Something

#!/bin/bashNIC="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 ${I} seconds"