Something

if=eth0;grep $if /proc/net/dev>l;i=`awk '{print substr($1,6)}' l`;o=`awk '{print $9}' l`;sleep 5;grep $if /proc/net/dev>l;echo "RX $(((`awk '{print substr($1,6)}' l`-$i)/1024)) KB/s"echo "TX $(((`awk '{print $9}' l`-$o)/1024)) KB/s"rm l