udevadm Command Linux: Complete Guide to Device Management and Udev Administration

The udevadm command is a powerful administrative tool in Linux systems that manages the udev device manager. It serves as the primary interface for controlling udev daemon behavior, monitoring hardware events, and managing device rules. Whether you’re a system administrator troubleshooting hardware issues or a developer working with device management, understanding udevadm is essential for effective Linux system administration.

What is udevadm?

udevadm is a command-line utility that provides administrative control over the udev device manager in Linux systems. The udev (userspace device) system dynamically manages device nodes in the /dev directory, handles device events, and applies rules for device configuration. The udevadm tool allows you to:

• Monitor real-time device events
• Query device information and properties
• Test and validate udev rules
• Trigger device events manually
• Control udev daemon behavior
• Settle pending device events

Basic Syntax and Structure

The general syntax for udevadm follows this pattern:

udevadm [command] [options] [arguments]

The main commands available in udevadm include:

• info – Query device information
• monitor – Monitor device events
• control – Control udev daemon
• settle – Wait for pending events
• trigger – Trigger device events
• test – Test udev rules
• test-builtin – Test built-in commands

Essential udevadm Commands with Examples

1. Monitoring Device Events (udevadm monitor)

The monitor command is invaluable for real-time hardware troubleshooting and understanding device behavior:

udevadm monitor

Example Output:

monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[1692963234.123456] add      /devices/pci0000:00/0000:00:14.0/usb2/2-1 (usb)
KERNEL[1692963234.124567] add      /devices/pci0000:00/0000:00:14.0/usb2/2-1/2-1:1.0 (usb)
UDEV  [1692963234.156789] add      /devices/pci0000:00/0000:00:14.0/usb2/2-1 (usb)
UDEV  [1692963234.167890] add      /devices/pci0000:00/0000:00:14.0/usb2/2-1/2-1:1.0 (usb)

To monitor only kernel events:

udevadm monitor --kernel

To monitor only udev events:

udevadm monitor --udev

2. Querying Device Information (udevadm info)

The info command provides detailed information about devices:

udevadm info --name=/dev/sda

Example Output:

P: /devices/pci0000:00/0000:00:17.0/ata1/host0/target0:0:0/0:0:0:0/block/sda
N: sda
S: disk/by-id/ata-ST1000LM035-1RK172_WL1234567890
S: disk/by-id/wwn-0x5000c500a1b2c3d4
S: disk/by-path/pci-0000:00:17.0-ata-1
E: DEVLINKS=/dev/disk/by-id/ata-ST1000LM035-1RK172_WL1234567890 /dev/disk/by-id/wwn-0x5000c500a1b2c3d4 /dev/disk/by-path/pci-0000:00:17.0-ata-1
E: DEVNAME=/dev/sda
E: DEVPATH=/devices/pci0000:00/0000:00:17.0/ata1/host0/target0:0:0/0:0:0:0/block/sda
E: DEVTYPE=disk
E: ID_ATA=1
E: ID_BUS=ata
E: ID_MODEL=ST1000LM035-1RK172
E: ID_SERIAL=ST1000LM035-1RK172_WL1234567890
E: MAJOR=8
E: MINOR=0
E: SUBSYSTEM=block

To query by device path:

udevadm info --path=/sys/class/net/eth0

To get only device attributes:

udevadm info --attribute-walk --name=/dev/sda

3. Triggering Device Events (udevadm trigger)

The trigger command manually triggers udev events, useful for reloading device rules:

# Trigger events for all devices
udevadm trigger

# Trigger events for specific subsystem
udevadm trigger --subsystem-match=block

# Trigger events with specific action
udevadm trigger --action=add

Example for USB devices:

udevadm trigger --subsystem-match=usb --action=add

4. Settling Pending Events (udevadm settle)

The settle command waits for all pending udev events to complete:

# Wait for all events to settle
udevadm settle

# Wait with timeout (in seconds)
udevadm settle --timeout=30

# Wait for specific number of events
udevadm settle --seq-start=100 --seq-end=200

5. Testing Udev Rules (udevadm test)

The test command helps debug and validate udev rules:

udevadm test /sys/class/net/eth0

Example Output:

calling: test
version 245
This program is for debugging only, it does not run any program
specified by a RUN key. It may show incorrect results, because
some values may be different, or not available at a simulation run.

Load module index
timestamp of '/etc/udev/rules.d' changed
Reading rules file: /lib/udev/rules.d/50-udev-default.rules
Reading rules file: /lib/udev/rules.d/60-persistent-storage.rules
Reading rules file: /lib/udev/rules.d/70-mouse.rules
...
ACTION=add
DEVPATH=/sys/class/net/eth0
SUBSYSTEM=net
...
run: 'sh -c 'test -e /proc/sys/kernel/hotplug && echo /sbin/udevd > /proc/sys/kernel/hotplug''

6. Controlling Udev Daemon (udevadm control)

The control command manages udev daemon behavior:

# Reload udev rules
udevadm control --reload

# Set log level
udevadm control --log-priority=debug

# Stop event execution
udevadm control --stop-exec-queue

# Start event execution
udevadm control --start-exec-queue

# Get daemon status
udevadm control --ping

Advanced udevadm Usage Scenarios

Hardware Troubleshooting

When troubleshooting hardware issues, combine multiple udevadm commands:

# Monitor events while plugging USB device
udevadm monitor --property &

# In another terminal, get device info
udevadm info --name=/dev/sdb --property

# Test rules for the device
udevadm test $(udevadm info --name=/dev/sdb --query=path)

Network Interface Management

For network interface management and troubleshooting:

# Get network interface information
udevadm info /sys/class/net/eth0

# Monitor network device events
udevadm monitor --subsystem-match=net

# Trigger network interface rules
udevadm trigger --subsystem-match=net --action=add

Storage Device Management

Managing storage devices with udevadm:

# List all block devices with properties
for device in /sys/class/block/*; do
    echo "Device: $(basename $device)"
    udevadm info --path="$device" --property | grep -E "(DEVNAME|ID_MODEL|ID_SERIAL)"
    echo "---"
done

Working with Udev Rules

udevadm helps create and test custom udev rules effectively:

Creating Custom Rules

First, identify device attributes:

udevadm info --attribute-walk --name=/dev/sdb

Create a rule file in /etc/udev/rules.d/:

# /etc/udev/rules.d/99-custom-usb.rules
SUBSYSTEM=="block", ATTRS{idVendor}=="1234", ATTRS{idProduct}=="5678", SYMLINK+="my-usb-drive"

Test the rule:

udevadm test /sys/block/sdb

Reload rules and trigger:

udevadm control --reload-rules
udevadm trigger --name-match=sdb

Debugging Rules

Enable debug logging for rule testing:

udevadm control --log-priority=debug
udevadm test --action=add /sys/block/sdb

Performance Optimization with udevadm

Event Processing Optimization

Monitor event processing performance:

# Check queue status
udevadm control --ping

# Monitor with timing information
udevadm monitor --property --timestamp

Batch Operations

For system initialization or bulk device management:

# Trigger all devices and wait for completion
udevadm trigger && udevadm settle --timeout=60

Common Use Cases and Solutions

USB Device Recognition Issues

# Monitor USB events
udevadm monitor --subsystem-match=usb

# Get USB device details
udevadm info --name=/dev/bus/usb/001/002 --attribute-walk

# Force USB device reinitialization
udevadm trigger --subsystem-match=usb --attr-match=idVendor=1234

Network Interface Naming

# Check current network interface rules
udevadm info /sys/class/net/eth0 | grep ID_NET_NAME

# Monitor network naming events
udevadm monitor --property | grep -E "(INTERFACE|ID_NET_NAME)"

Disk Mount Issues

# Check disk device properties
udevadm info --name=/dev/sdc --property | grep -E "(ID_FS|DEVLINKS)"

# Test disk-related rules
udevadm test /sys/block/sdc

Best Practices and Tips

Monitoring Best Practices

• Use --property flag with monitor for detailed event information
• Filter events by subsystem to reduce noise
• Always use settle after triggering events in scripts
• Set appropriate timeouts to prevent hanging

Rule Testing Guidelines

• Always test rules with udevadm test before applying
• Use verbose output for debugging complex rules
• Test both add and remove actions
• Validate symlinks and permissions after rule application

System Integration

# Create a monitoring script
#!/bin/bash
# monitor-hardware.sh

echo "Starting hardware monitoring..."
udevadm monitor --property | while read line; do
    if [[ $line == *"ACTION=add"* ]]; then
        echo "$(date): New device added - $line" >> /var/log/hardware-events.log
    fi
done

Troubleshooting Common Issues

Permission Problems

Most udevadm commands require root privileges. Use sudo when needed:

sudo udevadm control --reload-rules
sudo udevadm trigger

Service Dependencies

Ensure udev service is running:

systemctl status systemd-udevd
systemctl restart systemd-udevd

Rule Syntax Errors

Check rule syntax with test command:

udevadm test --action=add /sys/block/sda 2>&1 | grep -i error

Conclusion

The udevadm command is an indispensable tool for Linux system administrators and developers working with hardware device management. Its comprehensive feature set enables effective monitoring, troubleshooting, and configuration of device behavior through the udev system.

Key takeaways for mastering udevadm:

• Use monitor for real-time hardware event tracking
• Leverage info for detailed device property investigation
• Apply test for rule validation and debugging
• Utilize trigger and settle for controlled device management
• Combine commands for comprehensive hardware troubleshooting workflows

Whether you’re managing servers, workstations, or embedded systems, understanding udevadm will significantly enhance your ability to handle Linux device management challenges effectively. Practice with these examples and gradually incorporate udevadm into your system administration toolkit for robust hardware management capabilities.