Bash 中的反引号与大括号
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Backticks vs braces in Bash
提问by rojomoke
When I went to answer this question, I was going to use the ${}notation, as I've seen so many times on here that it's preferable to backticks.
当我去回答这个问题时，我打算使用${}符号，因为我在这里见过很多次，它比反引号更可取。
However, when I tried
然而，当我尝试
joulesFinal=${echo $joules2 \* $cpu | bc}
I got the message
我收到了消息
-bash: ${echo $joules * $cpu | bc}: bad substitution
but
但
joulesFinal=`echo $joules2 \* $cpu | bc`
works fine. So what other changes do I need to make?
工作正常。那么我还需要做哪些其他改变呢？
回答by fedorqui 'SO stop harming'
The ``is called Command Substitution and is equivalent to $()(parenthesis), while you are using ${}(curly braces).
在``被称为命令替换，相当于$()（括号），而你正在使用${}（大括号）。
So these are equal and mean "interpret the command placed inside":
所以这些是相等的，意思是“解释放置在里面的命令”：
joulesFinal=`echo $joules2 \* $cpu | bc`
joulesFinal=$(echo $joules2 \* $cpu | bc)
^ ^
( instead of { ) instead of }
While ${}expressions are used for variable substitution.
While${}表达式用于变量替换。
From man bash:
来自man bash：
Command substitutionallows the output of a command to replace the command name. There are two forms:
$(command) or `command`
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`
Also, ``are more difficult to handle, you cannot nest them for example. See comments below and also Why is $(...) preferred over ...(backticks)?.
此外，``更难以处理，例如您不能嵌套它们。请参阅下面的评论以及为什么 $(...) 优于...（反引号）？.
回答by Gunstick
They behave slightly differently in specific case:
在特定情况下，它们的行为略有不同：
$ echo "`echo \"test\" `"
test
$ echo "$(echo \"test\" )"
"test"
so backticks silently remove the double quotes.
所以反引号默默地删除双引号。
