cronlock

#!/bin/sh
# cronlock ensures there is only one copy of a cron running, preventing pile-ups
# prefix a crontab line with "cronlock " before the command and we'll do our best
# to bail if we're already in the processlist, but proceed as if we aren't there
# otherwise

cmdline=`ps -ocmd= -p $$`
parent=`ps -ocmd= -p \`ps -o ppid= -p $$\``

if [ $UID -eq 0 ]; then
	#root gets the nice directory
	lockdir="/var/lock/cronlock"
else
	#users each have their own so they can't lock out other users' crons
	lockdir="~/.cronlock/"
fi

#make the directory if it doesn't exist
if [ ! -d $lockdir ]; then
	mkdir $lockdir
fi

#when run from cron, our parent's commandline will always be the same
hash=`echo $parent $cmdline | md5sum | cut -d\  -f 1`
lockfile="$lockdir/cronlock-$hash"
if [ -f $lockfile ]; then
	# if you're using cronlock, you probably don't care when it collides?
	# so be quiet here and exit cleanly
	exit 0
else
	# the file doesn't exist, so make it and run!
	touch $lockfile || exit 1
	$@
fi