Running a sudo command automatically on startup

Question

In order to adjust the screenpad backlight on my ASUS Zenbook, I am using a kernel module I found here. Per his instructions, to make keybind shortcuts using a simple screenpad x command to adjust the brightness, I need to add sudo chmod a+w '/sys/class/leds/asus::screenpad/brightness' to 'rc.local', as the command is required with each reboot, and needs a password every time. By running automatically I could immediately use the custom keyboard shortcuts as they'd function normally with the drivers on Windows, without needing to run the command and enter my password each boot.

I'm a new Linux user, on Parrot OS. From what I've gathered, it's not recommend to use rc.local, and I should instead use either systemd, cronjob, or run it as process using the GUI startup applications menu.

I'm completely lost as to go about doing this with systemd or cronjob. I tried making a file called 'screenpad-perms.sh' and put it in /usr/local/bin, with just these lines in it based on what I've read:

#! /bin/bash

sudo chmod a+w '/sys/class/leds/asus::screenpad/brightness'

I then made it executable using chmod +x screenpad-perms.sh. Finally, I opened the GUI Autostart app and added it as a Login Script.

Restarted the PC but it doesn't work, typing screenpad x gets a permissions denied error unless I manually type sudo chmod a+w '/sys/class/leds/asus::screenpad/brightness' and enter my password; so it seems to not be executing.

Again apologies as I'm very new to Linux, just really hoping to get this screen working properly. What am I missing here?

Answer 1

If your system is using systemd, that is your best option for what you want to do. The systemd unit will already be executed as root, so sudo is not needed, and you can set it up to run during bootup without even needing anyone to be there to log in.

Here's one link with information on systemd: https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files

systemd unit files are more or less similar to a Microsoft *.INI file. They have [SectionHeadings] followed by Directive=Value line.

Here are the steps you will need:

1. Either load a root shell (sudo bash) or prefix most of the commands with sudo to run as root.

2. Create a shell script for the systemd service unit to execute. Typically, you will put the file in /usr/local/sbin. Let's call it /usr/local/sbin/fix-backlight.sh (as root):

   editor /usr/local/sbin/fix-backlight.sh

(Assuming editor launches your preferred editor and it creates the file if it does not exist.)

3. In the file, put (the #! MUST be the first line of the file):

   #!/bin/bash
   chmod a+w '/sys/class/leds/asus::screenpad/brightness'

4. Go ahead and save it and close your editor. Then make the file only read/write/executable by root (for security):

   chmod 0700 /usr/local/sbin/fix-backlight.sh

5. Create the systemd unit file (usually in /etc/systemd/system, but there are other locations; the link above gives more detail):

   editor /etc/systemd/system/fix-backlight.service

6. In the editor for that file, put:

   [Unit]
   Description=Fix perms for the 'screenpad x' backlight command
   [Service]
   ExecStart=/usr/local/sbin/fix-backlight.sh
   [Install]
   WantedBy=multi-user.target

7. Save and exit the editor. Test the unit:

   systemctl start fix-backlight.service

8. If all went well and from (a non-root) shell the 'screenpad x' cmmand is working, enable the unit to start on boot:

   systemctl enable fix-backlight.service

9. Go ahead then and reboot, and make sure it's all working now. (And if it does not and blows up the neighbor's cat, blame the dog!)

If needed, you can also systemctl disable fix-backlight.service to make it stop running at boot.

Answer 2

Maybe you need to enable this cmd inside the sudoers file, so this particular cmd won't need a password anymore. Sudoers is a config file, in which you can manage sudo related issues:

sudo EDITOR=nano visudo

to edit the file with nano via visudo (recommended) add:

[YOUR_USERNAME] ALL=(ALL) NOPASSWD:/bin/chmod u+w /sys/class/leds/asus::screenpad/brightness

insert your username without the brackets and add the correct location of your chmod with executing:

which chmod

for me it is /bin/chmod

basically you are allowing everyone logged in as you, to execute this command without a password prompt But please be careful, the sudoers file is very important,you can break your system if you save the file with a small typo. This is why you should use visudo to edit it, because it won't save the config file with errors. I'm changing my mac everytime I reboot this way, so it works for me.