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The special Fn keys typically trigger ACPI events, for which the ACPI daemon
can trigger actions.

Install ACPI daemon (`apt install acpid`) and make sure it is started and enabled. If it isn't,

    systemctl start  acpid
    systemctl enable acpid

You can read its manual page and inspect files under `/etc/acpi` to
easily figure it all out. But here is a summary.

Run `acpi_listen` and press the special Fn keys. For brightness keys, volume keys
and reconnecting the AC adapter, I get

    $ acpi_listen
     676AA15E-6A47- 000000bc 00000000
    video/brightnessdown BRTDN 00000087 00000000
     676AA15E-6A47- 000000bc 00000000
    video/brightnessup BRTUP 00000086 00000000
     676AA15E-6A47- 000000bc 00000000
     676AA15E-6A47- 000000bc 00000000
    button/volumedown VOLDN 00000080 00000000 K
    button/volumeup VOLUP 00000080 00000000 K
    ac_adapter ACPI0003:00 00000080 00000000
    battery PNP0C0A:00 00000080 00000001
    ac_adapter ACPI0003:00 00000080 00000001
    battery PNP0C0A:00 00000080 00000001

Ignore garbage lines, just take note of the relevant identifiers, and look inside
`/etc/acpi/acpi_handler.sh`.

The `set $*` statement turns each space delimited field into a positional
argument. I.e.,

    button/volumeup VOLUP 00000080 00000000 K
    ^^^^^^^^^^^^^^^ ^^^^^ ^^^^^^^^ ^^^^^^^^ ^
          $1         $2      $3       $4    $5

A working example:

```
if [ $# != 1 ]; then
        exit 1
fi
set $*

case "$1" in
    button/volumeup)      amixer -q set Master 5%+ ;;
    button/volumedown)    amixer -q set Master 5%- ;;
    video/brightnessup)   xbacklight -inc 6 ;;
    video/brightnessdown) xbacklight -dec 6 ;;
    ac_adapter)
        case "$4" in
            *1) adapter_connected ;;
            *0) adapter_disconnected ;;
        esac 
    ;;
esac
```

Then restart the daemon:

    systemctl restart acpid

And the bindings will work.

See also: [Arch Wiki: acpid](https://wiki.archlinux.org/title/Acpid).