How to use the command 'xev' (with examples)

The xev command is a powerful utility for monitoring and displaying X events, which occur in the X Window System, a foundational component of many Unix-like operating systems’ graphical interfaces. By intercepting these events, developers and system administrators can gain valuable insight into how different actions within the graphical interface are being processed. This can be particularly useful for debugging purposes, configuring custom input methods, or gaining a deeper understanding of user interactions with the system.

Monitor all occurring X events

Code:

xev

Motivation for using the example:

This example is useful when you want to observe all X events that occur for a specific window that xev creates by default. By running this command, you can see everything from mouse movements to keyboard presses and releases. It is invaluable for understanding how these events are captured and processed by the X server, especially when working on developing or debugging X applications.

Explanation:

• xev: Invokes the xev command, which creates a new window to monitor and print all X events it receives. By running xev without any options, you are setting up a session to monitor a wide range of events in a newly created window.

Example output:

Upon running xev, you will see outputs like:

KeyPress event, serial 32, synthetic NO, window 0x4000001,
    root 0x85, subw 0x0, time 1086856421, (95,122), root:(210,146),
    state 0x0, keycode 38 (keysym 0x61, a), same_screen YES,
    ...

This output provides details about a key press event, such as the exact character, its keycode, and the time of the event.

Monitor all X events of the root window instead of creating a new one

Code:

xev -root

Motivation for using the example:

Monitoring the root window’s events is essential for system-level observation and debugging. The root window encompasses the entire screen or virtual desktop space, allowing you to capture a broad spectrum of events affecting the entire display. This is particularly useful when tracking down issues related to display management or when debugging input devices that interact with the entire screen.

Explanation:

• xev: Starts the event monitoring session.
• -root: Directs xev to monitor X events on the root window, rather than a new window. This ensures you capture system-wide events rather than those restricted to a single application window.

Example output:

By running xev -root, you might see outputs such as:

MotionNotify event, serial 32, synthetic NO, window 0x6e00001,
    root 0x85, subw 0x0, time 1086932422, (834,839), root:(235,223),
    state 0x0, is_hint 0, same_screen YES,
    ...

This output details a motion notify event that captures mouse movement over the root window.

Monitor all X events of a particular window

Code:

xev -id window_id

Motivation for using the example:

Tracking events for a specific window is vital when you need to debug or develop applications that use X Window System. By specifying a particular window, you can efficiently monitor events like key presses, mouse movements, and interaction-specific nuances directly related to that window.

Explanation:

• xev: Initiates the X event monitoring utility.
• -id window_id: Only the window with the ID specified by window_id will be monitored. This requires you to know the ID of the window of interest beforehand, which can be retrieved using utilities such as xwininfo.

Example output:

When monitoring a specific window using a command like xev -id 0x3200003, the resulting output might look like:

ButtonPress event, serial 32, synthetic NO, window 0x3200003,
    root 0x85, subw 0x0, time 1087021469, (181,218), root:(297,240),
    state 0x0, button 1, same_screen YES
    ...

This output captures a button press event within the specified window, detailing the coordinates and button number involved.

Monitor X events from a given category (can be specified multiple times)

Code:

xev -event event_category

Motivation for using the example:

Sometimes you only need to monitor specific types of events, such as key presses or window focus changes, without the overhead of capturing all X events. This focused monitoring is beneficial for performance and clarity when debugging specific interactions or optimizing applications relying on particular event types.

Explanation:

• xev: begins the process to listen for X events.
• -event event_category: Allows you to specify the category of events you are interested in. This can include categories like Keyboard, Mouse, Focus, etc. Multiple -event options can be combined to capture more than one category of interest.

Example output:

Using a command like xev -event Keyboard, you might receive an output similar to:

KeyRelease event, serial 32, synthetic NO, window 0x4600001,
    root 0x85, subw 0x0, time 1088324671, (50,80), root:(160,98),
    state 0x0, keycode 38 (keysym 0x61, a), same_screen YES
    ...

This output pertains to key releases within the specified event category, highlighting the key released and contextual details.

Conclusion

By using xev, you can effectively monitor and debug the various X events occurring within your X Window System environment. Whether you’re interested in all events, those confined to specific windows, or even just particular categories, xev provides the flexibility needed for detailed event analysis, aiding in both application development and system troubleshooting.