Run jobs in parallel in bash

parallel.sh

#!/bin/bash

# Copied from https://milhouse.dev/2015/11/20/writing-a-process-pool-in-bash/

# Define the pool size (aka number of processes to run concurrently)
POOL_SIZE=4

parallel() {
    local proc procs
    declare -a procs=() # this declares procs as an array

    morework=true
    while $morework; do
        if [[ "${#procs[@]}" -lt "$POOL_SIZE" ]]; then
            read proc || { morework=false; continue ;}
            eval "$proc" &
            procs["${#procs[@]}"]="$!"
        fi

	for n in "${!procs[@]}"; do
            kill -0 "${procs[n]}" 2>/dev/null && continue
            unset procs[n]
        done
    done

    wait
}

# invoke the process pool (one task per line)
parallel <<EOF
sleep 2 && echo "hello 1"
sleep 1 && echo "hello 2"
echo "hello 3"
EOF