#!/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 [ $# -ne 0 ]; then
    ARG="$1"
else
    ARG="$PATH"
fi


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

if [ ! -d "/sys/class/net/${INTERFACE}" ]; then
    echo "Error: no such interface (${INTERFACE})."
    exit
fi
echo "$INTERFACE"

RX_BEFORE=$(cat /sys/class/net/${INTERFACE}/statistics/rx_bytes)
TX_BEFORE=$(cat /sys/class/net/${INTERFACE}/statistics/tx_bytes)
sleep 1
RX_AFTER=$(cat /sys/class/net/${INTERFACE}/statistics/rx_bytes)
TX_AFTER=$(cat /sys/class/net/${INTERFACE}/statistics/tx_bytes)

RX_RESULT=$(((${RX_AFTER:-0}-${RX_BEFORE:-0})/1024))
TX_RESULT=$(((${TX_AFTER:-0}-${TX_BEFORE:-0})/1024))

echo "RX $RX_RESULT KiB/s"
echo "TX $TX_RESULT KiB/s"