Backticks vs braces in Bash

bash command-substitution
rojomoke picture rojomoke · Mar 28, 2014 · Viewed 39.8k times · Source

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?

Answer

fedorqui 'SO stop harming' picture fedorqui 'SO stop harming' · Mar 28, 2014

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)?.

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