threads

#!/bin/bash

# number of threads
threads=10

# list of targets
targets=(foo bar baz)

# the function thaht's being threaded. "$1" is the current target
threadfn() {
  sleep "$((RANDOM % 10))"

  # print newline to the fifo at the end, so we know it finished
  echo >&3
}

trap 'trap -- INT; echo "waiting for jobs to complete..."; wait; exit' INT

if ! { fifo=$(mktemp -u) && mkfifo"$fifo"; }; then
  printf "error creating fifo \`%s'\n" "$fifo" >&2
  exit 1
fi

# tie fd to fifo, find total number of targets
exec 3<>"$fifo"
tot=${#targets[@]}

# launch initial threads
for ((cur=0; cur<threads; cur++)); do
  ((cur < tot)) && threadfn "${targets[cur]}" &
done

# stores number of finished threads
fin=0

# each time a line is written to the fifo, a thread finished
while read -r <&3; do
  # if there are more to launch, launch one
  if ((cur < tot)); then
    threadfn "${targets[cur]}" &
  fi

  # increment current and number of finished. if the number finished equals
  # the total, break the loop
  ((cur++, ++fin == tot)) && break
done

# close the fd/fifo
exec 3>&-