wait.sh

#!/bin/bash

function wait_until_job_vacancy() {
    local limit=${1:-4}
    while [[ ! "$(jobs -p | wc -l)" -lt $limit ]]
    do
        sleep 1
    done
}

function do_one_job {
    wait_until_job_vacancy

    local delay=$(echo $RANDOM | cut -c 2)
    echo "Job delay = $delay"
    sleep $delay &
}

while true
do
    do_one_job
done