detectnewhosts.sh

#!/bin/bash
LASTFILE=/var/lib/detectnewhosts.last
SUBNET=192.168.0.0/24
EMAILADDR=user@example.org
SLEEPTIME=60

echo -n "Detecting new hosts"
while true ; do
    echo -n "."
    nmap -n -sn -PR -oX - "$SUBNET" > "$LASTFILE.new"
    if [ -e "$LASTFILE" ] ; then
        DIFF=$(ndiff "$LASTFILE" "$LASTFILE.new" | grep '^\+.*:$')
        if [ ! -z "$DIFF" ] ; then
            echo
            echo "$(date): " $DIFF
            if [ ! -z "$EMAILADDR" ] ; then
                echo "$DIFF" | mail -s "New hosts detected" "$EMAILADDR"
            fi
        fi
    fi
    mv -f "$LASTFILE.new" "$LASTFILE"
    sleep "$SLEEPTIME"
done