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24
Dynamic Kernel Device
Management with udev
Since version 2.6, the kernel is capable of adding or removing 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.
24.1 The /dev Directory
The device nodes in the /dev directory provide access to the corresponding kernel
devices. With udev, the /dev directory reflects the current state of the kernel. Every
kernel device has one corresponding device file. If a device is disconnected from the
system, the device node is removed.
The content of the /dev directory is kept on a temporary file system and all files are
created from scratch at every system start-up. Manually created or changed files inten-
tionally do not survive a reboot. Static files and directories that should always be present
in the /dev directory regardless of the state of the corresponding kernel device can be
placed in the /lib/udev/devices directory. At system start-up, the contents of
that directory is copied to the /dev directory with the same ownership and permissions
as the files in /lib/udev/devices.
Dynamic Kernel Device Management with udev
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