#!/bin/bash
NIC="eth0"
TX_Bytes() { 
	/sbin/ifconfig "${NIC}"|egrep 'RX bytes'|awk '{print $2}'|cut -d':' -f2
}
start_bandwidth_usage=$(TX_Bytes)
sleep "${1}"
new_bandwidth_usage=$(TX_Bytes)
speed=$(($(($(($new_bandwidth_usage - $start_bandwidth_usage)) / ${1})) / 1024))
echo "${speed} KiB/s average in the past ${1} seconds"