#!/bin/bash
trap on_exit EXIT                    # run 'on_exit' on exit
on_exit() {
  [ -n "$LKD" ] && lock_queue -u 2>/dev/null   # unlock the queue on exit
}                                              # if the lock was set here

lock_queue() {        # <-- '-u' to remove lockfile
  local LOK="testlockfile"             # lock file name
  local -i MAX=240 SEC=0             # max seconds to gain a lock ; seconds waiting

  if [ -z "$1" ] ; then              # lock queue
	mkdir $LOK 2>/dev/null &&  LKD='t'
    while [ -z "$LKD" ] && (( SEC < MAX )) ; do      # lock file present
	    sleep 1                                        # wait a second
	    (( ++SEC ))                                    # accumulate seconds
		mkdir $LOK 2>/dev/null &&  LKD='t'
    done                                             # try again while locked for MAX secs
    [ -z "$LKD" ] && exit 1

  elif [ "$1" = '-u' ] ; then        # unlock queue
    'rmdir' "$LOK"                   # remove the lock
  fi
}

lock_queue
NO=$(cat testcounterfile)
echo "$NO"
(( ++NO ))
echo "$NO" > testcounterfile