root@choi:/usr/local/bin# vim netspeed
#!/bin/bash
if [ -z "$1" ]; then
echo
echo usage: $0 network-interface
echo
echo e.g. $0 eth0
echo
exit
fi
IF=$1
while true
do
R1=`cat /sys/class/net/$1/statistics/rx_bytes`
T1=`cat /sys/class/net/$1/statistics/tx_bytes`
sleep 1
R2=`cat /sys/class/net/$1/statistics/rx_bytes`
T2=`cat /sys/class/net/$1/statistics/tx_bytes`
TBPS=`expr $T2 - $T1`
RBPS=`expr $R2 - $R1`
TKBPS=`expr $TBPS / 1024`
RKBPS=`expr $RBPS / 1024`
echo "tx $1: $TKBPS kb/s rx $1: $RKBPS kb/s"
done

root@choi:/usr/local/bin# chmod +x netspeed
root@choi:/usr/local/bin# /usr/local/bin/netspeed eth0

Output Below.