Sample program for https://unix.stackexchange.com/questions/157689/how-to-capture-ordered-stdout-stderr-and-add-timestamp-prefixes

sample_program.sh

#!/bin/bash

function generator() {
	local I=1
	while [ $I -le 50 ]; do
		echo "Line $I"
		let I+=1
	done
}

function alternator() {
	local LN=0
	while read -r LINE; do
		test $((LN % 2)) -eq 0 && echo "$LINE" || echo "$LINE" 1>&2
		let LN+=1
	done
}

generator | alternator