D-Bus cronjobs
published on Tuesday, October 9, 2018
D-Bus is inter-process communication framework that allows accessing and controlling many applications such as NetworkManager, notify, pidgin, udisks, and many more. The reason to be interested in this as a user, is that some services may expose via their D-Bus API useful functionality and information that is not available via a command line interface.
However, if you try running a command that needs to communicate via the users session bus from a cronjob or SSH session, you may find that this won't work as expected without an extra bit of setup. This usually concerns user applications that are started within the desktop session, such as pidgin. The reason is that the environment is not initialized with the address of the desired session bus, and the session bus may not even be unique if multiple sessions are running for the same user.
The session bus address
Long story short, in order to run a cronjob that should interact with a program that is running within the user's desktop session, it is your responsibility to first set the DBUS_SESSION_BUS_ADDRESS environment variable. In many cases, this will simply be:
export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/$(id -u)/bus
However, the session bus may not be unique or the program may be running on a different session bus. For this reason, it is better to ask with which session bus the program is running. For this, you need to acquire its process ID first, e.g.:
program=pidgin PID=$(pgrep $program) export DBUS_SESSION_BUS_ADDRESS=$( grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ | cut -d= -f2-)
In other cases, you may want to talk to a program on a specific display number (this is not working at the moment on my system, but maybe it does the job for you):
display=0 machine=$(cat /var/lib/dbus-machine-id) session=~/.dbus/session-bus/${machine}-${display} source $session export DBUS_SESSION_BUS_ADDRESS DBUS_SESSION_BUS_PID
Turning it into a command
I recommend to encapsulate the above solutions as pre-commands that can be put in your PATH, e.g. /usr/local/bin or ~/.local/bin.
Guess session bus from currently logged in user id:
bin/talk_to_myself
#! /usr/bin/env bash export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/$(id -u)/bus "$@"
Determine session bus by program name:
bin/talk_to_program
#! /usr/bin/env bash program="$1" PID=$(pgrep "$program") export DBUS_SESSION_BUS_ADDRESS=$( grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ | cut -d= -f2-) "${@:2}"
Determine session bus by display number:
bin/talk_to_display
#! /usr/bin/env bash display="$1" machine=$(cat /var/lib/dbus-machine-id) session=~/.dbus/session-bus/${machine}-${display} source $session export DBUS_SESSION_BUS_ADDRESS DBUS_SESSION_BUS_PID "${@:2}"
Usage example
I have a purple-status python script that can be used to control the pidgin online status. It can be used manually from the command line, or e.g. within a cronjob to go offline at a specified time. Type crontab -e and enter:
45 5 * * * ~/bin/talk_to_program pidgin ~/bin/purple-status off
Note that the script depends on python-gobject.