#!/bin/bash

if [ "X$1" = "X-v" ]
then	verbose=true; shift
else	verbose=false
fi

dev=${1:-eth0}

previous=$(/sbin/ifconfig $dev | sed -n 's/.*RX bytes:\([0-9]*\) .*TX bytes:\([0-9]*\) .*/\1 \2/p')
rxprevious=${previous%% [0-9]*}
txprevious=${previous##[0-9]* }
while true
do
	sleep 10
	current=$(/sbin/ifconfig $dev | sed -n 's/.*RX bytes:\([0-9]*\) .*TX bytes:\([0-9]*\) .*/\1 \2/p')
	rxcurrent=${current%% [0-9]*}
	txcurrent=${current##[0-9]* }

	if [ $rxcurrent -ge $rxprevious ]
	then	rx=$(( $rxcurrent - $rxprevious ))
	else	rx=$(( 2 ** 32 - $rxprevious + $rxcurrent ))
	fi
	if [ $txcurrent -ge $txprevious ]
	then	tx=$(( $txcurrent - $txprevious ))
	else	tx=$(( 2 ** 32 - $txprevious + $txcurrent ))
	fi

	rx=$(( $rx / 10 ))
	tx=$(( $tx / 10 ))

	rxu=' '
	txu=' '

	if [ $rx -ge 10000 ]
	then	rxu=k; rx=$(( $rx / 1000 ))
	fi
	if [ $tx -ge 10000 ]
	then	txu=k; tx=$(( $tx / 1000 ))
	fi

	if [ $rx -ge 10000 ]
	then	rxu=M; rx=$(( $rx / 1000 ))
	fi
	if [ $tx -ge 10000 ]
	then	txu=M; tx=$(( $tx / 1000 ))
	fi

	printf "RX %5sB/s TX %5sB/s" "${rx}${rxu}" "${tx}${txu}"
	$verbose && printf "\t\trx:%10lu\ttx:%10lu" $rxcurrent $txcurrent
	printf "\n"

	rxprevious=$rxcurrent
	txprevious=$txcurrent
done