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At first, I tried to simulate Fn + F5 / F6 in order to control the monitor's brightness, but I totally failed.

So, I started to find out workarounds:

There is a built-in software called gnome-control-center, so I tried to find out the solution from it.

We can see that there is a bar controlling monitor's brightness. Drag to the left for darkening, and vice versa.

I've downloaded the source code for 12.04 [site], but I still couldn't find the code snippet which controls the monitor's brightness.

Could anyone help me? Thanks.

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you can try xbacklight, for example xbacklight -set 80 but typically this means that you have some problem with your drivers. Which driver did you have installed ? –  user2485710 May 6 at 17:50
@user2485710 Where can I find the driver name installed by system autimatically? –  Kevin Dong Nai Jia May 6 at 17:53
@user2485710 Oh, my graphics driver is NVIDIA 331.20. BTW, the driver provided by NVIDIA works totally fine. I just want to control the brightness from software in the background. –  Kevin Dong Nai Jia May 6 at 17:55
I don't have any recent experience with the Nvidia drivers, anyhow the way you are supposed to follow to install proprietary drivers under Ubuntu is to use Software & Updates -> Additional Drivers which is basically the old jockey-gtk . Other methods are likely to be unsupported and you are supposed to know what you are doing as you do it. Wait for someone experienced with Nvidia software. –  user2485710 May 6 at 18:05

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