/etc/udev/rules.d/ vs /lib/udev/rules.d/ - which to use and why?

Question

The Arch wiki recommended creating a file in /etc/udev/rules.d/ to fix my Steam controller issues. This worked! However, I noticed that there are no other rules in this folder except for the one I just created. Additionally, all other sources I've seen say to place udev rules in /lib/udev/rules.d/, and there are plenty of other udev rules in this folder too. Why are there two folders for seemingly the same thing, and which one should I use?

Answer 1

The difference between the two folders is that:

/lib/udev/rules.d

Contains default rules files. Do not edit these files.

/etc/udev/rules.d/*.rules

Contains customized rules files. You can modify these files or create them.

However, files with identical file names replace each other. Files in /etc have the highest priority and take precedence over files with the same name in /lib. This can be used to override the default rules.

Another thing, to disable the the rules file entirely, you can create a symlink in /etc with the same name as a rules file in /lib, pointing to /dev/null.

So to finish, Udev rules written by the administrator go in /etc/udev/rules.d/, and their file name has to end with .rules. The default udev rules are found in /lib/udev/rules.d/ (or /usr/lib/udev/rules.d). If there are two files by the same name under /lib and /etc, the ones in /etc take precedence.