How to run long time process on Udev event


Introduction

Udev is a device manager used by modern Linux systems. It detects hardware changes and manages device nodes in /dev directory. Udev provides a flexible way to run scripts or programs in response to device events. This feature can be used to perform various tasks like automounting, launching backup scripts, and running long-running processes. In this article, we will discuss how to run long time processes on Udev events and provide examples to illustrate concepts.

Understanding Udev Rules

Udev rules are files that specify how Udev should handle device events. A typical rule consists of four parts: a match, an action, a condition, and an optional comment. match identifies device or devices that rule should apply to. action specifies command or script to run when match is found. condition is an optional test that must be true for rule to match. comment is a description of rule.

Here is an example of a simple Udev rule −

# /etc/udev/rules.d/10-example.rules
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sda", RUN+="/usr/local/bin/myscript.sh"

This rule matches when a block device with kernel name "sda" is added and runs script "/usr/local/bin/myscript.sh". "RUN" keyword specifies that command should be run asynchronously.

Running a Long Time Process on Udev Event

To run a long time process on a Udev event, we need to modify Udev rule to launch process in background. This can be done using "&" character at end of command. This will run command in background and return control to shell immediately.

Here is an example of a Udev rule that launches a long-running process −

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

This rule matches when a block device with kernel name "sda" is added and runs script "/usr/local/bin/longtimeprocess.sh" in background.

In this example, we are assuming that script "/usr/local/bin/longtimeprocess.sh" will run for a long time. If script completes quickly, Udev event will be completed before script has finished running.

If we need to wait for long-running process to complete before completing Udev event, we can use "wait" command. "wait" command will wait for all background processes launched from current shell to complete.

Here is an example of a Udev rule that waits for a long-running process to complete −

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

This rule matches when a block device with kernel name "sda" is added and runs script "/usr/local/bin/waitlongtimeprocess.sh" in background. "wait" command waits for script to complete before completing Udev event.

Using Systemd to Manage Long Running Processes

Systemd is a system and service manager for Linux systems. It provides a way to manage long-running processes, including starting, stopping, and restarting them.

We can use systemd to manage long-running processes launched from Udev events. To do this, we need to create a systemd service unit file that describes long-running process. We can then modify Udev rule to start systemd service instead of launching process directly.

Here is an example of a systemd service unit file −

# /etc/systemd/system/longtimeprocess.service
[Unit]
Description=Long Time Process

[Service]
ExecStart=/usr/local/bin/longtimeprocess.sh
Restart=on-failure

[Install]
WantedBy=multi-user.target

This file describes a service called "longtimeprocess" that runs script "/usr/local/bin/longtimeprocess.sh". "Restart" option specifies that service should be restarted if it fails.

To start service from a Udev event, we can modify Udev rule to use "systemctl" command to start service −

# /etc/udev/rules.d/13-systemdlongtimeprocess.rules
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sda", RUN+="/bin/systemctl start longtimeprocess.service"

This rule matches when a block device with kernel name "sda" is added and starts "longtimeprocess.service" systemd service.

Using systemd to manage long-running processes provides many benefits. For example, systemd can automatically restart process if it fails or if system is restarted. It also provides a standardized way to manage and monitor long-running processes.

In addition to launching long-running processes, Udev rules can be used to perform a wide variety of tasks. For example, Udev rules can be used to automount USB drives, launch backup scripts when a new hard drive is detected, or update network configuration when a new network interface is added.

To create effective Udev rules, it is important to have a good understanding of devices and subsystems on your system. "udevadm" command can be used to query Udev database and inspect attributes of devices. For example, command "udevadm info -a -p /sys/class/block/sda" can be used to view attributes of "sda" block device.

When creating Udev rules, it is also important to test rules thoroughly to ensure that they work as expected. "udevadm" command can be used to simulate Udev events and test rules without actually plugging in a device. For example, command "udevadm test /devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda/sda1" can be used to test Udev rules that match "sda1" partition of first hard drive.

It is also important to use descriptive comments in Udev rules to make it easier to understand their purpose and function. This can be especially useful if multiple administrators are working on same system or if rules need to be updated in future.

Conclusion

In this article, we have discussed how to run long time processes on Udev events. We have provided examples of Udev rules that launch long-running processes and wait for them to complete. We have also discussed how to use systemd to manage long-running processes launched from Udev events.

When using Udev to launch long-running processes, it is important to ensure that process runs in background so that Udev event can complete quickly. It is also important to consider whether process should be managed by systemd to provide automatic restarts and standardized management.

Udev provides a flexible way to respond to hardware events and launch scripts or programs. By understanding Udev rules and using them effectively, we can automate many tasks and improve reliability and efficiency of our Linux systems.

Updated on: 03-Mar-2023

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