The modern recommendation is to use:
instead of the older:
mainly for readability and to avoid the need for escaping when nesting.
Which shells do not support the
$(...) form, and are any of them still in use today?
The original Bourne shell, csh or tcsh all do not support
$() and require
`` for command substitution.
The Bourne shell is no longer used as the default shell (/bin/sh) on many UNIX based operating systems. However, these shells are still supplied by the OS vendors as they are still used for historical reasons.
Both AIX and HP-UX have /bin/sh as a POSIX compliant shell. Bourne shell is still available on AIX as 'bsh' (but deprecated in 1995 with AIX 4.1) and HP-UX 11.11 '/usr/old/bin/sh' (Bourne was removed from 11.23). I'm not sure when the switch was made, but it was many years ago.
The Solaris 10 and earlier
/bin/sh does not support
It is still in use today because Solaris 10 is still in use, Sun did not want to replace it with a standard conforming version (up to and including Solaris 10) - and because most people consider
/bin/sh as the default shell and don't know how to portably execute a script with a POSIX conforming shell.
Sun's motivation for not changing
/bin/sh was to avoid breaking existing shell scripts that rely on the non-conforming behavior.