wait_max_concurrent.sh

#!/usr/bin/env bash

# make CTRL-C kill background jobs
# https://stackoverflow.com/a/2173421
trap "trap - SIGTERM  && kill -- -$$" SIGINT SIGTERM INT EXIT 

# wait if too many background jobs
wait_max_concurrent() {
	local n="$1"
	if [[ "$n" -le 0 ]]; then
		echo >&2 + wait
	    wait
	else
    	while [[ "$(jobs -p | wc -l)" -ge "$n" ]]; do
    		echo >&2 + wait -n
    		wait -n
    	done
	fi
}

# wait for all to terminate, and avoid the "Terminated" message on normal exit
cleanup(){ 
    wait_max_concurrent 0
    trap - SIGINT SIGTERM INT EXIT 
}


# important business logic
for i in 1 2 3 4 5 6 7 8 9 10; do 
  (
    echo start $i
    sleep 2
    echo end $i
  ) &
  wait_max_concurrent 5
done

# crimes
cleanup