This is a simple daemon for controlling displays' brightnesses. It consists of a daemon process listening for user requests (e.g. changing brightness) and a client to send commands to the daemon.

$ pipx install bctl

• control all displays, optionally blacklisting either class of screens or specific models
• monitor udev events and reinitialize on screen (dis)connections
• send desktop notifications on brightness change
• automatically synchronize screen brightnesses
• configure extra offsets to keep specific screen brightness higher or lower than the rest; could be useful e.g. with laptops whose displays tend to be dimmer.

Main reason for this program is to provide a simple, general-puropse central point for controlling brightness of all the connected screens simultaneously and keeping track of their current brightness levels.

Controlling laptops' internal screen is generally the easiest, as its device object is exposed under /sys/class/backlight/ dir -- it's the external displays that tend to be trickier.

For controlling external screens' brightness there are roughly two main methods, explained below. As the recommended method -- ddcutil -- takes in some cases non-trivial amount of time to execute (up to ~200ms), it can be slightly jarring to change brightness when spamming the key.

As this solution caches set brightness values there's no need to query it from ddcutil, making e.g. desktop notification generation simpler.

This section is more for documenting the possible methods. As long as ddcutil is installed on the system, feel free to skip.

This kernel module should detect the devices and expose 'em under /sys/class/backlight/, just like the laptop's internal display (e.g. /sys/class/backlight/amdgpu_bl0) is by default. This requires installation of ddcci-dkms package and loading ddcci kernel module.

Note as of '25 there are loads of issues w/ this kernel module's auto-detection logic, e.g. see issues 7, 42, 46

Current workaround seems to be manually enabling displays as per this reddit post:

• Before state (no external display devices listed/avail):

$ ls -l /sys/class/backlight
total 0
lrwxrwxrwx 1 root root 0 Sep  7 09:44 amdgpu_bl0 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/drm/card0/card0-eDP-1/amdgpu_bl0

• Enable manually:

$ echo 'ddcci 0x37' | sudo tee /sys/bus/i2c/devices/i2c-11/new_device

• After state (ddcci11 external screen is now avail):

$ ls -l /sys/class/backlight
total 0
lrwxrwxrwx 1 root root 0 Sep  7 09:44 amdgpu_bl0 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/drm/card0/card0-eDP-1/amdgpu_bl0
lrwxrwxrwx 1 root root 0 Sep  7 10:41 ddcci11 -> ../../devices/pci0000:00/0000:00:08.1/0000:07:00.0/i2c-11/11-0037/ddcci11/backlight/ddcci11

This is the recommended backend for controlling external displays. Requires i2c kernel module, but as of v1.4 "ddcutil installation should automatically install this file, making manual configuration unnecessary"

Note: arch wiki states:

Using ddcci and i2c-dev simultaneously may result in resource conflicts such as a Device or resource busy error

Meaning it's best to choose one of the options, not both.

As mentioned earlier, a daemon process needs to be started that keeps track of the displays and processes client commands. Easiest way to do so would be utilizing your OS's process manager. An example of a systemd user service file (e.g. ~/.config/systemd/user/bctld.service) would be:

[Unit]
Description=bctld aka brightness control daemon
PartOf=graphical-session.target
StartLimitIntervalSec=90
StartLimitBurst=5

[Service]
Type=simple
ExecStart=%h/.local/bin/bctld
Restart=on-failure
RestartSec=2
RestartPreventExitStatus=100

[Install]
WantedBy=graphical-session.target

Enable & start this unit by running

$ systemctl --user enable --now bctld.service

With daemon running, the client is used to send commands to the daemon. List available commands via bctl --help

Some examples:

• bctl up - bump brightness up by brightness_step config
• bctl down - bump brightness down by brightness_step config
• bctl up 20 - bump brightness up by 20%
• bctl down 20 - bump brightness down by 20%
• bctl delta 20 - bump brightness up by 20%
• bctl delta -- -20 - bump brightness down by 20%
• bctl set +20 - bump brightness up by 20%
• bctl set -- -20 - bump brightness down by 20%
• bctl set 55 - set brightness to 55%
• bctl get - return current brightness level in %; see the get_strategy config item in config.py to set how differing brightnesses get consolidated into a single int
• bctl get laptop - return our laptop/internal screen brightness; note "laptop" here can be either screen ID or our configured alias; "laptop" is an automatically added alias to all laptop/internal displays.
• bctl setvcp D6 01 - set vcp feature D6 to value 01 for all detected DDC displays; this is simply shortcut for ddcutil setvcp D6 01, but executed for all displays.

The daemon also registers signal handlers for SIGUSR1 & SIGUSR2, so sending said signals to the daemon process allows bumping brightness up or down by brightness_step respectively; e.g.: kill -s SIGUSR1 "$(pgrep -x bctld)" or killall -s SIGUSR1 bctld

The client and daemon communicate over a unix socket at $XDG_RUNTIME_DIR/bctl/bctld-ipc.sock. Note if XDG_RUNTIME_DIR is not defined, it defaults to /run/user/$UID. If using the provided client is too slow (e.g. for querying brightness), it's possible to talk to the daemon directly over this socket. For instance current brightness can be fetched via following command, which is equivalent to bctl get:

$ socat - UNIX-CONNECT:$XDG_RUNTIME_DIR/bctl/bctld-ipc.sock <<< '["get"]' | jq -re '.[1]'
75

Likewise, for setting brightness you might define a bash/zsh function similar to:

# Sets the screens' brightness level
#
# @param {digit|string}   percentage to set the brightness level to (without the percentage sign).
#                         may also prefix with + or - if delta change is wanted.
# @returns {void}
set_brightness() {
    local msg

    msg="${1%\%*}"  # strip trailing % if it was (mistakenly) given
    if [[ "$msg" =~ ^[0-9]{1,3}$ ]]; then
        msg="[\"set\",$msg]"
    elif [[ "$msg" =~ ^[-+][0-9]{1,3}$ ]]; then
        msg="[\"delta\",$((msg))]"
    else
        echo -e "illegal brightness arg provided: [$msg]" 1>&2
        return 1
    fi

    socat - UNIX-CONNECT:$XDG_RUNTIME_DIR/bctl/bctld-ipc.sock <<< "$msg"
}

...which is effectively same as bctl set "$1"

User configuration files are read from $XDG_CONFIG_HOME/bctl/*.json. The generated config will be the merger of all json files, with files ordered by the lexical order of their names. For full config list see the config.py file that defines the defaults and contains their descriptions, but the most important ones you might want to be aware of or change are:

Defines an event consumption window, meaning if say 'brightness up' key is spammed 5x during said window, ddcutil is invoked just once bumping up the brightness by 5x<brightness_step> value, as opposed to running ddcutil 5 times bumping 1x<brightness_step> each time.

This config sets the main backend for controlling the brightness. Available options:

• ddcutil - controls external displays via ddcutil, requires ddcutil to be on PATH, described above.
• raw - all displays are controlled via the device interfaces under /sys/class/backlight directory. In order to control external displays using this backend, you'd likely need the installation of ddcci kernel driver, described above.
• brightnessctl - all displays are controlled via brightnessctl program.
• brillo - all displays are controlled via brillo program.

This config sets the backend used only for controlling the internal display brightness, as that's not what ddcutil does. Only in effect if main_display_ctl=ddcutil and we're running on a laptop. Available options are raw | brightnessctl | brillo

Notification icon directory. Icon is chosen based on brightness level, and final used icon will be notify.icon.root_dir + notify.icon.brightness_{full,high,medium,low,off}.

Note either half of final value may be an empty string, so if you want to use single icon for all levels, set icon full path to notify.icon.root_dir and set notify.icon.brightness_{full,high,medium,low,off} values to an empty string.

Current implementation relies on listening for drm subsystem change action udev events. Some graphic cards (and/or monitors, unsure) are known to either not emit said events, emit them only sometimes, or emit different ones. Recommend you try debugging it via running $ udevadm monitor that starts listening for udev events, then connect or disconnect your monitor and see what events are printed out. With that info feel free to open an issue.

As a hacky workaround it's also possible to enable periodic polling by setting periodic_init_sec to seconds at which interval display detection should happen. Wouldn't set it to anything lower than 30.

Additionally you may opt out of udev monitoring altoghether (see config.py), and rely on your own custom detection; in that case daemon can be asked to re-initialize its state by sending init command via the client: $ bctl init

• offer ddcutil-service as an alternative backend for main_display_ctl; this one should really be preferred, as it plays nice with any other brightness control tool using the same service, meaning they're all aware of the brightness changes executed via it. Potential issue is with our syncing and offsets that could pose a challenge, especially if there's another service doing similar changes, which might lead to endless loop of fighting over control. Guess bctl could be the bigger man and not react to changes brought on by other parties. note on this to-be backend our own udev monitor should be switched off.

• https://github.com/digitaltrails/vdu_controls
• https://github.com/orhun/i3-workspace-brightness