systemd-udevd.service

The systemd-udevd.service unit primarily manages the systemd-udevd daemon. While users rarely invoke the daemon directly, as it's typically started and maintained by systemd, its underlying command invocation pattern is:

systemd-udevd [OPTIONS...]

However, it is predominantly managed as a systemd service, for example:
systemctl [COMMAND] systemd-udevd.service

--debug, -d
    Enables debug logging. This increases the verbosity of the daemon's output, which is useful for troubleshooting udev rules or device recognition issues.

--help, -h
    Displays a short help text and exits. Provides a brief overview of the available command-line options for the daemon.

--version, -V
    Prints version information and exits. Shows the installed version of the systemd-udevd daemon.

systemd-udevd.service is the systemd unit for the systemd-udevd daemon, which serves as the device manager for the Linux kernel. Its primary role is to manage device nodes in the /dev directory, ensuring they are created and removed dynamically as hardware is added or removed from the system.

It receives events directly from the kernel (via Netlink) when devices are discovered, connected, or disconnected. Upon receiving an event, systemd-udevd applies a set of rules (typically located in /lib/udev/rules.d/ and /etc/udev/rules.d/) to identify the device, assign appropriate permissions and ownership, create symbolic links, and trigger further actions such as loading kernel modules or executing scripts. This dynamic device management is crucial for modern Linux systems, enabling features like hot-plugging of USB devices, automatic network interface configuration, and consistent device naming across reboots. It superseded older systems like devfs and hotplug.

systemd-udevd is a fundamental component of a Linux system. Stopping or restarting this service directly without careful consideration can lead to severe system instability, including issues with device recognition, network interfaces, and file system mounting, as it dynamically manages the /dev directory. Changes to udev rules often require reloading them via udevadm control --reload-rules or triggering device events, rather than restarting the service itself.

Udev's behavior is controlled by rules files, typically located in /lib/udev/rules.d/ (for distribution-provided rules) and /etc/udev/rules.d/ (for local customizations). These rules are processed in lexical order (e.g., 10-local.rules before 99-custom.rules) and define how systemd-udevd should react to kernel device events, including setting device permissions, ownership, creating symlinks, and executing external programs.

systemd-udevd is responsible for populating the /dev directory, which is a virtual filesystem (devtmpfs) mounted at boot. It ensures that only necessary device nodes exist and are dynamically updated as hardware changes, preventing the need for a static, pre-populated /dev directory that was common in older Linux systems.

The concept of dynamic device management in Linux evolved from simpler systems like devfs and hotplug. devfs was deprecated due to issues with inconsistent device naming and permissions, while hotplug provided event handling but lacked a centralized device database.

udev was introduced around 2003 as a successor, providing a more robust and flexible way to manage /dev and handle hot-plugging. Initially, it ran as a standalone daemon. With the advent of systemd, udev's functionality was integrated directly into the systemd ecosystem, resulting in the systemd-udevd daemon and the systemd-udevd.service unit. This integration streamlined device event processing and made it an integral part of the system's boot process and runtime device management.

udevadm(8), udev(7), systemctl(1), systemd(1), systemd.service(5), systemd.special(7)