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I recently tried to change the default brightness of my ubuntu by echoing values to /sys/class/backlight/acpi_video0/brightness, and installing xbacklight, as suggested by answers to other questions. However, after booting ubuntu after doing the above, the screen just went blank.

After further checking, I found /sys/class/backlight folder is empty, and I have not been able to add a folder even under root permissions.

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marked as duplicate by Mitch Mar 13 '15 at 5:26

This question was marked as an exact duplicate of an existing question.

Sys is special, you won't be able to create folders there (answers in linked question explain why). Per Fabby's answer to similar question, that would indicate Kernel thinks you have a CRT, therefore you have knobs to adjust brightness and contrast. You may want to search supported hardware to see whether yours is there, or file a bug against kernel.

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