Question: I have installed elementary OS on my HP 2000-2128 notebook. But screen is faint and bright. To change contrast i have added `xgamma -gamma 0.7` to startup command. But on reboot it automatically changes to 1, so that at every reboot I have reset screen contrast to `xgamma -gamma 0.7` through terminal.

Answer: 
You have to pres `Esc` key once before the elementary logo appears.
Or, if you're not fast enough, edit grub file: 

`sudo nano /etc/default/grub` 

Comment (place a `#` before) the two timeout lines, just like in the image bellow: 

[![enter image description here](https://i.stack.imgur.com/6Nu83.png)](https://i.stack.imgur.com/6Nu83.png) 

`Ctrl` + `o` to save the modifications

`Ctrl` + `x` to exit nano

`sudo update-grub`

`sudo reboot`

Now you'll get the GRUB screen, with a 5 seconds delay and you'll be able to do your thing. To revert, just un-comment those two lines and run `update-grub` again. I recommend not modifying anything else, unless you know what you're doing...
You can hold down the (right) shift key while booting, before the elementary logo appears to get the Grub menu displayed.