#!/bin/sh

usage () {
	cat <<EOF>&2
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
}
 
[ $# -gt 1 ] && usage && exit 1
[ "$1" = "-h" ] && usage && exit

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 >&2 "Error: no such interface: ${interface}"
else
	path="/dev/shm/netspeed"
	time=$(date +%s)

	read -r rx < /sys/class/net/"$interface"/statistics/rx_bytes
	read -r tx < /sys/class/net/"$interface"/statistics/tx_bytes

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

	read -r 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

		printf "%s %s↓ %s %s↑" "$rx_rate" "$rx_unit" "$tx_rate" "$tx_unit"
	fi
fi