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

问题描述

当我去回答这个问题时，我打算使用 ${} 符号，因为我在这里见过很多次，它比反引号更可取.

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.

然而，当我尝试

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

我收到消息

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

但是

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

工作正常.那么我还需要进行哪些其他更改?

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

推荐答案

`` 称为 Command Substitution，相当于 $()(括号)，而您正在使用 ${}(花括号).

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

所以所有这些表达式都是相等的，意思是解释放置在里面的命令":

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

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

虽然 ${} 表达式用于变量替换.

While ${} expressions are used for variable substitution.

但是请注意，不推荐使用反引号，而 $() 与 POSIX 兼容，因此您应该更喜欢后者.

Note, though, that backticks are deprecated, while $() is POSIX compatible, so you should prefer the latter.

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

          $(command)
   or
          `command`

另外，`` 更难处理，例如你不能嵌套它们.请参阅下面的评论以及 为什么 $(...) 优于 ...(反引号)?.

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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