Send desktop notifications with cron

At work, I tend to check my mailbox too often. That's unnecessary. As an experiment, I have decided to check it once every hour. Since I don't want to spend "brain cycles" remembering when I should check my mailbox, I have added a cron task to display a notification every hour.

Desktop notifications on Linux can be displayed with a command line utility named notify-send:

$ notify-send "Hi ${USER}!" "it's time to check your email." --icon=dialog-information

However, when notify-send is invoked by cron, nothing is displayed. The script invoked by cron cannot communicate with the desktop environment because the DBUS_SESSION_BUS_ADDRESS variable is not set.

As a consequence, you have to retrieve the value of the DBUS_SESSION_BUS_ADDRESS for your session. You can do that by parsing the /proc/$PID/environ pseudo file of your window manager (i3 in my case):

$ grep -z DBUS_SESSION_BUS_ADDRESS /proc/$(pidof i3)/environ | cut -d= -f2-
unix:abstract=/tmp/dbus-ajRnZ5v7g9,guid=f8761603e1845a282e104e1a5515bcec

I wrote the following shell script named remind-email to grab the bus address of the DBUS session and display a desktop notification:

#!/bin/sh

export DBUS_SESSION_BUS_ADDRESS=$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$(pidof i3)/environ | cut -d= -f2-)
notify-send "Hi ${LOGNAME}!" "it's time to check your email." --icon=dialog-information

The script is invoked every hour by the following cron entry (use crontab -e to edit your cron tables):

0 * * * * /home/foo/bin/remind-email