"acpilight" is a backward-compatibile replacement for xbacklight that uses the ACPI interface to set the display brightness. On modern laptops "acpilight" can control both display and keyboard backlight uniformly on either X11, the console or Wayland.

On some modern laptops "XRandR" might lack the ability to set the display brightness. This capability was moved/unified to the kernel's ACPI interface, via /sys/class/backlight/.

"acpilight" provides a drop-in replacement for the xbacklight command that uses the ACPI interface instead of "XRandR", allowing old scripts to run. As a result, xbacklight can subsequently be used also from the console and Wayland (X11 is not used at all).

When paired with the ddcci-backlight kernel module, the backlight of most professional external monitors can be controlled as well.

If you have make available, to install the executable, man page, and udev rules; run:

sudo make install

Normally, users are prohibited to alter files in the sys filesystem. It's advisable (and recommended) to setup an "udev" rule to allow users in the "video" group to set the display brightness.

To do so, place a file in /etc/udev/rules.d/90-backlight.rules containing:

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

to setup the relevant permissions at boot time. These rules are very conservative: only the display brightness can be controlled, and nothing else. Do not set suid permissions on xbacklight! Similarly, running xbacklight under sudo is pointless (there's simply no reason to).

Keyboard backlight control is also available on certain laptop models via the "leds" subsystem. The following laptop models are known to have a controllable keyboard backlight:

• Lenovo ThinkPad: -ctrl tpacpi::kbd_backlight
• Apple MacBook Pro: -ctrl smc::kbd_backlight
• Dell Latitude/XPS: -ctrl dell::kbd_backlight
• Asus N-Series Notebooks: -ctrl asus::kbd_backlight

The following rules allow users in the video group to set the keyboard backlight as well:

SUBSYSTEM=="leds", ACTION=="add", KERNEL=="*::kbd_backlight", \
  RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness", \
  RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"

Please report if other laptops have working interfaces to control the keyboard backlight.

If you have an external monitor with which supports the DDC/CI protocol, backlight control can be enabled by installing the appropriate drivers and loading the ddcci-backlight module at boot time. On Debian, this is done by installing the ddcci-dkms package and then appending ddcci-backlight to /etc/modules.

The original "xbacklight" sets the display brightness of the current $DISPLAY, whereas "acpilight" ignores $DISPLAY completely and sets the brightness of the first device found in /sys/class/backlight by default. On systems with multiple panels, they might differ.

Use xbacklight -list to see the full list of available devices that can be controlled; then use the -ctrl flag to control them individually.

"acpilight" can always query, but might not necessarily be allowed to set the brightness of all listed devices depending on the current user and permissions. See the setup section above to set the permissions correctly.

"acpilight" doesn't fade: the requested brightness is set immediately by default. For smooth transitions, specify the number of fading steps (using -steps) or, more conveniently, the fading frame rate (-fps).

"acpilight" can be found at https://gitlab.com/wavexx/acpilight