How to use cron to display messages on the screen via notify-send

To debug this you can put

set -e -u

at the top of your shell script - it then ends with an error exit status when a command fails or an undefined variable is used.

Then you can call from the cron-job a wrapper script that calls the main script like this

sh -x main_script.sh || echo Failed with exit status: $?

With -x every line is printed out before it is executed. The output is mailed by the cron daemon to you.

You can also use a temp file when the output is too big:

sh -x main_script.sh > $TEMPFILE 2>&1
if [ $? -ne 0 ]; then echo Failed with exit status $? - see $TEMPFILE; fi

In case the exit status is > 128 the command was interrupted by a signal - e.g. someone 'killed' it, a segmentation fault occured or there was a out-of-memory situtation (how to get the signal from the exit status).

You need to set the DBUS_SESSION_BUS_ADDRESS variable. By default cron does not have access to the variable. To remedy this put the following script somewhere and call it when the user logs in, for example using awesome and the run_once function mentioned on the wiki. Any method will do, since it does not harm if the function is called more often than required.

#!/bin/sh

touch $HOME/.dbus/Xdbus
chmod 600 $HOME/.dbus/Xdbus
env | grep DBUS_SESSION_BUS_ADDRESS > $HOME/.dbus/Xdbus
echo 'export DBUS_SESSION_BUS_ADDRESS' >> $HOME/.dbus/Xdbus

exit 0

This creates a file containing the required Dbus evironment variable. Then in the script called by cron you import the variable by sourcing the script:

if [ -r "$HOME/.dbus/Xdbus" ]; then
  . "$HOME/.dbus/Xdbus"
fi

Here is an answer that uses the same mechanism.