How to Manage Devices with 'udevadm' in Linux (with examples)

The udevadm command in Linux is a powerful tool used for managing device nodes in the /dev directory. It is part of the Device Manager for the Linux kernel, known as udev. This tool helps you monitor, query, and manipulate the device events and the rules applied to them. Understanding how to use udevadm can aid system administrators in managing peripherals and other hardware on Linux systems efficiently.

Monitor all device events

Code:

sudo udevadm monitor

Motivation: Monitoring device events in real-time is crucial for diagnosing hardware issues or understanding exactly when devices are added or removed from the system. This command is particularly useful when you want to troubleshoot problems related to USB devices, storage, or any hardware connection.

Explanation:

• sudo: This command requires elevated permissions because it accesses sensitive parts of the system.
• udevadm: This is the command-line utility for interacting with udev.
• monitor: This option enables the monitoring of device events detected by the system.

Example output:

KERNEL[3193.559704] add      /devices/pci0000:00/0000:00:1a.0/usb1/1-1 (usb)
UDEV  [3193.563051] add      /devices/pci0000:00/0000:00:1a.0/usb1/1-1 (usb)

Print uevents sent out by the kernel

Code:

sudo udevadm monitor --kernel

Motivation: By focusing specifically on kernel events, system administrators can gain insights into the raw events being emitted directly by the kernel. This is useful when debugging issues at the kernel level or understanding the initial event stages before udev processes them.

Explanation:

• sudo: Executes the command with administrative privileges.
• udevadm: The device management command.
• monitor: Starts monitoring device events.
• --kernel: Ensures only kernel-emitted events are printed.

Example output:

KERNEL[3201.948113] add      /devices/pci0000:00/0000:00:1a.0/usb1/1-2 (usb)

Print device events after being processed by udev

Code:

sudo udevadm monitor --udev

Motivation: Understanding how udev processes kernel events and applies rules to them is necessary for verifying correct rule application and event processing. This command is particularly useful if you have customized udev rules and need to test if they are being executed as expected.

Explanation:

• sudo: Grants superuser permissions.
• udevadm: Calls the device management tool.
• monitor: Initiates monitoring of events.
• --udev: Displays only events processed through udev.

Example output:

UDEV  [3353.964821] add      /devices/pci0000:00/0000:00:1a.0/usb1/1-3 (usb)

List attributes of device /dev/sda

Code:

sudo udevadm info --attribute-walk /dev/sda

Motivation: Listing device attributes provides comprehensive details about the hardware and its configuration. This is valuable for system administrators who need to verify device parameters or when creating custom udev rules.

Explanation:

• sudo: Necessary to access deep system-level information.
• udevadm: Command to interact with udev.
• info: Displays detailed information about devices.
• --attribute-walk: Recursively lists all device attributes through the device hierarchy.
• /dev/sda: Specifies the particular device to query, in this case, the first hard drive.

Example output:

  looking at parent device '/devices/...':
    KERNELS=="pci0000:00"
    SUBSYSTEMS=="pci"
    DRIVERS==""
    ATTRS{irq}=="10"
    ATTRS{local_cpus}=="00000000"

Reload all udev rules

Code:

sudo udevadm control --reload-rules

Motivation: After updating or creating new udev rules, it’s essential to reload them for the changes to take effect. This command allows changes to be implemented without needing a system restart, which is crucial in environments where uptime is critical.

Explanation:

• sudo: Required to modify system configurations.
• udevadm: Executes the device manager command.
• control: Modifies udev control configurations.
• --reload-rules: This option tells udev to reload the rules files and apply them.

Example output:

There is no direct output as this command modifies system settings silently unless an error occurs.

Trigger all udev rules to run

Code:

sudo udevadm trigger

Motivation: After reloading rules, administrators can force udev to re-evaluate all devices against the new rule set instantly. This is useful for applying changes across all devices immediately without rebooting.

Explanation:

• sudo: Used to perform system-sensitive actions.
• udevadm: Command for device management.
• trigger: Executes all udev rules against devices, triggering them as if they have just been added.

Example output:

No immediate output; however, device changes will take effect and associated actions may start executing per the updated rules.

Test an event run by simulating loading of /dev/sda

Code:

sudo udevadm test /dev/sda

Motivation: Testing a device event without physically modifying the device setup is crucial for validating udev rules safely. This simulated approach is beneficial for ensuring rules work correctly and won’t misfire in a live environment.

Explanation:

• sudo: Grants necessary permissions for testing system changes.
• udevadm: The device interaction command.
• test: Simulates running udev for a device to verify rule correctness.
• /dev/sda: The target device for the test, typically a hard disk drive.

Example output:

Starting 'test' on '/dev/sda'

Conclusion:

The udevadm command is an essential utility for managing devices on Linux systems. By understanding its various use cases, system administrators can effectively monitor, configure, and troubleshoot hardware-related issues. Each use case provides a specific insight or control mechanism over how Linux handles device events, offering both diagnostic and operational advantages.