Set correct primary monitor for login screen when lightdm greeter starts up on Elementary OS

set-primary-monitor-pantheon-greeter

#!/bin/bash
#
# Created: 2017-07-02 16:20
# Updated: 2018-02-09 12:23
# Creator: Ryan Miller
# Website: http://devopsmachine.com/
# File:    /usr/local/bin/set-primary-monitor-pantheon-greeter
# 
# Set correct primary monitor for login screen when lightdm greeter starts up on Elementary OS.
#
# Make sure to add the following line into /usr/share/lightdm/lightdm.conf.d/40-pantheon-greeter.conf (without the #)
# greeter-setup-script=/usr/local/bin/set-primary-monitor-pantheon-greeter
# 

# Change this variable to your primary display (Run 'xrandr -q' to see available displays)
PRIMARY_DISPLAY='HDMI-0'

# Get list of connected monitors
x_out=$(xrandr -q | grep -e '\( \)connected' | awk -F' ' '{ print $1 }')

# Safety - Check if PRIMARY_DISPLAY is connected; if not, bail out of script
[[ $(grep ${PRIMARY_DISPLAY} <<< ${x_out}) ]] || exit 0
     
# Iterate over x_out and disable all monitors except for PRIMARY_DISPLAY
for monitor in ${x_out[@]} ; do
    if [[ ${monitor} = ${PRIMARY_DISPLAY} ]] ; then
        xrandr --output ${monitor} --primary
    else
        xrandr --output ${monitor} --off
    fi
done

Yes, it works like all other similar 'solutions' - until the next shutdown, or plug-in/plug-out the external monitor..

I was able to get it to work on Juno by also copying the monitors.xml file:

sudo cp /home/<_user_>/.config/monitors.xml /var/lib/lightdm/.config/

Reboot and it worked like a charm.

if the screen turn to black. Just make sure the computer boot failed several time you will get into Grub2 and enter to recovery mode after that remove 2 file above /usr/local/bin/set-primary-monitor-pantheon-greeter and /usr/share/lightdm/lightdm.conf.d/40-pantheon-greeter.conf now black screen gone. Also all solution doesn't work for me.