Control number of parallel 'threads' to be able execute and keep the in pool

parallel.sh

#!/bin/bash

# params
num=20 # total tests
threads=5 # parallel processes

# for script
readonly CONST_NAN_PID=-1

# return 0 (true) if PID is running, otherwise return 1 (false)
isPIDup() {
    if [ $1 -eq $CONST_NAN_PID ]; then
        return 1
    fi
    if ps -p $1 > /dev/null
    then
       return 0
    else
       return 1
    fi
}

# prepare
pool=()
for ((i=0;i<$threads;i++)); do
    pool[i]=$CONST_NAN_PID
done

i=0
while true; do

    thread=-1
    for pi in "${!pool[@]}"
    do
        pid=${pool[$pi]}
        if ! isPIDup $pid; then
            thread=$pi
            break
        fi
    done

    # if all process running and we didn't found idle thread -> check again later
    if [ $thread -eq -1 ]; then
        sleep 0.1
        continue
    fi

    # if we already tested all tests
    if [ $i -eq $num ] ; then
        break
    fi

    # start the test of 5 seconds
    executeTime=$[ ( $RANDOM % 5 )  + 1 ]
    sleep $executeTime &
    pid=$!
    echo "Running: $i - time: $executeTime, thread: $thread, pid: $pid"
    pool[$thread]=$pid

    # increase to next one
    i=$(( $i + 1 ))
done

# looks like we are fine
echo "DONE"
echo ""