Bash中的反引号vs括号 [英] Backticks vs braces in Bash

问题描述

当我去回答这个问题时，我将使用${}表示法，因为我已经在这里看到了很多次，所以比起反引号更合适.

但是，当我尝试

joulesFinal=${echo $joules2 \* $cpu | bc}

我收到了消息

-bash: ${echo $joules * $cpu | bc}: bad substitution

但是

joulesFinal=`echo $joules2 \* $cpu | bc`

正常工作.那么，我还需要进行哪些其他更改?

解决方案

在使用${}(花括号)时，``称为命令替换，等效于$()(括号). >

所以它们是相等的，意思是解释放置在其中的命令":

joulesFinal=`echo $joules2 \* $cpu | bc`
joulesFinal=$(echo $joules2 \* $cpu | bc)
             ^                          ^
       ( instead of {             ) instead of }

${}表达式用于变量替换.

---

来自man bash:

命令替换允许命令的输出替换 命令名称.有两种形式:

          $(command)
   or
          `command`

此外，``更难处理，例如，您不能嵌套它们.参见下面的评论，以及为什么$(...)比...(反引号)更受欢迎?.

When I went to answer this question, I was going to use the ${} notation, as I've seen so many times on here that it's preferable to backticks.

However, when I tried

joulesFinal=${echo $joules2 \* $cpu | bc}

I got the message

-bash: ${echo $joules * $cpu | bc}: bad substitution

but

joulesFinal=`echo $joules2 \* $cpu | bc`

works fine. So what other changes do I need to make?

解决方案

The `` is called Command Substitution and is equivalent to $() (parenthesis), while you are using ${} (curly braces).

So these are equal and mean "interpret the command placed inside":

joulesFinal=`echo $joules2 \* $cpu | bc`
joulesFinal=$(echo $joules2 \* $cpu | bc)
             ^                          ^
       ( instead of {             ) instead of }

While ${} expressions are used for variable substitution.

---

From man bash:

Command substitution allows the output of a command to replace the command name. There are two forms:

          $(command)
   or
          `command`

Also, `` are more difficult to handle, you cannot nest them for example. See comments below and also Why is $(...) preferred over ... (backticks)?.

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