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I tried re-installing kali linux for other reasons, and still don't see it

For some reason, running latest Kali linux, brightness slider is absent in the top right right-click menu where it usually is, like in my friend's laptop.

It's frustrating because I'm not able to change the brightness to get better battery life on my laptop. If someone could help me with it, that'd be great.

Perhaps it's best I mention right away that although I was able to install xbacklight by 'apt-get install xbacklight', the commands show no output and don't reduce brightness for my laptop at all.

I also have seen forums explaining how to manually edit /sys/class/backlight files to change the brightness, but I don't have any further directories to find that file from. The /sys/class/backlight folder doesn't have any file or subsequent folders inside.

The 'Battery & Lock' tab inside the settings app is also not there, and there's no option to change brightness in the Power tab.

Could someone explain a fix for me please?

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First check whether your laptop's back-light is detected or not by your OS kernel. The folder '/sys/class/backlight' should not be empty.

If your laptop back-light manufacture is Intel edit the following file to change brightness.

sudo vim /sys/class/backlight/intel_backlight/brightness

An empty '/sys/class/backlight/' folder indicate backlight hardware is not detected.

If so please add this two option "video.use_native_backlight=1 acpi_backlight=vendor" (without quotes) in grub command line before system boot up. Press 'e' to edit the grub command line before your system boot and add the above options after "quiet splash". After edit last part of your grub command line shall look like

....quiet splash video.use_native_backlight=1 acpi_backlight=vendor....

Then boot the system. You shall find some content under 'sys/class/backlight/' folder now. Now edit '/sys/class/backlight/intel_backlight/brightness' to change brightness. You may also try one grub option at a time.

If this solve the problem update grub options and regenerate grub configuration to make the change permanent. Once the backlight is detected by Operating system the slider option shall reappear (I'm not sure).

You may consult this article also https://askubuntu.com/questions/476664/cannot-change-backlight-brightness-ubuntu-14-04

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