Can't write to file /sys/class/backlight/acpi_video0/brightness (ubuntu)

Question

I am trying to change the brightness by overwriting the value on this file:

sudo echo 5 > /sys/class/backlight/acpi_video0/brightness
-bash: /sys/class/backlight/acpi_video0/brightness: Permission denied

It doesn't work even when using sudo. However if I switch to super-user with su, it works. Why is that?

This action is restricted to sudo users only. I found an answer [in this comment](https://askubuntu.com/questions/149054/how-to-change-lcd-brightness-from-command-line-or-via-script#comment1285547_469040): `The reason that this is set at su permissions is that a virus could conceivably make your screen dim and go bright at incredible speed ultimately damaging your hardware display. In the 90's I encountered a virus that would adjust the screen refresh Hertz so rapidly that your monitor would fry.` — Alexey Vol, Apr 03 '19 at 10:58

score 23 · Accepted Answer · answered Sep 19 '12 at 23:54

23

The error happens because sudo elevates permissions for the command (sudo echo 5) but not the redirection to write the file (> /sys/class/backlight/acpi_video0/brightness). The actual bash shell needs permission to write, which is why it fails with sudo but works as root.

You can work around this by running the tee command as root to write to the file:

echo 5 | sudo tee /sys/class/backlight/acpi_video0/brightness

Note that this will also echo "5" to your terminal. This is a normal side effect of the tee command.

answered Sep 19 '12 at 23:54

mguymon

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2
5

Credit goes to @duskwuff for the clever `tee` solution – mguymon Sep 20 '12 at 00:45
5

I can't take credit for coming up with that -- it's a bit of UNIX folklore I picked up from who-knows-where. – Sep 20 '12 at 02:15
2

not working for me with /sys/class/drm/card0/device/pp_sclk_od cannot change the value – alexela Sep 15 '17 at 20:25

Manuel Schmitzberger · Answer 2 · 2019-11-24T09:54:01.980

22

As written in the Arch wiki (link), by default, only root can change the brightness by this method. To allow users in the video group to change the brightness, a udev rule such as the following can be used (replace the <vendor> with your vendor id. E.g. acpi_video0, intel_backlight) :

% cat /etc/udev/rules.d/backlight.rules
ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="<vendor>", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="<vendor>", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"

Then you need to add your user to the video group.

usermod -aG video <user>

After that this should work:

echo 5 > /sys/class/backlight/<vendor>/brightness

edited Nov 24 '19 at 09:54

answered Jan 12 '19 at 11:59

Manuel Schmitzberger

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5

1

This method works for me, after replacing `acpi_video0` by `intel_backlight`. – Mehdi May 05 '19 at 10:06
1

Thx for your feedback. I've updated the answer. – Manuel Schmitzberger May 05 '19 at 11:27
I'm using debian 10, and the output of `groups` showed that my non-root account is already in `video`. Also, I had to reboot before this answer worked for me. – xdavidliu Apr 15 '20 at 01:07
This is the better way to do that. It must the accepted answer – Nicolas Perraut Jul 09 '20 at 15:19
Should be an accepted answer. Now I can run a custom script to adjust backlight with non-root user – Pham Dec 06 '20 at 03:11

score 2 · Answer 3 · answered May 23 '15 at 17:42

2

If you didn't want 5 to be echoed this also works:

sudo sh -c 'echo 5 > /sys/class/backlight/acpi_video0/brightness'

answered May 23 '15 at 17:42

raphael

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1

score 0 · Answer 4 · answered May 04 '20 at 19:56

0

The solution with tee is much better suited for scripting and such, but I found that simply editing the file with for example vim run using sudo works as well and is easier for me when I'm just doing it manually.

answered May 04 '20 at 19:56

Isti115

994
10
11

score 0 · Answer 5 · edited Nov 13 '13 at 22:10

0

I've been struggling with this problem on my VAIO VPCEG for quite a time. After doing everything mentioned in every forum I found something interesting:

After changing the boot parameter acpi_osi=Linux acpi_backlight=vendor and trying to manually change /sys/class/backlight/[vendor - in my case intel_backlight]/brightness, I realized that changing permission to this file from root to my user and restarting acpid service, this would allow me to use brightness keys flawlessly.

edited Nov 13 '13 at 22:10

answered Nov 13 '13 at 21:48

user273880

1

`"changing permission to this file from root to my user"` - this will get reset at next reboot. – TheEagle Jul 31 '21 at 21:11
@TheEagle why? how to make it persistent? – Vitaly Zdanevich Dec 24 '22 at 19:26

score 0 · Answer 6 · answered Jun 29 '15 at 13:19

0

the below solutions works fine for me..

i am posting it as answer so that others might get help:

change the permission:

sudo chmod a+rw /sys/class/backlight/intel_backlight/brightness

now change brightness:

echo 400 > /sys/class/backlight/intel_backlight/brightness

in your case it would be: /sys/class/backlight/acpi_video0/brightness

answered Jun 29 '15 at 13:19

noobdeveloper99

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1

1

Welcome to Super User! Please don't add "thanks" as answers. Invest some time in the site and you will gain sufficient [privileges](http://superuser.com/privileges) to upvote answers you like, which is the Super User way of saying thank you. – DavidPostill Jun 29 '15 at 13:51
In addition you didn't really answer the question, which was "Why is that?" – DavidPostill Jun 29 '15 at 13:52
thanks a lot.. for the information and really sorry.. just because of me.. you wasted your important time for guiding me..i will make sure this doesn't happen again – noobdeveloper99 Jun 29 '15 at 15:49
It is not good practice to allow everyone to read and write a system config file like that. – lindhe Dec 18 '15 at 15:15
I have some scenarios, where the screen starts functioning, yet the backlight value is still 0, – ransh Nov 26 '16 at 21:22
Also you have to redo this after every reboot – TheEagle Jul 31 '21 at 21:11

Can't write to file /sys/class/backlight/acpi_video0/brightness (ubuntu)

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