tcpdump rotator in shell script

dumper.sh

#!/bin/zsh

PID_FILE=$HOME/local/var/run/`basename $0`.pid
NIC=en0
DATA_DIR=$HOME/local/var/data
FNAME_PREFIX="dump"
FILTER="port not 80"

if [ "`id -u`" != "0" ]
then
    echo "need root privillege"
    exit
fi

if [ $# -ne 1 ]
then
    echo "syntax)" `basename $0` "[start|rotate|stop]"
    exit
fi

mkdir -p `dirname $PID_FILE`
mkdir -p $DATA_DIR

stop () {
    if [ -e $PID_FILE ]
    then
	kill `cat $PID_FILE`
	rm $PID_FILE
    fi
}

start ()
{
    SUFFIX=`date "+%Y%m%d%H%M%S"`
    tcpdump -s 0 -w $DATA_DIR/$FNAME_PREFIX-$SUFFIX.pcap -ni $NIC $FILTER 2> /dev/null &
    disown
    PID=$!
    echo $PID > $PID_FILE
}


case $1 in
    "start")
	start 
	;;
    "rotate")
	stop 
	start 
	;;
    "stop")
	stop 
	;;
    *)
	echo "syntax)" `basename $0` "[start|rotate|stop]"
	;;
esac