get rate from netdev stats

rate.sh

#!/bin/sh

if [ $# -lt 1 ]; then
        echo "Usage: $0 <ifname> [delay (s) default=10s]";
        exit 1;
fi
eth=$1
if [ "x$2" -eq "x" ]; then
        delay=10
else
        delay=$2
fi
arx=$(cat /sys/class/net/$eth/statistics/rx_bytes);
atx=$(cat /sys/class/net/$eth/statistics/tx_bytes);
sleep $delay;
btx=$(cat /sys/class/net/$eth/statistics/tx_bytes);
brx=$(cat /sys/class/net/$eth/statistics/rx_bytes);

echo rx: $(( ($brx - $arx)/$delay/1024 )) kBps
echo tx: $(( ($btx - $atx)/$delay/1024 )) kBps