Editing Udev (section)

==Operation==
[[File:Free and open-source-software display servers and UI toolkits.svg|thumb|350px|udev has been incorporated into [[systemd]] 183<ref name=":0"/> ]]

udev is a generic device manager running as a [[Daemon (computer software)|daemon]] on a Linux system and listening (via a [[netlink]] socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system. The udev package comes with an extensive set of rules that match against exported values of the event and properties of the discovered device. A matching rule will possibly name and create a device node and run configured programs to set up and configure the device.

udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of the order devices are discovered by the system.

In the past a common way to use udev on Linux systems was to let it send events through a socket to [[HAL (software)|HAL]], which would perform further device-specific actions. For example, HAL would notify other software running on the system that the new hardware had arrived by issuing a broadcast message on the [[D-Bus]] [[Inter-process communication|IPC]] system to all interested [[process (computing)|processes]]. In this way, desktops such as [[GNOME]] or [[K Desktop Environment 3]] could start the [[file browser]] to browse the file systems of newly attached [[USB flash drive]]s and [[Secure Digital|SD cards]].<ref>{{cite web
 |url         = http://w3.linux-magazine.com/issue/71/Dynamic_Device_Management_in_Udev.pdf
 |title       = Dynamic Device Management in Udev
 |publisher   = Linux Magazine
 |date        = 2006-10-01
 |access-date = 2008-07-14
}}{{Dead link|date=March 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

By the middle of 2011 HAL had been deprecated by most Linux distributions as well as by the KDE, GNOME<ref>{{cite web|url=http://wiki.debian.org/HALRemoval|title=HALRemoval|date=2011-06-28|access-date=2011-09-13}}</ref> and [[Xfce]]<ref>{{cite web|url=http://gezeiten.org/pre-2014/post/2010/01/Thunar-volman-and-the-deprecation-of-HAL|title=Thunar-volman and the deprecation of HAL in Xfce|date=2010-01-17|access-date=2017-12-25|archive-date=2017-12-26|archive-url=https://web.archive.org/web/20171226020945/http://gezeiten.org/pre-2014/post/2010/01/Thunar-volman-and-the-deprecation-of-HAL|url-status=dead}}</ref> desktop environments, among others. The functionality previously embodied in HAL has been integrated into udev itself, or moved to separate software such as udisks and [[upower]].

* udev provides low-level access to the linux device tree. Allows programs to enumerate devices and their properties and get notifications when devices come and go.
* dbus is a framework to allow programs to communicate with each other, securely, reliably, and with a high-level object-oriented programming interface.
* udisks (formerly known as DeviceKit-disks) is a daemon that sits on top of libudev and other kernel interfaces and provides a high-level interface to storage devices and is accessible via dbus to applications.
* upower (formerly known as DeviceKit-power) is a daemon that sits on top of libudev and other kernel interfaces and provides a high-level interface to power management and is accessible via dbus to applications.
* [[NetworkManager]] is a daemon that sits on top of libudev and other kernel interfaces (and a couple of other daemons) and provides a high-level interface to network configuration and setup and is accessible via dbus to apps.<ref>{{cite web |url=http://lists.freedesktop.org/archives/dbus/2010-April/012545.html |title=Relationship between udev, hal, Dbus and DeviceKit? |date=2010-04-25 |author=Lennart Poettering}}</ref>

udev receives messages from the kernel, and passes them on to subsystem daemons such as Network Manager. Applications talk to Network Manager over D-Bus.

:<chem>Kernel -> udev -> Network Manager <=> D-Bus <=> Firefox</chem>

HAL is obsolete and only used by legacy code. Ubuntu 10.04 shipped without HAL. Initially a new daemon DeviceKit was planned to replace certain aspects of HAL, but in March 2009, DeviceKit was deprecated in favor of adding the same code to udev as a package: udev-extras, and some functions have now moved to udev proper.