[OS] Time Logged in

#!/bin/bash
#almost the same as the auditorium one
if [ $# -ne 3 ]
then
    echo "USAGE: `basename $0` index month day"
    exit 1
fi
INDEX=$1
MONTH=$2
DAY=$3
#just like grep, with sed /p with can print the lines that contain the ceratin string with -n meaning printing only the lines of interest
time_logged_in=`last | grep ^$INDEX | sed -n '/$MONTH *$DAY /p' | awk 'print $10' | sed -e 's/(//' -e 's/)//' | awk -F: 'BEGIN {time=0} {time+=60*$1+$2} END {print time}' `
if [ -d loggedin.txt ]
then
    rm loggedin.txt
fi
touch loggedin.txt
echo "$time_logged_in" >> loggedin.txt
cat loggedin.txt