#! /bin/sh
#
# good lock using mkdir and directory, atomic

proc() {
  echo "Process $1 begin"
  mkdir lockdir 2>/dev/null
  if [ "$?" != 0 ]; then # mkdir failed, lock already exists
    echo "Process $1 can't lock"
  else # lock did not exist, now it does and we have it
    echo "Process $1 got lock"
  fi
  echo "Process $1 end"
}

rmdir lockdir 2>/dev/null

proc 1 & proc 2 & proc 3 &

wait

rmdir lockdir 2>/dev/null