Fixing brightness keys on Linux

If you have graphics card like mine, Intel GMA 4500MHD, with unfunctioning brightness key (e.g., Fn-brightness key up/down) you can fix it by using acpid handler.

First, install acpid package based on your GNU/Linux distribution. After installation, open journal log by executing

sudo journalctl -xf

Let it open and press your brightness key down or up, you will see in the log output something like this,

logger[30158]: ACPI group/action undefined: video/brightnessup / BRTUP
logger[28965]: ACPI group/action undefined: video/brightnessdown / BRTDN

Now, to fix this edit file /etc/acpi/handler.sh, add script below,

    video/brightnessdown)
    case "$2" in
    BRTDN)
        let x=`cat /sys/class/backlight/intel_backlight/actual_brightness`
        let x=$x-500000
        echo $x > /sys/class/backlight/intel_backlight/brightness
        logger "Brigtness down $x"
        ;;
    *)
        logger "ACPI action undefined: $2"
        ;;
    esac
    ;;
    video/brightnessup)
    case "$2" in
    BRTUP)
        let x=`cat /sys/class/backlight/intel_backlight/actual_brightness`
        let x=$x+500000
        echo $x > /sys/class/backlight/intel_backlight/brightness
        logger "Brigtness up $x"
        ;;
    *)
        logger "ACPI action undefined: $2"
        ;;
    esac
    ;;

What will it do was increasing/decreasing brightness by change its value in file descriptor /sys/class/backlight/intel_backlight/brightness. If you have different card try to change intel_brightness with other value. See list of /sys/class/blacklight directory for further examination.