light

Light - A Program to Control Backlight Controllers

• Introduction
• Examples
• Usage
  • Operation modes
  • Value modes
  • Target
  • Field
  • Controller modes
• Installation
  • Arch Linux
  • Manual
  • Permissions
• Origin & References

Introduction

Light is a program to control backlight controllers under GNU/Linux:

• Works, in particular when other software, e.g. xbacklight, does not
• Does not rely on X
• Automatically detects the best controller
• Possibility to set a minimum brightness value

Let's get started with a few examples, for details, see below for the full description of the different commands, options and how to access different controllers.

Examples

Get the current brightness in percent

light -G

or

 light

Increase brightness by 5 percent

light -A 5

Set the minimum cap to 2 in raw value on the acpi_video0 controller:

light -cr -s acpi_video0 -S 2

Try to set the brightness to 0 after that, it will be changed to the minimum 2:

light -r -s acpi_video0 -S 0

Find keyboard controllers:

light -k -L

Activate ScrollLock LED, here input15 is used, but this varies between different systems:

light -k -s "input15::scrolllock" -S 100

Usually, LEDs only take 0 or 1 in raw value (i.e. for off/on), so you can instead write:

light -kr -s "input15::scrolllock" -S 1

Verify by reading back the max brightness, you should get a value of 1:

light -kr -m -s "input15::scrolllock

Usage

Commands

• -G: Get (read) brightness/data from controllers/files
• -S VAL: Set (write)brightness/data to controllers/files
• -A VAL: Like -S, but adds the given value
• -U VAL: Like -S, but subtracts the given value
• -O: Save the current brightness for later use (usually used on shutdown)
• -I: Restore the previously saved brightness (usually used on boot)
• -L: List available controllers, see below -k option as well

Without any options (below) the commands operate on the brightness of an automatically selected controller. Values are given in percent, unless the below r option is also given.

Note: like most UNIX applications, light only gives output on errors. If something goes wrong try the verbosity option -v VAL:

• 0: No debug output
• 1: Errors
• 2: Errors, warnings
• 3: Errors, warnings, notices

Options

Values may be given, or presented, in percent or raw mode. Raw mode is the format specific to the controller. The default is in percent, but raw mode may be required for precise control, or when the steps are very few, e.g. for most keyboard backlight controllers.

• -p: Percent, default
• -r: Raw mode

By default the screen is the active target for all commands, use -k to select the keyboard instead. In either case you may need to select a different controller, see below.

• -l: Act on screen backlight, default
• -k: Act on keyboard backlight and LEDs

By default commands act on the brightness property, which is read+write. The maximum brightness is a read-only property. The minimum brightness cap is a feature implemented to protect against setting brightness too low, since some controllers make the screen go pitch black at 0%. For controllers like that it is recommended to set this value.

• -b: Current brightness of selected controller, default
• -m: Max. brightness of selected controller
• -c: Min. brightness (cap) of selected controller (recommend raw mode)

Controller is automatically done to select the controller with maximum precision. It can however also be done manually and we recommend the -L and -Lk commands to list available controllers:

• -a: Automatic controller selection
• -s ARG: Manual controller selection

Note: Without the -s flag on every command light will default to automatic controller selection.

Installation

Arch Linux

If you run Arch Linux, there exists 2 packages;

• light-git - For the absolutely latest version
• light-tag - For the latest tagged release

We recommend you go with light-git as you might miss important features and bugfixes if you do not.

Manual

We recommended downloading a versioned tarball from the relases page on GitHub. Download and untar the archive:

tar xf light-x.yy.tar.gz
cd light-x.yy/
./configure && make
sudo make install

However, should you want to try the latest GitHub source you first need to clone the repository and run the autogen.sh script. This requires automake and autoconf to be installed on your system.

./autogen.sh
./configure && make
sudo make install

The configure script and Makefile.in files are not part of GIT because they are generated at release time with make release.

Permissions

Optional: Instead of SUID root you can set up udev rules to manage sysfs permissions, you may skip the make install step and instead copy the file 90-backlight.rules to /etc/udev/rules.d/:

ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"

Origin & References

Copyright (C) 2012-2018 Fredrik Haikarainen

This is free software, see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE