#!/bin/bash
delay=5
while sleep $delay ; do
    ifconfig eth0
done \
| awk -v delay=$delay '
BEGIN{
    rx=0;
    tx=0;
    FS="[\t :]+";
    count=1;
}
/RX bytes:/{
    count--;
    if(count==0){
        printf "---------- ----------\n";
        printf "RX bytes/s TX bytes/s\n";
        printf "---------- ----------\n";
        count=20;
    }
    nrx=$4;
    ntx=$9;
    printf "%10d %10d\n",(nrx-rx)/delay,(ntx-tx)/delay;
    rx=nrx;
    tx=ntx;
}
'