Modify external monitor settings like brightness and contrast, from debian

external-monitor-control.sh

# add the repository to get ddcutil
sudo add-apt-repository ppa:rockowitz/ddcutil
# update 
sudo apt-get update
# install ddcutil
sudo apt install ddcutil


# change brightness of the primary montior to 80
sudo ddcutil setvcp 10 80

# change contract of the primary monitor to 80
sudo ddcutil setvcp 12 80

# change settion of secondary monitor
sudo ddcutil -d 2 setvcp 12 80


# or, change the brighness of the OS, in case you can't change the montior.
# see connected displays
xrandr -q | grep " connected" 

# set bightness of the video output going to the monitor (OS level change)
xrandr --output DP-1 --brightness 0.5