Does anyone know what the output from getevent means?

Does Anyone Know What the Output from `getevent` Means?

The `getevent` command is a powerful tool for debugging input devices on Linux systems. It provides a raw stream of events that occur when you interact with your keyboard, mouse, touchscreen, or other input devices. However, the output of `getevent` can be quite cryptic, leaving many users wondering what exactly those numbers and codes mean.

Understanding the `getevent` Output

The output from `getevent` is a series of lines, each representing a single input event. Each line consists of several fields, separated by spaces:

Field Breakdown

• Timestamp: The time the event occurred, expressed in seconds since the epoch.
• Type: The type of event, typically 0, 1, 2, or 3.
  • 0: Synonymous event, typically used for synchronization purposes.
  • 1: Key press or release.
  • 2: Absolute axis event (e.g., touchscreen coordinates).
  • 3: Relative axis event (e.g., mouse movement).
• Code: The specific event code. This code varies depending on the type of event and the device.
• Value: The value associated with the event. This value also varies depending on the event type and code.

Decoding Event Codes

The real challenge in understanding `getevent` output lies in deciphering the event codes. These codes are defined in the Linux kernel and are not necessarily intuitive.

Event Code References

There are a few resources that can help you decode these event codes:

• Linux Input Device documentation: The official Linux kernel documentation provides a comprehensive list of event codes, but it can be overwhelming for beginners.
• `evdev` library documentation: The `evdev` library provides an interface for interacting with input devices. Its documentation includes a section on event codes, with more user-friendly descriptions.
• Online event code databases: There are several online databases, such as [https://www.kernel.org/doc/html/latest/input/input.html](https://www.kernel.org/doc/html/latest/input/input.html), that list event codes along with their meanings.

Example: Mouse Events

Let’s look at an example of `getevent` output for a mouse:

1625306841.358965 0 0 0
1625306841.362008 1 272 1
1625306841.362017 1 272 0
1625306841.363493 0 0 0
1625306841.363561 1 330 1
1625306841.363577 1 330 0
1625306841.365021 0 0 0
1625306841.365094 3 2 1
1625306841.365110 3 5 1
1625306841.365113 3 1 3
1625306841.365130 3 1 1
1625306841.365154 0 0 0
1625306841.367275 3 1 20
1625306841.367281 3 5 12
1625306841.367285 3 2 3
1625306841.367303 3 2 1
1625306841.367321 0 0 0

Decoding the Output

Here’s a breakdown of the events in this example:

Tools for Analyzing `getevent` Output

While deciphering the output manually can be a daunting task, there are several tools available to help you understand `getevent` output:

• `evtest` command: This command provides a more user-friendly interface for interacting with input devices. It can display events in a more readable format and also allows you to test different devices and events.
• `xev` command: This command is primarily used for debugging X11 applications. It can also capture and display input events, making it a useful tool for analyzing `getevent` output.
• GUI tools: There are various GUI tools available, such as “Input Devices” (found under Settings > Mouse & Touchpad on many Linux distributions), that can display and analyze input device events.

Conclusion

While the output from `getevent` can be challenging to interpret at first glance, understanding its format and event codes empowers you to troubleshoot input device issues and gain deeper insights into how your Linux system interacts with the physical world. By leveraging available tools and resources, you can confidently decipher the language of `getevent` and effectively analyze your input device events.