newpgrp.sh

newpgrp() {
    # Non-interactive shells don't have job control.  Spawn an interactive shell
    # to create a new process group.  The inner non-interactive shell is there
    # to avoid interactive side-effects later on.
    $BASH --norc -ic '$BASH -c \"\$@\" -- "$@"' -- "$@"
}

# Notes on things that don't work:
# set -i
# You can't turn job control on this way
# $BASH -is <<EOF
# This will try to run prompt commands from the environment and may create history entries or potentially even truncate the user's history file.

setsid.sh

# This is sort of portable, right?
setsid() {
    local setsid=$(mktemp -t setsid) exec

    if test $1 = exec; then
        exec=exec; shift
    fi

    cc -x c - -o $setsid <<-'EOF' && $exec $setsid "$@"
	#include <unistd.h>
	#include <stdlib.h>
	#include <stdio.h>

	int main(int argc, char *argv[]) {
	    unlink(argv[0]);
	    setsid();
	    execvp(argv[1], argv+1);
	    perror(argv[1]);
	    exit(1);
	}
	EOF
}

test-newpgrp.sh

#!/usr/bin/env bash

if test "$1" = report; then
    ps -o pid,pgid -p $$

else
    . newpgrp.sh

    echo plain
    $BASH $0 report

    echo -ic -c
    newpgrp $BASH $0 report
fi