#!/bin/sh

if [ "$1" = "-h" ] || [ $# -gt 1 ]; then
	cat<<EOF
Usage: ${0##*/} [INTERFACE]

Return up and down transmission speed on given interface. If not given, use the
first online interface returned by ifconfig (lo is ignored).
EOF
	exit
fi

if [ -n "$1" ]; then
	interface="$1"
else
	interface="$(ifconfig | grep -vm1 '^lo\|^ \|^$' | cut -f1 -d':')"
	## AWK alternative
	# ifconfig | awk -F: '!/^ / && !/^$/ && $1!="lo" {print $1;exit}'
fi

if [ ! -d "/sys/class/net/${interface}" ]; then
	echo "Error: no such interface: ${interface}" >&2
else
	path="/dev/shm/netspeed"
	time=$(date +%s)

	read rx < /sys/class/net/${interface}/statistics/rx_bytes
	read tx < /sys/class/net/${interface}/statistics/tx_bytes

	if [ ! -f "$path" ]; then
		echo "$time $rx $tx" > "$path"
		chmod 0666 "$path"
	fi

	read time_old rx_old tx_old < "$path"
	echo "$time $rx $tx" > "$path"
	time_diff=$(($time - $time_old))

	if [ "$time_diff" -gt 0 ]; then
		rx_rate=$((($rx - $rx_old) / $time_diff))
		tx_rate=$((($tx - $tx_old) / $time_diff))

		[ "$rx_rate" -gt 1024 ] && rx_rate=$(($rx_rate / 1024)) && rx_unit=K
		[ "$rx_rate" -gt 1024 ] && rx_rate=$(($rx_rate / 1024)) && rx_unit=M

		[ "$tx_rate" -gt 1024 ] && tx_rate=$(($tx_rate / 1024)) && tx_unit=K
		[ "$tx_rate" -gt 1024 ] && tx_rate=$(($tx_rate / 1024)) && tx_unit=M

		echo -n "$rx_rate $rx_unit↓ $tx_rate $tx_unit↑"
	fi
fi