20

I need to run some program within crontab , but how can the program know about dbus session id ? it's only available for programs launched by session managers.

5 Answers 5

27

The problem is somewhat similar to accessing the X display and finding the location of the X cookie file. (Also, refer to these questions if you want to launch a GUI program on the user's display.)

Dbus stores the session address in a file in ~/.dbus/session-bus. The name of the file is $machine_id-$display_number, where $machine_id is a randomly generated number stored in /var/lib/dbus/machine-id and $display_number is the X display number ($DISPLAY is :$display_number or :$display_number.$screen_number). The file in ~/.dbus/session-bus is parseable by a shell and contains definitions for DBUS_SESSION_BUS_ADDRESS and DBUS_SESSION_BUS_PID.

dbus_session_file=~/.dbus/session-bus/$(cat /var/lib/dbus/machine-id)-0
if [ -e "$dbus_session_file" ]; then
  . "$dbus_session_file"
  export DBUS_SESSION_BUS_ADDRESS DBUS_SESSION_BUS_PID
  dbus-send …
fi

Beware that there's no guarantee that the dbus daemon is still available. The user may have logged out.

An alternative method is to find the PID of a process in the desktop session, and obtain the dbus address from its environment.

export $(</proc/$pid/environ tr \\0 \\n | grep -E '^DBUS_SESSION_BUS_ADDRESS=')

If the crontab is running as root and you want to communicate with the session of whatever user is logged in on the console, see Can I launch a graphical program on another user's desktop as root?

1
  • One caveat to this: dbus may store the session bus address in $HOME/.dbus/session-bus/, but dbus-broker does not. So the first method won't work on any system that's migrated away from the original dbus implementation to dbus-broker. (That list consists of at least: Fedora; most likely RHEL an CentOS as well.)
    – FeRD
    Aug 16, 2021 at 21:07
4

I can't comment on Vincenzo's answer, but I find his answer works best for me on KDE4.

I've had to slightly modify the command though. For me it's:

ps -u yourlogin e | grep -Eo 'dbus-daemon.*ADDRESS=unix:abstract=/tmp/dbus-[A-Za-z0-9]{10}' | tail -c35

Notice ADDRESS in capital letters.

2

I think if you know the pid of the gnome session manager, then you read the environment from /proc filesystem.

GNOME_SESSION_PID=<PID_OF_GNOME_SESSION> 
READ_SESSION_COOKIE="$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$GNOME_SESSION_PID/environ|cut -d= -f2-)"

Use the SESSION id then, with other programs like notify-send or dbus* tools.

Cheers.

1
  • very impressive , actually i'm running KDE4 , so i looked for plasma-desktop instead, thanks !
    – daisy
    Jan 7, 2012 at 0:13
1
ps -u yourlogin e | grep -Eo 'dbus-daemon.*address=unix:abstract=/tmp/dbus-[A-Za-z0-9]{10}' | tail -c35
1

Yes DBus must have a x session. Like earlier answers, 'DBUS_SESSION_BUS_ADDRESS' must have a value before starting . However, you could use 'dbus-launch' to create this value. Following snippet could be put in a script called from crontab.

if test -z "$DBUS_SESSION_BUS_ADDRESS" ; then
  eval `dbus-launch --sh-syntax`
fi
echo "D-Bus daemon address is:"
echo "$DBUS_SESSION_BUS_ADDRESS"

# -E to export DBUS_SESSION_BUS_ADDRESS. Crucial to make DBUS work.
sudo -u <user> -E <program> 

One setback with this solution is that the session might hang around after you've stopped your software.

2
  • +1 for the -E flag to sudo
    – seren
    Dec 14, 2020 at 22:30
  • Correction: Dbus doesn't require a session. (How could it require an X session, when so many distros have migrated to Wayland?) dbus-launch requires a session manager, true. There's a separate utility, dbus-run-session, that can be used to run terminal commands in their own session bus. In a crontab that's probably a better choice. Either way, this won't merely "create [the DBUS_SESSION_BUS_ADDRESS] value", it'll create an entirely separate session bus instance. If you notice, you get a different value for DBUS_SESSION_BUS_ADDRESS every time you run it.
    – FeRD
    Sep 14, 2021 at 19:22

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