In Ubuntu 18.04 with Gnome, what is the command that is used to change the display brightness (backlight)?
You can use these commands in most if not all versions of gnome:
# Increase brightness: gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp # Decrease brightness: gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepDown
If you have multiple monitors, you may be interested in this script which dims up to three monitors after 5 minutes of inactivity until mouse is moved or key is pressed. This is convenient when watching a movie on a second to fourth monitor (ie Big Screen TV).
At the very least the script will help you learn
xrandr in general and how to change brightness using
xrandr in great detail.
xrandr can do this. Type in xrandr to find the name of the display that's connected it will likely be named whatever_interface_you_use-1. Then use
xrandr --output the_display_name --brightness brightness_level
With brightness-level being a number between 0 and 1. Don't type in 0 unless you want your screen to go black.