Sunday, March 8, 2009

Using DBus on Xubuntu 8.04

This post is obsolete has been superseded by my 2012 dbus tutorial series.

DBus is a system that permits different applications to exchange information. Tutorial Reference Other Reference.
Sometimes, DBus crashes upon restart from a suspend or hibernation. These bash commands will help you figure out if it has crashed, and how to restart it.

$ps -e | grep `cat /var/run/dbus/pid` # Confirm if DBus is running by checking for the PID number in the list of live processes.
                                      # If DBus is running, this will return the process number. 
                                      # If not, it will return nothing.
$sudo rm /var/run/dbus/pid            # Remove the stale pid file so DBus can be restarted.
$sudo dbus-daemon                     # Start DBus again.

A python script uses DBus to see if the network connection is available by asking Network Manager:

#! /usr/bin/env python
import dbus
bus = dbus.SystemBus()
item = 'org.freedesktop.NetworkManager'
eth0_path = '/org/freedesktop/NetworkManager/Devices/eth0'
eth1_path = '/org/freedesktop/NetworkManager/Devices/eth1'
interface = 'org.freedesktop.NetworkManager.Devices'

# There are two possible network interfaces: eth0 (wired) and eth1 (wireless).
eth0 = dbus.Interface(bus.get_object(item, eth0_path), interface)
if eth0.getLinkActive(): print('The wired network is up') # getLinkActive() is a boolean, TRUE if the network link is active
eth1 = dbus.Interface(bus.get_object(item, eth1_path), interface)
if eth1.getLinkActive(): print('The wireless network is up')

This shell script does exactly the same thing, using the same DBus call:
# This shell script checks Network Manager if the network is up, using dbus as the communications medium.
# There are two possible network interfaces: eth0 (wired) and eth1 (wireless). Of course, you may need to alter these to meet your own circumstances.
# The basic format of dbus-send is: dbus-send --system --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Devices/eth0 --print-reply org.freedesktop.NetworkManager.Devices.eth0.getLinkActive

result_eth0=`dbus-send --system --dest=$DEST $PPATH'/eth0' --print-reply $DEVICE'.eth0.getLinkActive'`
shaved_eth0=`echo $result_eth0 | cut -d ' ' -f8`
if [ $shaved_eth0 = 'true' ]; then echo 'The wired network is up'; fi

result_eth1=`dbus-send --system --dest=$DEST $PPATH'/eth1' --print-reply $DEVICE'.eth1.getLinkActive'`
shaved_eth1=`echo $result_eth1 | cut -d ' ' -f8`
if [ $shaved_eth1 = 'true' ]; then echo 'The wireless network is up'; fi

A Python script that queries Network Manager to get the list of wireless networks.

import dbus
item = 'org.freedesktop.NetworkManager'
path = '/org/freedesktop/NetworkManager'
interface = item + '.Device'

network_list = []
bus = dbus.SystemBus()

# Create a Network Manager interface and get the list of network devices
event = dbus.Interface(bus.get_object(item, path), interface)

# Create an interface for each device
# Query each interface to see if it's wireless
# Query each wireless interface for the networks it sees
for device in event.getDevices():
    device_interface = dbus.Interface(bus.get_object(item, device), interface)
    if device_interface.getType() == 2:  # 0 unknown, 1 wired, 2 wireless

# Reformat the network names in the list to be more readable
if network_list:
    for entry in network_list:
        #print entry    # String of path/to/network_name
        entry_list = entry.split('/')
        print entry_list[-1]  # String of network_name

A Python listener that catches the changes in wireless signal strength using both available methods.

import dbus, gobject
from dbus.mainloop.glib import DBusGMainLoop

def print_device_strength1(*args):  #Use the received signals
    signal_strength = args[1]
    print ('Signal Strength (Method 1): ' + str(signal_strength) + '%')

def print_device_strength2(*args, **kwargs):  #Use the received signals
    signal_strength = args[1]
    print ('Signal Strength (Method 2): ' + str(signal_strength) + '%')

DBusGMainLoop(set_as_default=True)    # Set up the event loop before connecting to the bus
bus_object = dbus.SystemBus()

# The variables you need. I used the shell command 'dbus-monitor --system' to find this information
sender = 'org.freedesktop.NetworkManager'
path = '/org/freedesktop/NetworkManager'
interface = sender
member = 'DeviceStrengthChanged'

# Method 1 - bus_object.proxy_object.connect_to_signal(method, action, filter, message_parts)
proxy_object = bus_object.get_object(sender, path)
proxy_object.connect_to_signal(member, print_device_strength1, dbus_interface = interface)

# Method 2 - bus_object.add_signal_receiver(action, [filters])
bus_object.add_signal_receiver(print_device_strength2, dbus_interface = interface, member_keyword = member)

# Start the loop
loop = gobject.MainLoop()

Thunar responds beautifully to D-Bus. Introspection is fully set up, so it's easy to use with the d-feet application. Useful for launching programs, opening folders and windows, and manipulating the trash. Launching a program by this method means that the the window manager launches the program, not the script or terminal, so the program can remain open after the script or terminal terminates.

#!/usr/bin/env python
import dbus
item = ('org.xfce.Thunar')
path = ('/org/xfce/FileManager')
interface = ('org.xfce.FileManager')
event = dbus.Interface(dbus.SessionBus().get_object(item, path), interface)

# These three lines at the end of the script open the file's 'properties' window
display = (':0')         # The current session screen
uri = ('/home/me/')
event.DisplayFileProperties(uri, display)

# These three lines at the end of the script launch a new application
display = (':0')         # The current session screen
uri = ('/usr/bin/gftp-gtk')
event.Launch(uri, display)

# These four lines at the end of the script open a folder window and optionally select a file
display = (':0')         # The current session screen
uri = ('/home/me/.cron')
filename = ('anacrontab.daily')
event.DisplayFolderAndSelect(uri, filename, display)

A sample hal script.

"""This python 2.5 script uses dbus to check if the lid switch is open.
Based on an original python script at"""
import dbus

dest = 'org.freedesktop.Hal'
hal_path = '/org/freedesktop/Hal/Manager'
hal_interface = 'org.freedesktop.Hal.Manager'
udi_interface = 'org.freedesktop.Hal.Device'

# Get the list of possible input switches. The return is a list of paths.
bus = dbus.SystemBus()
hal = dbus.Interface(bus.get_object(dest, hal_path), hal_interface)
list_of_udi_paths = hal.FindDeviceByCapability('input.switch')

# Filter the list for the word 'lid'. Print the status for each one.
for udi_path in list_of_udi_paths:
    udi = dbus.Interface(bus.get_object(dest, udi_path), udi_interface)
    if udi.GetProperty('button.type') == "lid":
        # The button.state.value is FALSE if the lid is open.
        if udi.GetProperty('button.state.value'): print ('Lid is closed')
        else: print ('Lid is open') 
    else: print ('Problem: I could not find the lid switch. Sorry.')


  • The D-feet application is very handy for exploring DBus, and figuring out how to communicate with it. It's available in the Ubuntu repositories.
  • More information on the destination/path/interface settings is available from each application's XML config files, found in the /etc/dbus-1/system.d and /session.d directories.
  • The system-tools-backends DBus interfaces look promising, with methods for network interfaces, time, users and groups, and more. But I couldn't get any of it to work. One hint suggested that the DBus message must be sent by root instead of user.
  • xfce4-terminal has a Launch method, seemingly for launching items in the terminal (source code). I can see how that would be handy, but I couldn't get it to work.

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