touch /tmp/login1.txt /tmp/login2.txt
while [ true ]
do
        who | gawk '{ print $1 }' > /tmp/login2.txt
        comm -13 /tmp/login1.txt /tmp/login2.txt
        #Just a bit easier to read
        #diff /tmp/login1.txt /tmp/login2.txt
        cat /tmp/login2.txt > /tmp/login1.txt
        sleep 1
done