Shell script to display an interface's bandwidth usage (Tx/Rx) every second

bw_usage.sh

#!/bin/bash
secs=1
interface=eth0
echo "Displaying bandwidth on $interface every $secs seconds, ctrl-c to stop"
rx_bytes_prev=`ifconfig eth1|grep 'RX bytes'| sed -n 's/^\s*RX bytes:\([0-9]*\).*/\1/p'`
tx_bytes_prev=`ifconfig eth1|grep 'TX bytes'| sed -n 's/.*TX bytes:\([0-9]*\).*/\1/p'`
while [ true ] ; do
    sleep $secs
    rx_bytes=`ifconfig eth1|grep 'RX bytes'| sed -n 's/^\s*RX bytes:\([0-9]*\).*/\1/p'`
    tx_bytes=`ifconfig eth1|grep 'TX bytes'| sed -n 's/.*TX bytes:\([0-9]*\).*/\1/p'`

    rx_mbps=`echo "scale=2;($rx_bytes - $rx_bytes_prev) * 8 / 1000 / 1000" | bc`
    tx_mbps=`echo "scale=2;($tx_bytes - $tx_bytes_prev) * 8 / 1000 / 1000" | bc`

    echo "Rx $rx_mbps Mbps - Tx $tx_mbps Mbps"

    tx_bytes_prev=$tx_bytes
    rx_bytes_prev=$rx_bytes
done