notify-send to other user on the same system

If you have the other users password you can do sudo -u somedude notify-send Hello. somedude must have an X-session started.

Edit: Found this script for use with cron:

#!/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin
export DISPLAY=:0.0
export $(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$(pgrep -session)/environ )
TIME=$1
shift
/usr/bin/notify-send -t 36000 $1 $2

If the users are on terminals the following commands will be of use -

write user [tty]

or to send to all users

wall

I stumbled upon the following project that implements a user notify: tinynotify-send

I needed similar tool to send notification to all sessions. Here is my solution: https://unix.stackexchange.com/a/230062/93227

It scans /proc to find all session busses and then it executes notify-send on each of it (once per bus). All arguments are passed unchanged to real notify-send.

#!/bin/bash

/bin/grep -sozZe '^DBUS_SESSION_BUS_ADDRESS=[a-zA-Z0-9:=,/-]*$' /proc/*/environ \
| /usr/bin/php -r '
        $busses = array();
        array_shift($argv);
        while($ln = fgets(STDIN)) {
                list($f, $env) = explode("\0", $ln, 2);
                if (file_exists($f)) {
                        $user = fileowner($f);
                        $busses[$user][trim($env)] = true;
                }
        }
        foreach ($busses as $user => $user_busses) {
                foreach ($user_busses as $env => $true) {
                        if (pcntl_fork()) {
                                posix_seteuid($user);
                                $env_array = array("DBUS_SESSION_BUS_ADDRESS" => preg_replace("/^DBUS_SESSION_BUS_ADDRESS=/", "", $env));
                                pcntl_exec("/usr/bin/notify-send", $argv, $env_array);
                        }
                }
        }
' -- "$@"