How to run long time process on Udev event

Udev is a device manager used by modern Linux systems to detect hardware changes and manage device nodes in the /dev directory. It provides a flexible way to run scripts or programs in response to device events, such as automounting drives, launching backup scripts, or running long-running processes. This article explains how to handle long-time processes triggered by Udev events.

Understanding Udev Rules

Udev rules are configuration files that specify how Udev should respond to device events. Each rule consists of match conditions that identify devices, actions to take when conditions are met, and optional attributes for fine-tuning behavior.

A basic Udev rule structure follows this pattern

# Match conditions, Action to take
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sda", RUN+="/usr/local/bin/myscript.sh"

This rule triggers when a block device named sda is added to the system, executing the specified script. The RUN directive runs the command asynchronously.

Running Long-Time Processes on Udev Events

When launching long-running processes from Udev events, you must ensure they don't block the Udev event processing. Here are three approaches

Background Process Execution

Use the & operator to run processes in the background

# /etc/udev/rules.d/11-longprocess.rules
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd*", RUN+="/usr/local/bin/longprocess.sh &"

This immediately returns control to Udev while the process continues running in the background.

Process with Wait

If you need Udev to wait for the process to complete

# /etc/udev/rules.d/12-waitprocess.rules  
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd*", RUN+="/usr/local/bin/process.sh & wait"

The wait command ensures the Udev event doesn't complete until the background process finishes.

Using Systemd for Process Management

The recommended approach for long-running processes is to use systemd services, which provide better process management, logging, and restart capabilities.

Create a Systemd Service

# /etc/systemd/system/device-handler.service
[Unit]
Description=Device Handler Long Process
After=multi-user.target

[Service]
Type=simple
ExecStart=/usr/local/bin/device-handler.sh
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

Trigger Service from Udev Rule

# /etc/udev/rules.d/13-systemd-trigger.rules
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd*", RUN+="/bin/systemctl start device-handler.service"

This approach provides automatic restarts on failure, proper logging through journald, and standardized service management.

Best Practices and Testing

Use udevadm to inspect device attributes and test rules

# View device attributes
udevadm info -a -p /sys/class/block/sda

# Test a rule without triggering actual events  
udevadm test /sys/class/block/sda

# Monitor Udev events in real-time
udevadm monitor

Always include descriptive comments in your rules for maintainability

# Launch backup process when external USB drive is connected
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd*", ATTRS{removable}=="1", RUN+="/usr/local/bin/backup.sh &"

Common Use Cases

Conclusion

Running long-time processes on Udev events requires careful consideration to avoid blocking device management. Use background execution for simple cases and systemd services for robust, manageable long-running processes. Proper testing with udevadm ensures reliable automation of hardware-triggered tasks in Linux systems.