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

lockflag=0

lock() {
  local n=5
  while [ "$n" -gt 0 ]; do
    echo "$$ locking"
    mkdir lockdir 2>/dev/null
    if [ "$?" = 0 ]; then lockflag=1; break; fi # got the lock
    n=$((n-1))
    sleep 1
  done
}

unlock() {
  echo "$$ unlocking"
  rmdir lockdir 2>/dev/null
  lockflag=0
}

lock

if [ "$lockflag" = 1 ]; then
 echo "$$  got lock, working..."
 # do some work here...
 sleep 3
 unlock
else
 echo "$$ failed"
fi