#!/bin/sh
###############################################################################
#
#
#   Helper for periodically executed shell scripts (e.g. from cron(8)),
#   where you want to be absolutely sure that only one copy of script
#   is running at time.  It also tries to handle common signals.  Although,
#   can't handle cases where machine is crashed or when scripts exits before
#   "__unlock" or without calling it.  Be aware of that, if until lock file
#   is not deleted, your script WILL NOT run.
#
#
#      Written by  Sulev-Madis Silber <madis555 at hot dot ee>
#   Last modified  Tue Oct 24 18:37:18 2006
#
#
###############################################################################
#
#  Example #1
###############
#
#   #!/bin/sh
#
#   # Source in our helper
#   . THIS_FILE
#
#
#   # Optionally, set lockfile, instead of default "$0.lock"
#   __lockfile my.lock
#
#
#   # Lock script
#   __lock
#
#   # It surely runs long, but safely
#   sleep 3600
#
#   # Unlock script
#   __unlock
#
#
#  Example #2
###############
#
#   #!/bin/sh
#
#   # Source in our helper
#   . THIS_FILE
#
#
#   # It surely runs long, but safely
#   __run sleep 3600
#
#
###############################################################################


__LOCKFILE="$0.lock"


__lockfile()
{
	if [ "$1" ]
	then
		__LOCKFILE=$1
	else
		echo 'Locker: fatal error' >&2
		exit 1
	fi
}


__lock()
{
	if [ -f "$__LOCKFILE" ]
	then
		#echo "$0 (PID $$) aborted, last session is still running" >&2
		exit 1
	fi
	
	touch "$__LOCKFILE"
}


__unlock()
{
	rm -f "$__LOCKFILE"
}


__run()
{
	__lock
	$*
	__unlock
}


trap '__unlock; exit' 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 24 25 26 27 30 31