Question: Like a lot of other users, I have problem with the brightness settings on my laptop. But there is `xbacklight` which works like a charm.

How do I bind `xbacklight -dec 10` to Fn+Up?

I have tried to use custom shortcut with no success.

Answer: 
According with [this post](https://askubuntu.com/a/80961/147044), in general there is no way to know that `Fn` was pressed and the keycode of key-combinations including **`Fn` depends on the individual keyboard design**. In fact, when you press only the the `Fn` key, this doesn't send any data to the computer.

So unfortunately, you can't bind this combination: `Fn`+`Up`. More about: [Fn key](http://en.wikipedia.org/wiki/Fn_key).

What you can do is to use another key combination to [add a custom keyboard shortcut](https://askubuntu.com/a/69161/147044).
You can use a software called "Autokey": it will start after the login if you want and supports scripting & key combinations.  
You can install it from the Ubuntu repository (`apt-get install autokey`).