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i want to edit actual_brightness with gedit in (/sys/class/backlight/acpi_video0) to change brightness level. my way is pressing alt+f2 then gksudo gedit then enter the password after editing the number and press save
its give me (you don't have the permission necessary to save the file)

note:i cannot change brightness level from settings

where is the mistake?

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  • First of all what is you video adapter? Please edit your question and add output of lspci -k | grep -EA2 'VGA|3D' terminal command. And what is your computer model?
    – Pilot6
    Jul 8, 2015 at 20:51
  • possible duplicate of cat showing file content but mousepad and abiword won't
    – muru
    Jul 9, 2015 at 8:46
  • Whatever I said about /proc is equally applicable to /sys.
    – muru
    Jul 9, 2015 at 8:46

3 Answers 3

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run ll /sys/class/backlight/acpi_video0 and you'll probably get something like this

% ll /sys/class/backlight/intel_backlight/
total 0
-r--r--r-- 1 root root 4096 Jul  6 14:01 actual_brightness
-rw-r--r-- 1 root root 4096 Jul  6 14:01 bl_power
-rw-r--r-- 1 root root 4096 Jul  6 14:01 brightness
lrwxrwxrwx 1 root root    0 Jul  6 14:01 device -> ../../card0-eDP-1
-r--r--r-- 1 root root 4096 Jul  6 14:01 max_brightness
drwxr-xr-x 2 root root    0 Jul  6 14:01 power
lrwxrwxrwx 1 root root    0 Jul  6 14:01 subsystem -> ../../../../../../../class/backlight
-r--r--r-- 1 root root 4096 Jul  6 14:01 type
-rw-r--r-- 1 root root 4096 Jul  6 14:01 uevent

actual_brightness is read-only. I'm guessing you're supposed to write to brightness, and actual_brightness will be automatically updated when/if your write to brightness causes an update.

Before you do this, though, try xbacklight.

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  • did you try xbacklight? Jul 6, 2015 at 23:00
  • you mean xbacklight file ? no i don't have it ... see the picture i.imgur.com/7e5NPcM.png Jul 6, 2015 at 23:53
  • the program xbacklight Jul 7, 2015 at 0:06
  • by mistake i delete my first replay it was (after i edit brightness file it say: could not create backup file while saving . save any way ? when press save any way it ask me again same question and repeat it) Jul 7, 2015 at 0:13
  • seriously, use xbacklight. it will probably work. Jul 7, 2015 at 0:16
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To write a number to a system file you do it like this:

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

or

sudo tee /sys/class/backlight/acpi_video0/brightness <<< "5"

But in your case it may be not acpi_video0 but intel_backlight.

Depending on your laptop model and video adapter this issue can be fixed by adding kernel boot parameters.

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2 possibilities I can think of:

  • Sudo ain't giving you "full" root rights. Sudo is just executing stuff with root's rights while still being you - roughly speaking. (It also does not require root password, but sudo password, this might even be the password of the user who is "sudo-ing") It might fix your problem if you try to become root: do "su" in the terminal and enter the roots password. You will see that your prompt will change to root@PCNAME. Then try editing the file with gedit (launch via this terminal session ofcourse) again.

  • If this does not work, it's simply not a permission problem (afaik) - the file is read-only. Part of the unix-philosophy is "everything is a file". Even devices, even not really "existing" stuff. It does not always make sense to have something writable. Example: No matter how much permissions you have, you never will be able to really mount an iso read-writeable. So: Try achieving the same without writing the "file" ;-)

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