Change permission on file in /sys folder I am trying to turn off the backlight on the soft keys (home, menu, back and search).

My phone is a HTC Evo 3D running Ice Cream Sandwich.  Before, I had Gingerbread installed and I could use "Screen Filter" app to turn the lights off. After the upgrade, "Screen Filter" no longer works.

On one forum it says that if you edit the file max_brightness in folder 

/sys/class/leds/button-backlight

from 255 to 0 and then switch the screen off and on again then the key lights stay off.

However, the file is read-only.

I want to make the file writable. Do I need to root my phone to do this?
Yes, your phone must be rooted because to access that file the /system needs to be mounted as write-able

Once you obtain root you can run this command inside a shell:
su
chmod 777 /sys/class/leds/button-backlight/max_brightness

Then just navigate to that file and edit it using a file browser. What the chmod 777 command does is change the permissions on that file so that you can edit it. After you finish I would suggest changing the permissions back to what they were.

The original permissions were probably 0666.
So it would be like this: chmod 0666 /sys/class/leds/button-backlight/max_brightness

To encompass t0mm13b's remark below, go here for information on how to make the file write-able at boot, instead of having a script be run at each boot that makes it write-able.