###################### brightlight v8 ######################

Overview

This is brightlight, version v8. brightlight is a program that can get and set the screen backlight brightness on Linux systems using the kernel sysfs interface. I wrote this program because the backlight keys on my laptop's keyboard didn't work after I installed Linux on it, and the bash script I was using to perform the task wasn't very flexible nor portable.

This program requires libbsd or a BSD-compatible implementation of strtonum().

Please note that versions less than or equal to 5 are licenced under the GPL, version 2 or later, while versions 6 and later are licenced under an ISC-like licence.

Installation

Fedora

A package is available in Fedora 25 or later.

sudo dnf install brightlight

Source

brightlight can be compiled by issuing 'make'. Compiler options can be changed by editing the Makefile. The default compiler is gcc, however clang works too. You can then place the resulting binary somewhere in your $PATH. The default linker flags link libbsd for the strtonum() function, however it is possible to obtain the strtonum() source seperately and link it into the final executable.

You can change the path #defined by the preprocessor macro DEFAULT_CTRL_DIR to suit your own system. It must point to the directory where the files 'brightness' and 'max_brightness' reside. On my laptop, for example, this path is "/sys/class/backlight/intel_backlight/", however yours may be different. The compile-time default can always be overridden using the -f flag (see below).

By default, the files in sysfs are owned by user and group root, so the brightness may only be set by root (though any user may read it). One way around this would be to install brightlight setuid root, however this is not recommended, as a regular user could use the -f option to read and write files owned by other users. It is, however, possible to change the permissions on these files in sysfs, which could be set at boot time using the "anacron" functionality in some cron implementations, systemd-tmpfiles or /etc/rc.local to a group which would permit a controlled subset of users to change the backlight brightness without the requirement for a setuid root executable.

Usage

Options:

-v, --version          Print program version and exit.
-h, --help             Show this help message.
-p, --percentage       Read or write the brightness level as a percentage
                       (0 to 100) instead of the internal scale the kernel
                       uses (such as e.g. 0 to 7812).
-r, --read             Read the backlight brightness level.
-w, --write <val>      Set the backlight brightness level to <val>, where
                       <val> is a positive integer.
-i, --increment <val>  Increment/increase the backlight brightness level by
                       <val>, where <val> is a positive integer.
    --increase <val>   Same as --increment.
-d, --decrement <val>  Decrement/decrease the backlight brightness level by
                       <val>, where <val> is a positive integer.
    --decrease <val>   Same as --decrement.
-f, --file <path>      Specify alternative path to backlight control
                       directory. This is likely to be a subdirectory under
                       "/sys/class/backlight/".
-m, --maximum          Show maximum brightness level of the screen
                       backlight on the kernel's scale. The compile-time
                       default control directory is used if -f or --file is
                       not specified. The -p and --percentage flags are
                       ignored when this option is specified.

The flags -r, -w, -m, -i, -d and their corresponding long options are mutually exclusive. If none of these flags are explicitly specified then -r is the default.

Changelog:

v1, 07/02/2016 - First version of brighlight. v2, 26/04/2016 - Tidied up program internals, added new operations and long options. v3, 31/07/2016 - Tidied up program logic and internals, added build system. v4, 08/01/2017 - Minor changes to fix bugs found by clang static analyzer. v5, 21/10/2017 - Tweak to the Makefile, contributed by Igor Gnatenko. v6, 25/07/2018 - From-scratch rewrite, relicence under an ISC licence. v7, 08/10/2018 - Bugfix release. v8, 28/11/2019 - Make the read operation the default when none is otherwise specified.

License

See LICENSE.txt.