#!/usr/bin/ksh

LOCKFILE=/tmp/lockfile

#---At the start of your script check to see that a lockfile exists
if [[ -f $LOCKFILE ]]
then
    #----If the lockfile does exist then check that the process is still running
    #     since it may have aborted and left the lockfile behind
    if ps -p $(<$LOCKFILE) >/dev/null
    then
        echo job is still running
        exit
    fi
fi

#---Must be okay to run, so create the lockfile containing the process id
echo $$ > $LOCKFILE

#----Rest of script goes here
:

#----end
rm $LOCKFILE