#!/bin/bash

queue_file='/tmp/nbody.queue'
cmd="./test_cmd"
lock_file="/tmp/nbody.lock"

remove_first_in_queue() {
  tail -n +2 "$queue_file" > "$queue_file.tmp" && mv "$queue_file.tmp" "$queue_file"
}

run_command() {
  (
    flock -n 9 || exit 1
    eval "$cmd"
    remove_first_in_queue
  ) 9>"$lock_file"
}

append_user_to_queue() {
  user_is_in_queue=$(grep "$USER" "$queue_file")

  if [ -z "$user_is_in_queue" ]; then
    echo "$USER" >> $queue_file
  fi
}

first_in_queue=$(head -1 "$queue_file")

if [ "$first_in_queue" == "$USER" ]; then
  run_command
else
  append_user_to_queue
fi