output the value of any scripy in ubuntu unity panel

netusage.sh

#!/bin/bash
echo $(($(cat /proc/net/dev | awk '{print $2}' | tail -n1) + $(cat /proc/net/dev | awk '{print $2}' | tail -n2 | head -n1)))

panel_indicator.py

#!/usr/bin/env python3
import gi
gi.require_version('AppIndicator3', '0.1')
gi.require_version('Gtk', '3.0')
import threading
import subprocess

from gi.repository import Gtk
from gi.repository import AppIndicator3 as appindicator
from gi.repository import GLib

class PanelIndicator:
    def __init__(self, script_file, update_interval):

        self.ind = appindicator.Indicator.new(
                    "Test",
                    "go-down-symbolic",
                    appindicator.IndicatorCategory.APPLICATION_STATUS)
        self.ind.set_status (appindicator.IndicatorStatus.ACTIVE)

        self.menu = Gtk.Menu()
        #
        item = Gtk.MenuItem()
        item.set_label("Exit")
        item.connect("activate", self.quit)
        self.menu.append(item)
        #
        self.menu.show_all()
        self.ind.set_menu(self.menu)

        self.script_file = script_file
        self.update_interval = update_interval
        self.data = 0

        self.main()

    def set_value(self, price):
        self.ind.set_label(price, "")
        self.ind.set_status(appindicator.IndicatorStatus.ATTENTION)

    def get_new_value(self):
        """runs periodically"""
        pass

    def update_value(self):
        val = self.get_new_value()
        self.set_value(val)
        return True

    def f1(self):
        """ runs once"""
        GLib.timeout_add_seconds(self.update_interval, self.update_value)

    def main(self):
        threading.Thread(target=self.f1).start()
        Gtk.main()

    def quit(self, widget):
        Gtk.main_quit()

class MyIndicator(PanelIndicator):
    def get_new_value(self):
        def script_runner():
            out, err = subprocess.Popen(["bash", self.script_file], stdout=subprocess.PIPE).communicate()
            out = out.strip().decode('utf-8')
            return out

        old_val = self.data
        self.data = int(script_runner())
        val = (self.data*1.0 - old_val)/10**3
        val /= self.update_interval

        if val/10**3 >= 1.0:
            val /= 10**3
            val = round(val, 1)
            val = str(val) + " MB"
        else:
            val = round(val, 1)
            val = str(val) + " KB"

        return val

if __name__ == '__main__':
    indicator = MyIndicator("/home/user/bin/netusage.sh", 1.5)