Access to user's session D-bus from their cron commands

This is likely due to the fact that the DBUS_SESSION_BUS_ADDRESS environment variable isn't propagated to the cron environment.

At least under Gnome, the bus isn't made "discoverable" (as documented in the "AUTOMATIC LAUNCHING" section of the dbus-launch(1) man page) via files in $HOME/.dbus/session-bus. This leaves anything run in your crontab without a way to discover $DBUS_SESSION_BUS_ADDRESS and contact the session D-Bus.

I'll take your word for it that it worked in the past, possibly due to use of $HOME/.dbus or the existence of actual /tmp/dbus-$TMPNAM files referenced in $DBUS_SESSION_BUS_ADDRESS (which is normally set to something resembling unix:abstract=/tmp/dbus-GkJdpPD4sk,guid=0001e69e075e5e2). As the dbus-cleanup-sockets(1) man page explains:

On Linux, this program is essentially useless, because D-Bus defaults to using "abstract sockets" that exist only in memory and don't have a corresponding file in /tmp.

However, we can use a variation on the idea presented in an ubuntuforum post, Attach to existing DBUS session over SSH, to discover the session D-Bus of an existing user session on the local machine from within the cron environment and set $DBUS_SESSION_BUS_ADDRESS accordingly.

While the technique used there discovers the environment from commonly-running processes like nautilus, pulseaudio, and trackerd, and requires that one or more of them be running in the active session, I recommend a more basic approach.

All of the common desktop environment session managers (gnome-session, lxsession, and kded4) have $DBUS_SESSION_BUS_ADDRESS set in their environment, even though they're started before dbus-daemon and have lower PIDs. So, it makes the most sense to just use whatever session manager corresponds to your desktop environment.

I wrote the following script, placed in $HOME/bin/test-crontab-dbus.sh, to test access to the existing session bus:

#!/bin/sh
SESSION_MANAGER=lxsession
OUTFILE=/tmp/${USER}-cron-dbus.txt

export $(cat /proc/$(pgrep "$SESSION_MANAGER" -u "$USER")/environ \
  |egrep -z '^DBUS_SESSION_BUS_ADDRESS=')

date >> $OUTFILE
dbus-send --session --dest=org.freedesktop.DBus \
   / org.freedesktop.DBus.GetId 2>> $OUTFILE
if test "$?" -eq 0; then
    echo "Success contacting session bus!" >> $OUTFILE
fi

The SESSION_MANAGER=lxsession above is appropriate for a main desktop session running under LXDE, under Gnome you'd set SESSION_MANAGER=gnome-session, and under KDE you'd use SESSION_MANAGER=kded4.

If the job in the crontab has access to the session bus, you'll see something like the following in the output:

Fri Dec 18 15:27:02 EST 2015
Success contacting session bus!

Otherwise, you'll see the error message output by dbus-send.

Obviously, you can substitute any other method of testing connectivity to the session bus, including whatever operation you actually need to perform via a cron job.