[m-dev.] Command substitution in shell scripts

[Julien Fischer]

Hi all,

Back in 2020, Zoltan ran shellcheck over some of the shell scripts in the
Mercury system.

Reference:

     <https://lists.mercurylang.org/archives/reviews/2020-May/021355.html>

One of the issues raised in that then is whether we should use $(...)
for command substitution in place of `...`. (shellcheck emits a warning
about the latter begin a legacy construct.)  Are there an objections to
us moving to the $(...) syntax?  (As Peter points out in the above 
thread, the $(...) syntax is defined in POSIX sh.)

The background here is that I have been attempting to get the bootcheck
script to pass through shellcheck more cleanly.

Julien.