A really simple daemon for adjusting monitor brightness using DDC/CI. Requires `ddcutil` to be installed.

brightness-daemon.service

[Unit]
Description=Simple Brightness Daemon

[Service]
ExecStart=/usr/bin/brightness-daemon.sh
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

brightness-daemon.sh

#!/usr/bin/bash

FIFO_NAME="/tmp/brightness-daemon-fifo"
DDCUTIL_FLAGS="--async --nousb"
NUM_DISPLAYS=2
BRIGHTNESS_MIN=20
BRIGHTNESS_MAX=100

################################

set -eEo pipefail

get_brightness() {
    /usr/bin/ddcutil $DDCUTIL_FLAGS --brief getvcp 10 \
        | /usr/bin/sed -r 's/VCP 10 C ([0-9]*) 100/\1/g'
}

CURRENT_BRIGHTNESS=$(get_brightness)
echo Current brightness is $CURRENT_BRIGHTNESS%.

if [[ ! -p $FIFO_NAME ]]; then
    /usr/bin/mkfifo $FIFO_NAME --mode 666
fi

while true; do
    read BRIGHTNESS_OFFSET < $FIFO_NAME
    CURRENT_BRIGHTNESS=$((CURRENT_BRIGHTNESS + BRIGHTNESS_OFFSET))
    if ((CURRENT_BRIGHTNESS < BRIGHTNESS_MIN)); then
        CURRENT_BRIGHTNESS=$BRIGHTNESS_MIN
    fi
    if ((CURRENT_BRIGHTNESS > BRIGHTNESS_MAX)); then
        CURRENT_BRIGHTNESS=$BRIGHTNESS_MAX
    fi
    echo Setting brightness to $CURRENT_BRIGHTNESS%...
    for i in $(seq 1 $NUM_DISPLAYS); do
        /usr/bin/ddcutil ${DDCUTIL_FLAGS} --display $i setvcp 10 $CURRENT_BRIGHTNESS
    done
done