mjhjmtu

You need to set the variables in the crontab itself:

DISPLAY=:0.0
XAUTHORITY=/home/matrix/.Xauthority

# m h dom mon dow command 
* * * * * /usr/bin/notify-send "hello"

No sudo needed, at least not on my system.

The safest way to get X session related environmental variables is to get them from the environment of a process of the user who is logged on to X. Here is an adaptation of the script that I use for exactly the same purpose (although DBUS_SESSION_BUS_ADDRESS doesn't seem to be a problem for me on Debian):

X=Xorg # works for the given X command
copy_envs="DISPLAY XAUTHORITY DBUS_SESSION_BUS_ADDRESS"

tty=$(ps h -o tty -C $X | head -1)
[ -z "$tty" ] && exit 1

# calling who with LANG empty ensures a consistent date format
who_line=$(LANG= who -u | grep "^[^ ]+[ ]+$tty")

x_user=$(echo $who_line | cut -d ' ' -f 1) # the user associated with the tty
pid=$(echo $who_line | cut -d ' ' -f 7) # the user's logon process

for env_name in $copy_envs
do
 # if the variable is not set in the process environment, ensure it does not remain exported here
 unset "$env_name"

 # use the same line as is in the environ file to export the variable
 export "$(grep -az "^$env_name=" /proc/$pid/environ)" >/dev/null
done

sudo -u "$x_user" notify-send "hello"

This sends to message to the first X user it finds, although you could add a loop to send it to all users. Hope this helps.

Update

It seems that updates to the utmp format cause who to print a display instead of a tty in its second column. This actually makes things easier, previously it only printed the display in the comment at the end and I decided this wasn't safe to rely on for the original answer. If this is the case, try this:

X=Xorg # works for the given X command
copy_envs="DISPLAY XAUTHORITY DBUS_SESSION_BUS_ADDRESS"

# calling who with LANG empty ensures a consistent date format
who_line=$(LANG= who -u | awk '$2 ~ ":[0-9]"')

x_user=$(echo $who_line | cut -d ' ' -f 1) # the user associated with the tty
pid=$(echo $who_line | cut -d ' ' -f 7) # the user's logon process

for env_name in $copy_envs
do
 # if the variable is not set in the process environment, ensure it does not remain exported here
 unset "$env_name"

 # use the same line as is in the environ file to export the variable
 export "$(grep -az "^$env_name=" /proc/$pid/environ)" >/dev/null
done

sudo -u "$x_user" notify-send "hello"

This is sufficient to make notify-send work for me in a cronjob on Ubuntu Trusty:

#!/bin/bash
export DISPLAY=$(who -u | awk '/s:[0-9]s/ print $2')

It simply exports the DISPLAY for the user the cronjob is running as. It works for me without setting XAUTHORITY or DBUS_SESSION_BUS_ADDRESS.

For those on Linux who are comfortable installing Python packages, I just released a notify-send-headless program which is working well for me. It searches /proc for the required username and environment variables and then runs notify-send with these variables (it will use sudo to switch to the required user if necessary).

You could also make a script:

#!/usr/bin/env bash
runuser -l [yourusername] -c 'DISPLAY=:0 notify-send "hey there user"'

Then run it with sudo. However, since crontab -e runs all commands with the user that created it, the following should suffice when called without sudo:

#!/usr/bin/env bash
DISPLAY=:0 notify-send "hey there user"

At least it does for me. It all seems to be dependent on the environment configuration.

This one-liner worked for me in Manjaro with Cronie:

# Note: "1000" would be your user id, the output of... "id -u <username>" 
10 * * * * pj DISPLAY=:0 DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus notify-send 'Hello world!' 'This is an example notification.'

Without the very ugly DBUS_blah_blah it doesn't work at all. I also found journalctl -xb -u cronie helpful. I'm not familiar with Cronie yet, but created my "crontab" as /etc/cron.d/mycronjobs and I'm not sure if that filename is required, or if it just reads everything in the cron.d directory.

I found the solution here https://wiki.archlinux.org/index.php/Desktop_notifications

I use this script in cron to post MPD now playing to twitter every hour

#!/bin/bash
export DISPLAY=":0.0"
msg=$(mpc current -h 192.168.1.33)
twitter set "#MPD Server nowplaying $msg : http://cirrus.turtil.net:9001"
#ttytter -status="#MPD Server nowplaying $msg. http://cirrus.turtil.net:9001"

exit 

similar script using notify-send

#!/bin/bash
export DISPLAY=":0.0"
notify-send -i ~/.icons/48arch.png 'OS- Archlinux x86_64 : DWM Window Manager' 'Installed on Sun Apr 21 2013 at 18:17:22' 
exit

you may be experiencing problems as KDE uses its own notify-deamon IIRC.

For what its worth....

I had to use ALL of the following on Debian Jessie to get this to work...

export DISPLAY=:0.0
export HOME=/home/$user
source "$HOME/.dbus/session-bus/*-0"

Leaving out any one of these caused it to stop working.

Using sudo :

sudo -u $currentxuser notify-send $message

Tip :

We can get the current x user by this command

ps auxw | grep -i screen | grep -v grep | cut -f 1 -d ' '

In addition...

currentxuser=$(ps auxw | grep -i screen | grep -v grep | cut -f 1 -d ' ')
echo $currentxuser

Good to know :

Cron running under root does not have access to x thus all gui commands will not be displayed, one simple solution is to add root to authorized x user for the current x user with this command

from the x user shell

xhost local:root

or

sudo -u $currentxuser xhost local:root

Here is a less complex script than what Graeme provided. His script didn't work for me, $who_line was always empty. My script doesn't waste so much time with finding a process. Instead, it just tries all and pick the last useful value found. I'm running xubuntu 14.04 and have some lxc containers running which probably confuse this kind of scripts.

env="$(
 ps -C init -o uid,pid | while read u p; do
 [ "$u" = "`id -u`" ] || continue
 grep -az '^DBUS_SESSION_BUS_ADDRESS=' /proc/$p/environ | xargs -0
 done | tail -1
)"

export "$env"

notify-send "test"