monitor network speed shell script

usage(){
echo "Usage: $0 [-i INTERFACE] [-s INTERVAL] [-c COUNT]"
echo
echo "-i INTERFACE"
echo " The interface to monitor, default is eth0."
echo "-s INTERVAL"
echo " The time to wait in seconds between measurements, default is 3 seconds."
echo "-c COUNT"
echo " The number of times to measure, default is 10 times."
exit 3
}

readargs(){
while [ "$#" -gt 0 ] ; do
case "$1" in
-i)
if [ "$2" ] ; then
interface="$2"
shift ; shift
else
echo "Missing a value for $1."
echo
shift
usage
fi
;;
-s)
if [ "$2" ] ; then
sleep="$2"
shift ; shift
else
echo "Missing a value for $1."
echo
shift
usage
fi
;;
-c)
if [ "$2" ] ; then
counter="$2"
shift ; shift
else
echo "Missing a value for $1."
echo
shift
usage
fi
;;
*)
echo "Unknown option $1."
echo
shift
usage
;;
esac
done
}

checkargs(){
if [ ! "$interface" ] ; then
interface="eth0"
fi
if [ ! "$sleep" ] ; then
sleep="3"
fi
if [ ! "$counter" ] ; then
counter="10"
fi
}

printrxbytes(){
/sbin/ifconfig "$interface" | grep "RX bytes" | cut -d: -f2 | awk '{ print $1 }'
}

printtxbytes(){
/sbin/ifconfig "$interface" | grep "TX bytes" | cut -d: -f3 | awk '{ print $1 }'
}

bytestohumanreadable(){
multiplier="0"
number="$1"
while [ "$number" -ge 1024 ] ; do
multiplier=$(($multiplier+1))
number=$(($number/1024))
done
case "$multiplier" in
1)
echo "$number Kb"
;;
2)
echo "$number Mb"
;;
3)
echo "$number Gb"
;;
4)
echo "$number Tb"
;;
*)
echo "$1 b"
;;
esac
}

printresults(){
while [ "$counter" -ge 0 ] ; do
counter=$(($counter - 1))
if [ "$rxbytes" ] ; then
oldrxbytes="$rxbytes"
oldtxbytes="$txbytes"
fi
rxbytes=$(printrxbytes)
txbytes=$(printtxbytes)
if [ "$oldrxbytes" -a "$rxbytes" -a "$oldtxbytes" -a "$txbytes" ] ; then
echo "RXbytes = $(bytestohumanreadable $(($rxbytes - $oldrxbytes))) TXbytes = $(bytestohumanreadable $(($txbytes - $oldtxbytes)))"
else
echo "Monitoring $interface every $sleep seconds. (RXbyte total = $(bytestohumanreadable $rxbytes) TXbytes total = $(bytestohumanreadable $txbytes))"
fi
sleep "$sleep"
done
}