udevtest Command in Linux

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udevtest is a command-line utility in Linux used for testing udev rules and debugging device event handling. Udev is the device manager for the Linux kernel, responsible for managing device nodes in /dev and handling dynamic device events.

udevtest simulates how udev processes a device event, allowing administrators to verify rules before applying them to a live system. This helps prevent misconfigurations that could lead to device recognition failures.

Table of Contents

Here is a comprehensive guide to the options available with the udevtest command −

• Understanding udevtest Command
• Syntax of udevtest Command
• udevtest Command Options
• How to Use udevtest Command in Linux?
• Examples of udevtest Command in Linux
• Understanding udevtest Command Output
• Troubleshooting with udevtest Command
• Differences between udevtest and udevadm test
• Best Practices while Using udevtest Command

Understanding udevtest Command

The udevtest command in Linux was historically used as a debugging and testing utility for the udev device manager. Its primary purpose was to simulate the execution of udev rules without actually applying them to the system. This feature allowed system administrators and developers to assess whether a given set of udev rules would correctly handle device events before deploying those rules in a live environment.

When a device is connected to the system, udev processes rules defined in /etc/udev/rules.d/, determining how the device should be named, symlinked, or assigned permissions. Using udevtest, administrators could test these rules in isolation, ensuring they behaved as expected without directly altering device configurations.

Purpose of udevtest

• Test udev rules without affecting the running system.
• Debug device recognition issues by simulating events.
• Verify environment variables and attributes assigned to a device.
• Check if custom rules (in /etc/udev/rules.d/) are correctly applied.

Syntax of udevtest Command

The basic syntax of udevtest is −

udevtest [options] <devpath>

Where −

• <devpath> is the device path in /sys (e.g., /sys/block/sda).
• [options] modify the behavior of the command.

udevtest Command Options

The following table highlights the options frequently used with udevtest along with their brief descriptions –

How to Use udevtest Command in Linux?

The way udevtest operated was relatively straightforward. When invoked with a specific device path under /sys, it would trigger udev processing for that device while logging the rule evaluation process. This allowed administrators to observe how rules matched against device attributes such as vendor ID, product ID, or bus type.

By examining the debug output, users could fine-tune udev rules, avoiding misconfigurations that might lead to devices failing to initialize properly or being assigned incorrect parameters. This aspect of udevtest was particularly useful when working with complex rule sets that involved multiple conditions and assignment operations. Instead of applying a rule blindly and hoping for the desired outcome, administrators could proactively debug and refine it.

Examples of udevtest Command in Linux

The following set of examples demonstrate the usage of udevtest command –

Basic udevtest Execution

Simulate how udev processes a device event for /dev/sda −

udevtest /sys/block/sda

Output Explanation −

• Shows how rules are matched.
• Displays actions taken (e.g., symlink creation, permissions).

Debug Mode

Enable debug output for detailed analysis −

udevtest --debug /sys/block/sda

Output Explanation −

• Includes internal udev processing steps.
• Useful for diagnosing rule-matching issues.

Print Environment Variables

Check environment variables set during rule processing −

udevtest --env /sys/block/sda

Output Explanation −

• Lists variables like ID_MODEL, ID_SERIAL.
• Helps verify if rules correctly use these variables.

Attribute Walk

Print all attributes of a device and its parents −

udevtest --attribute-walk /sys/block/sda

Output Explanation −

• Useful for writing rules based on device hierarchy.
• Shows attributes like size, vendor, model.

Simulate a Custom Device Node Name

Test how a rule behaves for a specific /dev name −

udevtest --name=sda1 /sys/block/sda/sda1

Output Explanation −

• Simulates the device node /dev/sda1.
• Verifies if naming rules work as expected.

Understanding udevtest Command Output

A typical udevtest output includes −

• Rule matches − Which rules were applied.
• Actions taken − Symlinks, permissions, or commands executed.
• Environment variables − Used in rule evaluation.
• Errors − If a rule fails to apply.

Example Output Breakdown −

udevtest /sys/block/sda

• parse_file − reading '/etc/udev/rules.d/10-local.rules' as rules file
• udev_rules_apply_to_event − RUN 'usb_id --export %p' /etc/udev/rules.d/60-persistent-storage.rules:5
• udev_device_new_from_syspath − device 0x55a1b2e3b2e0 has devpath '/block/sda'
• udev_node_add − creating device node '/dev/sda', major = '8', minor = '0', mode = '0660', uid = '0', gid = '6'
• udev_node_add − creating symlink '/dev/disk/by-id/ata-ST1000DM003-1CH162_W123456' to '/dev/sda'

Troubleshooting with udevtest Command

Issue − Rule Not Applied

• Check syntax − Ensure rules are correctly formatted.
• Verify path − Confirm the /sys path exists.
• Debug mode − Use --debug to see why a rule is skipped.

Issue − Incorrect Permissions

• Check GROUP/MODE in rules − Ensure they are correctly set.
• Test with --env − Verify environment variables.

Issue − Missing Symlinks

• Check SYMLINK rules − Ensure they are properly defined.
• Use --attribute-walk − Confirm device attributes match the rule.

Differences between udevtest and udevadm test

udevtest is older and deprecated in favor of udevadm test. udevadm test provides more features and better integration.

In later versions of Linux, udevtest was deprecated and replaced with more modern alternatives such as udevadm test. The newer command follows the same principles but provides additional flexibility and improved diagnostics. udevadm test enhances debugging capabilities by enabling targeted evaluations of specific devices or configurations while offering more detailed logging output.

This shift was part of a broader effort to consolidate device management utilities under the udevadm framework, ensuring a more unified and standardized approach to handling device interactions within Linux. Despite its deprecation, knowledge of udevtest remains valuable for those managing legacy Linux distributions or exploring the historical evolution of udev-based device management.

For modern systems, prefer udevadm test, but understanding udevtest helps in legacy environments. Always test rules thoroughly before deploying them in production.

Example of udevadm test equivalent −

udevadm test /sys/block/sda

Best Practices while Using udevtest Command

Given below is a set of best practices that you should follow while using udevtest command –

• Test rules before applying: Always use udevtest or udevadm test before reloading udev.
• Use --debug for detailed logs: Helps diagnose complex issues.
• Check dmesg for kernel events: Useful if udev doesn't detect a device.
• Backup rules: Before modifying, keep a backup of /etc/udev/rules.d/.

Conclusion

udevtest is a powerful tool for debugging udev rules and simulating device events. Although largely replaced by udevadm test, it remains useful in older systems. By mastering udevtest, administrators can ensure proper device management and avoid misconfigurations.