Backlight

This page documents methods for controlling backlight (aka screen brightness) and tips to control it via hotkeys.

Kernel native

Some laptops, using a recent enough kernel, will automatically handle increasing and decreasing the backlight using the hot keys. The following tools will allow scripting or controlling the backlight using other means, if desired.

Desktop Environment native

Some desktop environments will handle querying and setting the backlight, including configuring the backlight keys. These include at least: Plasma (KDE) and XFCE. It may be needed to configure or start some desktop environment-specific services.

xbacklight

xbacklight uses X to change the light settings. This can be inconvenient in some situations, e.g. for use with the services.actkbd service, which doesn't know about the X session. It, though, has an history of being more compatible with different hardware, especially newer hardware^{[citation needed]}

To install xbacklight globally, add this to your configuration.nix.

  environment.systemPackages = with pkgs; [ xorg.xbacklight ];

Alternatively, use nix-env -iA nixos.xorg.xbacklight to install it to your user profile.

light

light does not use X to change the light settings. This can be used in situations where the X service isn't available. While it does not use X, it will need some privileges to work.

To enable the use of light, add this to your configuration.nix and make sure that your user is a member of the video group.

  programs.light.enable = true;

The following commands will allow you to test light:

• light -U 30 — the screen should become darker.
• light -A 30 — the screen should become brighter.

Be careful using light -U, as you might turn your backlight completely off! You will not be able to see what you're typing anymore.

brightnessctl

brightnessctl (homepage) is another option, which, like light, would work even without X or on Wayland.

You can use it by simply installing the package. Since brightnessctl supports the systemd-logind API it should work out of the box (i.e. without installing any udev rules or using a setuid wrapper).

/sys/class/backlight/...

The /sys/class/backlight/*/brightness files are a built-in way to set brightness. Use them e.g. with:

  sudo tee /sys/class/backlight/intel_backlight/brightness <<< 300

to set the brightness to 300, where the maximum is stored in brightness_max. You can set file permissions e.g. with a udev rule, if you don't want to use sudo. Here is an example udev rule, where you will likely have to replace intel_backlight, with the name in your /sys/class/backlight/:

  services.udev.extraRules = ''
    ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="intel_backlight", MODE="0666", RUN+="${pkgs.coreutils}/bin/chmod a+w /sys/class/backlight/%k/brightness"
  '';

Tips

Key mapping

While controlling the backlight via the command line is useful, it would be preferable to control it using key bindings. This is especially true considering most laptops have backlight control keys.

There are two main choices to add key bindings, using a system-level service like actkbd or using an X session tool, either provided by your Desktop environment, Window manager or a tool like xbindkeys.

Depending on the tools that work for controlling the backlight you will be able to choose one of those options.

	light	xbacklight
System service	Yes	With hacks
X session	Yes	Yes

Follows, an example mapping for use with actkbd:

Note: This was verified to work with

• NixOS 18.03 on a Lenovo T440 and
• NixOS 19.09pre173166.373488e6f4c on a Lenovo X240

it may work on other models, but is unconfirmed. See actkbd for details on finding out the proper key bindings. Also check if the path to the light binary is correct.

  programs.light.enable = true;
  services.actkbd = {
    enable = true;
    bindings = [
      { keys = [ 224 ]; events = [ "key" ]; command = "/run/current-system/sw/bin/light -A 10"; }
      { keys = [ 225 ]; events = [ "key" ]; command = "/run/current-system/sw/bin/light -U 10"; }
    ];
  };

OLED Screens

OLED screens do not have a backlight, but their perceived brightness can be changed via xrandr:

• xrandr --output <output> --brightness .5 - dim to 50%
• xrandr --output <output> --brightness 1 - no dimming

See also

• Arch Linux wiki page about the backlight