$() or `` is called command subsitution
They are the same. Command subsitution allows the output of a command to
replace the command identifier.()
- Placing a list of commands between parentheses causes a subshell environment
to be created, and each of the commands in list to be executed in that subshell.
Since the list is executed in a subshell, variable assignments do not remain
in effect after the subshell completes.
{}
- Placing a list of commands between curly braces causes the list to be executed
in the current shell context. No subshell is created. The semicolon (or newline)
following list is required.$(cmd) substitutes the result of cmd as a string, whereas (cmd; cmd) run a list of commands in a subprocess.
If you want to put the output of one or more commands into a variable use the $( cmd ) form.
However if you want to run a number of commands and treat them as a single unit use the () form.
The latter is useful when you want to run a set of commands in the background.