== How to do locking in shell scripts

To do simple locking in shell scripts, you need an atomic system call
that fails if the target already exists, and that is directly exposed
in a utility program. Ignoring for a moment [[what System V did to _ln_
SystemVLn]], there are two good candidates on modern Unix systems:
_link(2)_ and _mkdir(2)_.

_Mkdir_ is actually the better of the two (because you don't have to
fiddle around with temporary files), but _ln_ is the more common one,
probably for two reasons:

# with _mkdir_ you can't atomically associate some extra information,
  like a PID, with the lock. You can put a PID file inside the directory,
  but there will be a time when it doesn't exist although you have
  the lock.
# there was no _mkdir(2)_ system call in V7; it was only added in UCB
  BSD, and didn't make it into System V until fairly late. So for a
  fair time _mkdir_ wasn't actually suitable for this, and everyone
  got into the habit of using _ln_.

General locking with _mkdir_ is simple:

>  mkdir LOCK 2>/dev/null || exit 0
>  echo running locked
>  rmdir LOCK

Locking with _ln_ ([[ignoring System V SystemVLn]]) is somewhat more
complicated:

>  echo $$ >$$
>  if ln $$ LOCK 2>/dev/null; then
>    echo running locked
>    rm $$ LOCK
>  else
>    rm $$
>  fi

In both cases you really want a private directory for the locks. If you
have to use a shared directory like _/tmp_, I believe the _ln_ approach
is the only one that can stand up to hostile attacks (provided that you
use something like _mktemp_ to securely create the temporary file).