The backticks/gravemarks have been deprecated in favor of $()
for command substitution because $()
can easily nest within itself as in $(echo foo$(echo bar))
. There are other differences such as how backslashes are parsed in the backtick/gravemark version, etc.
See BashFAQ/082 for several reasons to always prefer the $(…) syntax.
Also see the POSIX spec for detailed information on the various differences.