notify-send to all users

notify-send-all

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

XUSERS=($(who|grep -E "\(:[0-9](\.[0-9])*\)"|awk '{print $1$5}'|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!"