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 operating modes, value conversion and how to operate 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

Light has 5 different criteria on flags to use: operation modes, value mode, target, field and controller mode. Flags from these different modes can never be used in conjunction, i.e. you cannot get and set a value at the same time, that requires different invocations.

Note: like most UNIX applications, light only print errors if you are using it incorrectly. If something goes wrong, and you can't figure out why, try setting the verbosity flag with -v:

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

Operation modes

The operation modes describe what you want to do.

• -G: Which reads/gets brightness/data from controllers/files
• -S: Which writes/sets brightness/data to controllers/files
• -A: Which does like -S but instead adds the value
• -U: Which does like -S but instead 'subtracts the 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 the available controllers

When used by themselves operate on the brightness of a controller that is selected automatically. S, A and U needs another argument -- except for the main 4 criteria -- which is the value to set/add/subtract. This can be specified either in percent or in raw values, but remember to specify the value mode (read below) if you want to write raw values.

Value modes

The value mode specify in what unit you want to read or write values in. The default one (if not specified) is in percent, the other one is raw mode and should always be used when you need very precise values (or only have a controller with a very small amount of brightness levels).

• -p: Percent
• -r: Raw mode

Remember, this is the unit that will be used when you set, get, add or subtract brightness values.

Target

You can choose which target to act on:

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

Field

As you can not only handle the brightness of controllers, you may also specify a field to read/write from/to:

• -b: Current brightness of selected controller
• -m: Maximum brightness of selected controller
• -c: Minimum brightness (cap) of selected controller

The minimum brightness is a feature implemented as some controllers make the screen go pitch black at 0%, if you have a controller like that, it is recommended to set this value (in percent or raw mode). These values are saved in raw mode though, so if you specify it in percent it might not be too accurate depending on your controller.

Controller modes

Finally, you can either use the built-in controller selection to get the controller with the maximum precision, or you can specify one manually with the -s flag. The -a flag will force automatic mode and is default. Use -L to get a list of controllers to use with the -s flag (to specify which controller to use).

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