#!/bin/sh

# shellthreads - pseudo threads in bourne shell

# Problem:
#   All jobs in a shell program are executed like a batch, one after
#   another. But what if you have jobs that could be executed
#   concurrently? Imagine a list of downloads, fetched with wget.
#   Most likely a single wget job wouldn't saturate your bandwidth.
#   Running more than one wget would utilise your bandwidth more
#   efficiently. You could start the jobs with ,,wget url &'', but
#   that would need your constant attention. Could a shell program
#   do it for you in a clean way?
# Answer:
#   Yes! Even tho it's kind of ugly. Enjoy :).

set -u # Exit at undefined variables.
set -e # Exit if a command fails.

# This could be a list of urls for wget or so.
joblist="a b c d e f g h"

# Maximum number of worker threads running at the same time.
max_threads="3"

# Time in seconds the main loop sleeps until it checks
# the number of running worker threads again.
mainloop_sleep="1"

tempdir="/tmp/$(basename $0)_$$"

emergency_cleanup() {
  echo "mainloop $$: emergency exit, killing running threads"
  for file in $(find "$tempdir" -maxdepth 1 -type f -name "thread-*.pid") ; do
    pid_to_kill=$(cat $file)
    echo "mainloop $$: emergency exit, killing thread $pid_to_kill"
    kill -15 $pid_to_kill && rm $file
  done
  #rm -rf "$tempdir"
  exit 1
}

# This is a worker thread. After the job is done, we exit.
if [ $# -gt 0 ] ; then # Needed because of ,,set -u''.
  if [ $1 = thread ] ; then
    echo "$$" > "$tempdir/thread-$$.pid"
    echo "thread $$: starting job $2"
    # Here we do the job, wich is given in $2 from the main loop
    sleep 8
    # The job is done, time to clean up and exit.
    echo "thread $$, job $2, finished"
    rm "$tempdir/thread-$$.pid"
    exit
  fi
fi

trap "emergency_cleanup" 2 3 15
mkdir -p $tempdir

# This loop will run until we run out of jobs.
while [ "$joblist" ] ; do
  running_threads="$(find "$tempdir" -maxdepth 1 -type f -name "thread-*.pid" | wc -l)"
  # Start as many worker threads as requested in $max_threads.
  if [ $running_threads -lt $max_threads ] ; then
    # Get the current job and remove it from the joblist, then start a new thread.
    current_job="$(echo $joblist | awk '{print $1}')"
    joblist="$(echo $joblist | sed 's/'$current_job'//')"
    echo "mainloop $$: $running_threads running threads, starting new thread with job $current_job"
    $0 thread $current_job &
  else
    echo "mainloop $$: enough threads running already"
  fi
  sleep $mainloop_sleep
done

# All jobs are spawned, now we wait until they finish, then
# we clean up.
until [ $(find "$tempdir" -maxdepth 1 -type f -name "thread-*.pid" | wc -l) = 0 ] ; do
  echo "mainloop $$: out of jobs, waiting for worker threads to finish"
  sleep $mainloop_sleep
done
rm -r "$tempdir"