run-tasks-in-parallel.bash

#!/usr/bin/env bash

set -euo pipefail

declare -r IFS=$'\n'

declare -r SPAWNED_LIST=$(mktemp)
declare -r SPAWNED_LIMIT=20

task() {
  sleep $[RANDOM % 5]
}

task-in-parallel() {
  local -r number=${number}

  echo "Spawning task ${number}"
  task &
  echo "${!}" >> "${SPAWNED_LIST}"
}

can-spawn() {
  while read pid; do
    kill -0 ${pid} 2> /dev/null || sed --in-place "/${pid}/d" "${SPAWNED_LIST}"
  done < "${SPAWNED_LIST}"

  test $(wc --lines < "${SPAWNED_LIST}") -lt ${SPAWNED_LIMIT}
}

wait-for-spawn-slot() {
  while true; do
    can-spawn && break || continue
  done
}

for number in {1..100}; do
  wait-for-spawn-slot
  task-in-parallel ${number}
done

wait

echo "All done!"

@henriquemoody : Quite nice! wait-for-spawn-slot() & can-spawn() seems like a as-fast-as-i-can-busy-loop, though?