[jonaf]

Technically, you're right that it's atomic to create a file. But creating a lock using a file can be deceptive and is a common pitfall in my experience. I have seen a lot of shell scripts take this form:

  if [ ! -f $FILE ] ; then
   touch $FILE
   # do something dangerous, assuming I have a lock
   rm $FILE
  fi

The problem here is, of course, that I've checked whether the file exists, but another process (even a concurrent execution of the same script) could remove $FILE after I've checked that it doesn't exist. Now I (or any other process) can happily proceed to create $FILE, thinking that no one else is executing simultaneously. Actually, if I ran two executions of this script at about the same time, they could both pass this check and executed the (mistakenly expectedly) "synchronized" block.

Of course, you don't have to use flock(1) to make this operation atomic. It just handles a lot of the extra work that I don't want to have to think about, even if I did set `noclobber` or something like that.