Bandwidth monitor bash script

#!/bin/bash

##
# Bandwidth me!
# Bandwidth monitor script written by Manfred Dellkrantz in 2012.
# I run it every five minutes using crontab.
#   */5 * * * * /home/whatever/bwme.sh >> /home/whatever/bwme.log
# Outputs totals by the hour, day, week, month and year.
# Also outputs some preliminary numbers
##

function byte {
        echo $1|awk '
                BEGIN {
                        split("B,KiB,MiB,GiB,TiB,PiB,EiB,ZiB,YiB", Units, ",");
                }
                {
                        u = 1;
                        while ($1 >= 1024) {
                                $1 = $1 / 1024;
                                u += 1
                        }
                        $1 = sprintf("%.2f %s", $1, Units[u]);
                        print $0;
                }
        '
}

mkdir $0.lock || exit 1

#echo "## Running" `date`

net=(`grep eth0 /proc/net/dev`)

in=${net[1]}
out=${net[9]}
let tot=$in+$out

year=`date +%Y`
month=`date +%B`
week=`date +%W`
day=`date +%d`
hour=`date +%H`
min=`date +%M`

if [ -f $0.db ]; then
        data=(`cat $0.db`);
else
        data=($in $out $year $month $week $day $hour $min 0 0 0 0 0 0)
fi

let oTot=${data[0]}+${data[1]}
if (( $oTot > $tot )); then
        echo "## Has there been a reboot?"
        dIn=$in
        dOut=$out
else
        let dIn=$in-${data[0]}
        let dOut=$out-${data[1]}
fi

data[0]=$in
data[1]=$out
for i in `seq 8 13`; do
        let data[$i]=data[$i]+$dIn+$dOut
done

if [ "${data[2]}" != "$year" ]; then
        echo "End of year ${data[2]}:" `byte ${data[8]}`
        data[2]=$year
        data[8]=0
fi
if [ "${data[3]}" != "$month" ]; then
        echo "End of month ${data[3]}: `byte ${data[9]}`, so far for year ${data[2]}: `byte ${data[8]}`"
        data[3]=$month
        data[9]=0
fi
if [ "${data[4]}" != "$week" ]; then
        echo "End of week ${data[4]}:" `byte ${data[10]}`
        data[4]=$week
        data[10]=0
fi
if [ "${data[5]}" != "$day" ]; then
        echo "End of day ${data[5]}: `byte ${data[11]}`, so far for week ${data[4]}: `byte ${data[10]}`, ${data[3]}: `byte ${data[9]}`"
        data[5]=$day
        data[11]=0
fi
if [ "${data[6]}" != "$hour" ]; then
        echo "End of hour ${data[6]}: `byte ${data[12]}`, so far for day ${data[5]}: `byte ${data[11]}`"
        data[6]=$hour
        data[12]=0
fi
if [ "${data[7]}" != "$min" ]; then
#        echo "End of minute ${data[7]}: `byte ${data[13]}`, so far for hour ${data[6]}: `byte ${data[12]}`"
        data[7]=$min
        data[13]=0
fi

echo ${data[@]}>$0.db

rmdir $0.lock