The udevadm command is a powerful utility in Linux for managing device events and rules. It allows administrators to interact with the udev system, which is responsible for dynamically managing device nodes in the /dev directory. Understanding and utilizing udevadm is crucial for system administrators who need to configure how the kernel handles device additions and removals, ensuring devices are correctly recognized and configured upon connection.

Key Udevadm Commands and Usage

Here are some essential udevadm commands for common device management tasks:

Reloading Udev Rules and Triggering Events

When you make changes to udev rules, you need to inform the system to re-read them. The following command reloads the rules and then triggers events for all devices, ensuring your new configurations are applied.

# Reload udev rules without reboot
udevadm control --reload-rules && udevadm trigger

Getting Device Sysfs Path

The sysfs filesystem provides a hierarchical view of devices and kernel objects. This command helps you find the sysfs path for a specific device node, which is often needed for more detailed inspection.

# Get sysfs path of a device
udevadm info -q path -n /dev/sda

Listing Device Attributes

To understand a device's properties and how udev sees it, you can list its attributes. This command retrieves the sysfs path first and then displays all associated attributes.

# Get a list of attributes for a specific device
udevadm info -a -p  $(udevadm info -q path -n /dev/sdb)

Monitoring Udev Events

The monitor command is invaluable for debugging. It allows you to watch events as they happen in real-time, showing the actions taken by the kernel and udev for each event. This is particularly useful when troubleshooting device recognition issues.

# Get an entry for each action taken by KERNEL and UDEV
udevadm monitor --property

Further Resources

• systemd-logind Documentation - Related to device management and user sessions.
• udevadm man page - Official manual page for udevadm.
• Kernel Sysfs Rules - Understanding how sysfs interacts with udev.