A posix shell compliant script for sending a notification to all logged in users with a display in use (tested on void, put in an sbin path, needs to run as root)

notify-send-all

#!/bin/sh
verbose_echo()
{
  [ -n "${VERBOSE##[NFnf]*}" ] && \
    echo "$@"
}

LOGGED_USERS=$(who | awk '{print $1 " "}')
for user in $LOGGED_USERS; do

  # grab the user's running DISPLAY
  disp=$(ps -u "$user" -o pid= |
    xargs -I{} cat /proc/{}/environ 2>/dev/null |
    tr '\0' '\n' |
    grep -m1 '^DISPLAY=')
  # check used X DISPLAY exists for the logged in user
  if [ -n "$disp" ]; then
    verbose_echo attempting to notify "$user" on "$disp"
    # run notify-send with passed command
    sudo -u "$user" "$disp" /bin/notify-send "$@"
  fi
done