brightness-daemon.service

# vim: ft=systemd.unit
#
# Systemd service file for the brightness-daemon script.
#
# Author: Lorenzo Murarotto <lnzmrr@gmail.com>
# License: MIT

[Unit]
Description=Daemon to sync the brightness level of external monitors.
After=dbus.socket

[Service]
Type=simple
ExecStart=%h/.local/bin/brightness-daemon
Restart=always

[Install]
WantedBy=default.target

brightness-daemon.sh

#!/usr/bin/env bash
# vim: ft=bash
#
# Keep the external monitor brightness in sync with the DBUS brightness.
#
# Author: Lorenzo Murarotto <lnzmrr@gmail.com>
# License: MIT
# Dependencies: ddcutil

set -o errexit  # Exit on error.
set -o nounset  # Trigger error when expanding unset variables.
set -o pipefail # Use last non-zero exit code in a pipeline.

DBUS_PATH="/org/gnome/SettingsDaemon/Power"
DBUS_TYPE="signal"
DBUS_MEMBER="PropertiesChanged"
DBUS_INTERFACE="org.freedesktop.DBus.Properties"
DBUS_GET_METHOD="org.freedesktop.DBus.Properties.Get"
DBUS_ARG0="org.gnome.SettingsDaemon.Power.Screen"

# Wait time in seconds before propagating the brightness change.
# On my system the ddcutil command takes a while to execute, so if the
# brightness changes in quick succession, it will start lagging behind.
WAIT_TIME_SECONDS=1

function update_brightness() {
    echo "Setting external monitor brightness to $1"
    ddcutil setvcp 10 "$1"
}

function schedule_brightness_update() {
    sleep $WAIT_TIME_SECONDS
    update_brightness "$1"
}

update_brightness "$(
    dbus-send \
        --session \
        --dest=org.gnome.SettingsDaemon.Power \
        --type=method_call \
        --print-reply \
        $DBUS_PATH \
        $DBUS_GET_METHOD \
        string:"$DBUS_ARG0" \
        string:"Brightness" |
        grep -oP '(?<=variant       int32 )\d+'
)"

dbus-monitor --monitor "type=${DBUS_TYPE},interface=${DBUS_INTERFACE},member=${DBUS_MEMBER},arg0=${DBUS_ARG0}" |
    while read -r line; do
        if [[ "$line" =~ variant[[:space:]]+int32[[:space:]]+([[:digit:]]+) ]]; then
            if [[ -n "${last_pid:-}" ]]; then kill "$last_pid" 2>/dev/null || true; fi
            schedule_brightness_update "${BASH_REMATCH[1]}" &
            last_pid=$!
        fi
    done

Tested on Ubuntu 24.04 LTS

Save brightness-daemon.service to $HOME/.config/systemd/user/brightness-daemon.service
Save brightness-daemon.sh to $HOME/.local/bin/brightness-daemon (note no sh extension)
Exec systemctl --user enable --now brightness-daemon.service