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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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