitution, i.e. allows the output of a command to replace the command itself and can be nested.
$(...)允许命令替换，即允许命令的输出替换命令本身并且可以嵌套。
