#!/bin/sh
update() {
	sum=0
	for arg; do
		read -r i <"$arg"
		sum=$((sum + i))
	done
	cache=/tmp/${1##*/}
	[ -f "$cache" ] && read -r old <"$cache" || old=0
	printf %d\\n "$sum" >"$cache"
	printf %d\\n $((sum - old))
}
rx=$(update /sys/class/net/[ew]*/statistics/rx_bytes)
tx=$(update /sys/class/net/[ew]*/statistics/tx_bytes)
printf "%5sB %5sB\\n" $(numfmt --to=iec $rx $tx)