Which directory contains volatile udev rules?

The directory that contains volatile udev rules is indeed /run/udev/rules.d. This directory is used for runtime or temporary rules that are created during the current boot session. These rules can change dynamically as devices are added or removed, making them volatile in nature.

The /run directory is a temporary filesystem (tmpfs) that is cleared on boot, so any rules stored there are not persistent across reboots. This is ideal for rules that need to adapt to changes in hardware availability or configuration at runtime, rather than being statically defined.

In contrast, other directories like /etc/udev/rules.d and /lib/udev/rules.d contain persistent rules. The /etc/udev/rules.d directory usually has user-defined or custom rules, while /lib/udev/rules.d contains the default rules provided by the system. These persistent rules are loaded during system initialization and remain unchanged until modified by the user or system updates. Therefore, for rules that are intended to be transient and only applicable during the current session, /run/udev/rules.d is the appropriate location.