Show a notification across all running X displays

You can send a message to all consoles with the command wall.

For sending notifications under X there is notify-send which sends a notification to the current user on the current display. (From your question, I guess you already know this one.) You can build upon this with some bash scripting. Basically you have to find out which users are on which X-Displays. Once you got this info you can use notify-send like this:

DISPLAY=:0 sudo -u fschmitt notify-send "Message"

Where fschmitt is the user at display 0. You can parse the output of the "who" command to find all users and their displays. The output looks like this

[edinburgh:~]$ who
markmerk3 tty7         2010-09-23 10:59 (:0)
markmerk3 pts/1        2010-09-30 13:30 (:0.0)
fschmitt pts/2        2010-10-08 11:44 (ip-77-25-137-234.web.vodafone.de)
markmerk3 pts/0        2010-09-29 18:51 (:0.0)
seamonkey pts/6        2010-09-27 15:50 (:1.0)
markmerk3 pts/5        2010-09-27 14:04 (:0.0)
seamonkey tty8         2010-09-27 15:49 (:1)
markmerk3 pts/13       2010-09-28 17:23 (:0.0)
markmerk3 pts/3        2010-10-05 10:40 (:0.0)

You see, there are two users running X sessions, markmerk3 at display 0 and seamonkey at display 1. I think you need to grep for tty[0-9]* then assure that at the end of the line there is (:[0-9.]*) to get rid of console logins and extract the display id from the string between the parentheses.

This thread is a bit old, sorry, but I hope I still can add something useful to the topic. (also Josef Kufner wrote a nice script, it was just a little bit too long for my taste, and it uses PHP).

I also needed a tool as described in the original question (to send a message to all active X-users). And based on the answers here, I wrote this little bash-only script, which searches for active X-users (using who), and then running notify-send for every active user.

And the best: you can use my script exactly like "notify-send", with all of its parameters! ;-)

notify-send-all:

#!/bin/bash
PATH=/usr/bin:/bin

XUSERS=($(who|grep -E "\(:[0-9](\.[0-9])*\)"|awk '{print $1$NF}'|sort -u))
for XUSER in "${XUSERS[@]}"; do
    NAME=(${XUSER/(/ })
    DISPLAY=${NAME[1]/)/}
    DBUS_ADDRESS=unix:path=/run/user/$(id -u ${NAME[0]})/bus
    sudo -u ${NAME[0]} DISPLAY=${DISPLAY} \
                       DBUS_SESSION_BUS_ADDRESS=${DBUS_ADDRESS} \
                       PATH=${PATH} \
                       notify-send "$@"
done

Copy the above code into a file named notify-send-all, make it executable and copy it to /usr/local/bin or /usr/bin (as you like). Then run it e.g. as root in a console session like this:

notify-send-all -t 10000 "Warning" "The hovercraft is full of eels!"

I'm using it several months now, on different machines, and didn't have any problems so far, and I've tested it with MATE and Cinnamon desktops. Also successfully running it within cron and anacron.

I wrote this script for/under Arch Linux, so please tell me if you're having problems on another Linux distributions or desktops.

I needed this too for some system-wide notifications. Here is my solution. 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, $env_array);
                        }
                }
        }
' -- "$@"