#!/usr/bin/env bash
CACHE="/tmp/waybar-net"
iface=$(ip route get 8.8.8.8 2>/dev/null | awk '{for(i=1;i<=NF;i++) if($i=="dev") print $(i+1)}' | head -1)

if [ -z "$iface" ]; then
  echo "↑0B ↓0B"
  exit 0
fi

rx=$(cat /sys/class/net/$iface/statistics/rx_bytes 2>/dev/null || echo 0)
tx=$(cat /sys/class/net/$iface/statistics/tx_bytes 2>/dev/null || echo 0)
now=$(date +%s%3N)

if [ -f "$CACHE" ]; then
  read -r prev_rx prev_tx prev_time < "$CACHE"
  dt=$(( now - prev_time ))
  [ "$dt" -le 0 ] && dt=1000
  up=$(( (tx - prev_tx) * 1000 / dt ))
  down=$(( (rx - prev_rx) * 1000 / dt ))
else
  up=0; down=0
fi

echo "$rx $tx $now" > "$CACHE"

fmt() {
  local b=$1
  if   [ "$b" -ge 1073741824 ]; then printf "%4s" "$(( b / 1073741824 ))G"
  elif [ "$b" -ge 1048576 ];    then printf "%4s" "$(( b / 1048576 ))M"
  elif [ "$b" -ge 1024 ];       then printf "%4s" "$(( b / 1024 ))K"
  else printf "%4s" "${b}B"
  fi
}

echo "↑$(fmt "$up")/s ↓$(fmt "$down")/s"