Backticks vs braces in Bash

Question

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?

Accepted Answer

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

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 }
```

While `${}` expressions are used for variable substitution.

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

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)?.

Backticks vs braces in Bash

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