POSIX compliant simplified implementation of foreman (Procfile) in shell script.

procman.sh

#!/usr/bin/env sh

# Halt on error or undefined variables.
set -eu

# Prefix input with a given string.
prefix() {
	while IFS= read -r output; do
		printf '%s\t%s\n' "$1" "$output"
	done
}

# Print an error message and exit.
fatal() {
	echo "procman: $1" >&2
	exit 1
}

# Polyfill for mktemp.
if ! command -v mktemp >/dev/null 2>&1; then
	mktemp() {
		echo "/tmp/procfile.$$"
	}
fi

# Read the Procfile from the first argument or use the default.
procfile=${1=./Procfile}

# Check the Procfile exists and is a file.
if ! test -f "$procfile"; then
	fatal "$procfile is not a file"
fi

# Don't leave processes hanging around.
pids="--"
trap 'test "$pids" = "--" || kill $pids 2>/dev/null' EXIT QUIT TERM INT HUP

# Create a cleaned up temporary version of the Procfile.
temp_procfile="$(mktemp)"
sed -n '/^[a-z][a-z0-9_-]*: .*/ { s/#.*//; p; }' "$procfile" >"$temp_procfile"

# Read each line of the Procfile, split the line into a
# prefix and command, and launch the command in the
# background, collecting each PID.
while IFS= read -r proc; do
	prefix="${proc%%:*}"
	cmd="${proc#*:}"

	{ sh -c -- "$cmd" 2>&1 | prefix "$prefix"; } &
	pids="$pids -$!"
done <"$temp_procfile"

# Wait for all processes to finish.
wait