The kernel can add or remove almost any device in the running system. Changes in device state (whether a device is plugged in or removed) need to be propagated to userspace. Devices need to be configured as soon as they are plugged in and discovered. Users of a certain device need to be informed about any state changes of this device. udev provides the needed infrastructure to dynamically maintain the device node files and symbolic links in the /dev directory. udev rules provide a way to plug external tools into the kernel device event processing. This enables you to customize udev device handling, for example, by adding certain scripts to execute as part of kernel device handling, or request and import additional data to evaluate during device handling.