udevd Command in Linux
Historically, Linux administrators interacted with hardware using static device nodes in the /dev directory. In early Unix-like systems, these device files were created manually or with very simple scripts. As systems grew more complex and hardware became hot-pluggable for example, with the advent of USB the need arose for a dynamic system that could react to hardware events in real time.
udev was developed as the new mechanism to achieve this. It listens for kernel-generated events (known as "uevents") on a netlink socket, and based on customizable rules, it creates or removes device nodes, assigns permissions, creates persistent symlinks, and even triggers custom scripts. At the very heart of this process is the udevd daemon.
Table of Contents
Here is a comprehensive guide to the options available with the udevd command −
- Understanding the udevd Command
- How to Use udevd Command in Linux
- Common Options Used with udevd Command
- Configuration Files and Environment Variables
Understanding the udevd Command
Prior to udev, systems often used tools such as devfs or hotplug. These earlier mechanisms were less flexible and did not offer the complete dynamic handling that modern hardware configurations require. With udev, Linux moved to a system where devices are treated as events rather than static files. This is the conceptual leap that gives udevd its importance.
How to Use udevd Command in Linux?
Unlike many other tools, udevd is not typically used interactively by end users during normal operation. It generally runs as a background daemon and is managed through the system's init system (e.g., systemd). However, udevd does provide several command-line options that are useful for debugging, testing, or custom system setups.
Given below is the general syntax for the udevd command −
udevd [OPTIONS]
In many modern systems, particularly those based on systemd, you may see it referenced as systemd-udevd. When exploring its behavior, the following options are commonly available −
Common Options Used with udevd Command
Highlighted below are some of the common options used with udevd command along with a brief description –
- --help : Prints a short help text describing usage and available options.
udevd --help
This will output a summary of the command options.
--version : Displays version information for the udevd daemon. This can help determine which version of udev is running on your system.
udevd --version
--nofork : This option tells udevd not to fork into the background and to run in the foreground instead. This is especially useful for debugging because it allows you to see all output (including debugging messages) directly on the terminal.
sudo udevd --nofork
Running udevd this way will prevent it from detaching from the console, allowing you to observe its real-time log output.
--debug : Enables debug logging. When combined with --nofork, you can see verbose debugging information on standard output. This option can greatly help in diagnosing problems related to device event processing.
sudo udevd --nofork --debug
This command will run udevd in the foreground with extensive debug information.
--log-priority=<level> or similar variants : Some implementations support specifying a log priority level. With this option, you can control the granularity of log messages (e.g., info, warning, debug). The exact name of the option may vary with the version of udevd.
sudo udevd --log-priority=debug
It tells udevd to log at the debug level, providing more detailed output.
It is important to note that while the above options are common, the exact list of command-line parameters depends on the version and the specific Linux distribution. In systems using systemd, udevd is usually invoked indirectly via the systemd-udevd service unit, and many of these options are configured in the unit file or through environment files rather than via the command line at runtime.
For example, many people rarely run −
sudo systemd-udevd --nofork --debug
It directly from the command line because the service is managed by systemd. Instead, you might temporarily override the defaults for debugging purposes.
Configuration Files and Environment Variables
While command-line options give you direct control over the behavior of udevd at startup, much of its configuration is also driven by configuration files. These files allow system administrators to fine-tune how udevd processes events and applies rules.
udev.conf
Historically, udevd read configuration values from a configuration file (typically found at /etc/udev/udev.conf). Although more modern systems often integrate this configuration into systemd or other mechanisms, many of the same parameters persist −
- udev_log : In older versions, this parameter controls the log level. For example, setting
udev_log="debug"
In the configuration file would result in more extensive logging output. In newer environments, you might control logging via udevd command-line options or through journal configuration.
udev Rules
One of the primary functions of udevd is to examine device events and apply rules. Udev rules are written in files ending with a .rules suffix. These files are usually located in −
A simple rule might look like this −
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="1234", ATTR{idProduct}=="abcd",
RUN+="/usr/local/bin/usb_notify.sh"
Running udevd: Practical Examples
Because udevd typically runs as a background daemon, most system administrators interact with it indirectly via systemctl (in systemd systems) or through its log output. Nevertheless, there are scenarios where running udevd manually with specific options is very useful.
Running udevd in the Foreground for Debugging
If you suspect that udev rules are not being processed correctly or that a hardware event is failing to trigger the expected action, you may wish to run udevd in the foreground with debugging enabled. For example −
sudo udevd --nofork --debug
Explanation −
- The --nofork option forces udevd to remain in the foreground instead of forking and running as a daemon.
- The --debug option increases logging verbosity, so you can see detailed information about each device event as it is processed.
Starting udevd Normally with Systemd
On most modern Linux systems, udevd is integrated into systemd and managed as a service. In such systems, you would start, stop, or restart udevd using systemd commands. For example −
sudo systemctl restart systemd-udevd.service
Explanation −
- This command tells systemd to restart the udev daemon.
- Restarting the service is often necessary after modifying udev rule files or the corresponding configuration settings.
You can then check the serviceâs log output with −
journalctl -u systemd-udevd.service
This log will show you how udevd is processing events in real time and will include any debug output if youâve configured the logging level appropriately.
Conclusion
The udevd command (or systemd-udevd in many modern Linux distributions) is a critical component of Linuxâs dynamic device management system. Its responsibilities include listening for hardware events from the kernel, processing these events through a set of flexible rules, and executing the necessary actions to ensure that devices are brought online with the correct permissions, naming, and configuration.