Dueling rsync scripts?

[Monty J. Harder]

If you're going to do this, you need to deal with the possibility that the
script will die unceremoniously, leaving the lock file behind.  This adds a
trap, makes the lock file identify the machine, PID, and date/time the lock
was asserted, and examines that information and attempts to determine
whether the lock is really valid rather than blindly assuming that it is.

* #!/bin/bash
Lock=**/some-where/rsync-is-running*
*Log=**/some-where/rsync.log*
*if [ -f $**Lock** ]
 then
  read <$**Lock** Machine PID RDate
  if [ `uname -n` = "$Machine" ]
   then
    if [ "x`ps -ocmd= -p $PID`" = "x$0" ]
     then
      echo "`date` - An instance of rsync is already running on this
server:  `ps -fp $PID`" >>$Log
      exit
     else
      echo "`date` Ignoring invalid lock entry: `cat **$**Lock**`"
    fi #[ "x`ps -ocmd= -p $PID`" = "x$0" ]
   else
      echo "`date` - An instance of rsync appears to be already running on
$Machine: `cat **$**Lock**`" >>**$Log*
*       exit
   fi #[ `uname -n` = "$Machine" ]
fi #**[ -f $**Lock** ]
# If we make it this far, actually do it.
**echo "`date` - Starting synchronization" >> **$Log**trap 'rm **$**Lock**;
exit' 1 2 3 4 5 6 7 8 10 11 12 13 14 15
echo `uname -n` $$ `date` >**$**Lock*
*# ... your script here ...
echo "`date` - Finished synchronization" >> **$Log*
*rm **$**Lock*

You could probably even use rcmd to validate the alleged PID of the process
on the other machine, if you wanted to go to the trouble.  Or just have one
server be the designated initiator of the rsync, which simplifies things
greatly.
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