because it exec doesn't return to the program, unless theres an error
That's because exec doesn't create a new process. It replaces the calling program with the specified program, running it in the same process.
In Linux/Unix/POSIX based systems, the PID returned by fork to the parent is the child you wait for (or kill, if need be).
my $pid = fork();
unless defined $pid {
warn "Can't fork: $!\n";
...; # do something else
exit;
}
if ($pid == 0) {
# child
exec @cmd_and_parameters;
die "Could not exec $cmd_and_parameters[0]: $!\n";
} else {
# parent
...; # do other things
waitpid($pid,0) == $pid or die "Error waiting for child: $!\n";
...; # do more (if needed)
}
Using, for example, Parallel::ForkManager will make this easier, as well as hide any ugly details for OSs that do not have fork (like MS Windows).
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