Add this little bit of code to your script:
if [ ! -v DBUS_SESSION_BUS_ADDRESS ]; then pid=$(pgrep -u $LOGNAME cinnamon-sessio) eval "export $(\grep -z DBUS_SESSION_BUS_ADDRESS /proc/$pid/environ)" fi export DISPLAY=:0 notify-send "Hello!"
Modified from one of the comments here (note the shortened “cinnamon-sessio” — that’s intentional!): http://askubuntu.com/questions/298608/notify-send-doesnt-work-from-crontab
I only added the export DISPLAY as that was required on older versions of notify-send.