i was searching the forum and found some discussion about exec node and the usage of sudo but that unfortunately didn't help me with my issue:
I have the need to call a command in "sudo su" or root mode within exec node of node red.
Details:
I have a 5'' DSI TFT touch display from Waveshare on my raspberry pi, for which i want to control the brightness out of node red.
For controlling the brightness Waveshare proposes a call "echo xxx > /sys/class/backlight//brightness" in root mode, with "xxx" reflecting the level of brightness 0...255.
Generating different levels of brightness, depending on dashboard state - no issue.
Transferring the brightness level in node red to a system call in "sudo su" mode - stuck.
If i try to access the target file from command line in normal user mode or with "sudo" i get a permission denied error. The same is true, when calling it like this within the exec-node.
I already found out that root mode can be achieved with "sudo su" in command line. If i switch to root mode with "sudo su "first and then call "echo xxx > /sys/class/backlight//brightness" it works as expected.
Unfortunately i have no clue, how to achieve this within an exec node or node red. I tried "sudo su && echo" as command already, but didn't succeed.
Any idea how i could solve the challenge ? I am really stuck.
I believe the problem is that if you do sudo echo hello > filename, the echo command is executed as root but the redirection (the > ) is not.
What happens if you run this from the command line? echo xxx | sudo tee /sys/class/backlight/brightness
(Obviously use a sensible value instead of "xxx")
Thank You for your quick reply. Your idea works on command line. (i missed a path level in my first unsuccessful tests...). Looks like a more elegant solution than mine below.
I think i found a solution meanwhile myself. So we have two solutions to choose from:
Option 1:
echo "echo xxx > /sys/class/backlight/<display-id>/brightness" | sudo su
In the exec node i distribute the command like the following:
Ok, that's an ingenious approach.
I do wonder if the root shell will be left hanging after it's written to the file.
I almost never use sudo su, and definitely not for a background process, it just feels wrong.
By the way, I notice that in your original post your command was
echo xxx > /sys/class/backlight//brightness
So I suggested
echo xxx | sudo tee /sys/class/backlight/brightness
But now your command is
echo xxx > /sys/class/backlight/display-id/brightness
So did you test my version with the display id ?
Edit, the forum removed the angle brackets round displayid, so we may have been confusing one another
