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I'd like to read small text file, located at: /sys/kernel/debug/vgaswitcheroo/switch without root permissions. It contains information about graphic cards (dedicated ATI Radeon and integrated Intel HD3000). I wrote a simple script displaying infomation I need, but it works only with root privileges. I already tried changing file & folder permissions, but when I restart my computer, the problem remains. Is it possible to permanently change that file permissions or owner?

Why do I need that? To bind that script with keyboard shortcut and get following Ubuntu 'dialog' after pressing key combination:

image

What is inside switch file? As I said before, it's just simple text file, with following content:

0:IGD:+:Pwr:0000:00:02.0

1:DIS: :Off:0000:01:00.0

My script status-radeon.sh

#!/bin/bash
CONTENT=`cat /sys/kernel/debug/vgaswitcheroo/switch | cut -d ':' -f 1-4 `
DISPLAY=:0 notify-send -t 4000 -i /home/Greg/Pictures/ati_radeon.png "GPU status" "$CONTENT"
echo "$CONTENT"

My /etc/sudoers.tmp content, without comments starting with #. It starts with nano editor, but I don't think it's relevant.

Defaults        env_reset                                             
Defaults        mail_badpass Defaults       
secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
root    ALL=(ALL:ALL) ALL 
Greg    ALL=(ALL:ALL)   ALL 
Greg    ALL= NOPASSWD: /home/Greg/Scripts/status-radeon.sh
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2 Answers

up vote 0 down vote accepted

Changing the permissions of system folders is not a good idea. Why don't you just setup a crontab for root that copies the file's contents somewhere else and have your script read from that?

$ sudo crontab -e

Add a line like this:

 */5 * * * * cp /sys/kernel/debug/vgaswitcheroo/switch /home/YOUR_USER/switch

Now, every time you reboot, that file will be copied to your home directory (remember to replace YOUR_USER with your actual username) and you can access it there.

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I've added at the end of file line: @reboot cp /sys/kernel/debug/vgaswitcheroo/switch /home/Greg/Scripts/switch. After saving file and rebooting computer, no new file was added to /home/Greg/Scripts/. – gogowitczak Nov 23 '12 at 15:38
@gogowitczak, not sure what's wrong here and I don't want to debug it in this question. Try posting a new question about why a crontab fails to run and I would be happy to help you there. Also, try adding a similar crontab with a simple commmand, for example @reboot ls /usr > /home/Greg/usr_contents and check if that is executed after reboot. – terdon Nov 23 '12 at 16:15
@gogowitczak, see updated answer. – terdon Nov 23 '12 at 18:08
When I ran script with sudo, it works fine (see screenshot). After adding your modifications, when I execute script ./status-radeon.sh, it prompts for root password. – gogowitczak Nov 23 '12 at 19:14
@gogowitczak, sorry, my updated answer is just wrong (and will be deleted), I tried it on my machine and it worked because I still had root priviledges (idiot). Maybe the problem with cron is that it is running before the file is created. Try replacing the @reboot line with */5 * * * * cp /sys/kernel/debug/vgaswitcheroo/switch /home/YOUR_USER/switch to make it run every 5 minutes. – terdon Nov 23 '12 at 19:27
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up vote 1 down vote

You can use sudo to allow some users (e.g., you) to run the script as root

$ visudo

and then add

username  ALL= NOPASSWD: /path/to/your/script

For a short introduction to sudo: http://www.sudo.ws/sudo/intro.html

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It's not working. I've added at the end of file: Greg ALL=NOPASSWD: /home/Greg/Scripts/status-radeon.sh and rebooted my machine. – gogowitczak Nov 23 '12 at 15:23
No need to reboot. And what do you mean by "it's not working". Any error? Which system are you using? BTW there is a space between the = and NOPASSWD: – Matteo Nov 23 '12 at 15:25
@Matteo: I've added the missing space. Error: cat: /sys/kernel/debug/vgaswitcheroo/switch: Access denied. I'm using Ubuntu 12.10 x64. @terdon: I've ran successfully sudo chmod 4755 status-radeon.sh, still not working. – gogowitczak Nov 23 '12 at 16:33
@gogowitczak, hmm,I think we are falling afoul of the XY Problem. What exactly are you trying to do here? Why do you need to read that file? There should be a workaround... – terdon Nov 23 '12 at 16:51
@terdon - I've added a detailed info to original post. – gogowitczak Nov 23 '12 at 17:48

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