interception-tools

interception-tools is a set of utilities to control and customize the behavior of keyboard input mappings.

Interception-tools operates at a lower level compared to other similar tools (xcape, xmodmap) by using libevdev and libudev(3). This makes it one of the only options available for customizing the keyboard behavior across X11, Wayland, and the Linux console.

Installation

Install interception-tools.

Many plugins are available:

interception-caps2esc to switch CapsLock with Ctrl/Esc
interception-caps2esc-delay-git^AUR
interception-caps2esc-nocaps-git^AUR
interception-dual-function-keys to modify the behavior of a key when held.
interception-hideaway^AUR
interception-k2k-git^AUR
interception-ralt2hyper^AUR
interception-space2meta^AUR
interception-uswitch^AUR
interception-vimproved-git^AUR
interception-xswitch^AUR

Configuration

The tools comprise commandline tools and a systemd service.

A configuration in /etc/interception/udevmon.yaml needs to be added before starting the packaged udevmon.service.

Note: Configuration files found on /etc/interception/udevmon.d/ are read first, and fallbacks to /etc/interception/udevmon.yaml.

How it works

Interception-tool makes use of libevdev, which according to its wiki is essentially a read(2) on steroids for /dev/input/eventX devices. It sits in between the kernel and the process handling an event. In the simplest scenario would look like this:

kernel | libevdev | evtest

For X.Org input modules, the stack would look like this:

kernel | libevdev | xf86-input-evdev | X server | X client

For Wayland, the stack would look like this:

kernel | libevdev | Compositor | Wayland client

In other words, libevdev is so low level that it does not have knowledge of X/Wayland clients.

Practical examples

Interception-tools makes 4 utilities available:

intercept: redirect device input events to stdout,
mux: combine streams of input events,
udevmon: monitor input devices for launching tasks,
uinput: redirect device input events from stdin to virtual device.

Increase niceness

Since the tool is going to be sitting down at the lowest level of the device inputs, make sure it will behave consistently by increasing udevmon priority:

# nice -n -20 udevmon -c udevmon.yaml > udevmon.log 2> udevmon.err &

Tip: The udevmon systemd service runs with Nice=-20.

Simple redirection

The simplest way or redirecting the event to the stdin (without doing nothing) is:

$ intercept -g DEVNODE | uinput -d DEVNODE

where DEVNODE is the path to the actual device: e.g. /dev/input/by-path/platform-i8042-serio-0-event-kbd.

Embed commands

To actually perform an operation in between the key event and the input, simply pipe it in between intercept and uinput.

E.g. with the interception-caps2esc plugin installed:

$ intercept -g DEVNODE | caps2esc | uinput -d DEVNODE

If we omitted the -g flag, then device event would have been just observed, not grabbed.

Feed as YAML

This way of intercepting the input can quickly become sub-optimal, this is where udevmon comes in handy. udevmon accepts a YAML configuration with a list of jobs (sh commands by default) to be executed.

In case the device matches a given description:

$ udevmon -c caps2esc.conf.yml

- JOB: intercept -g DEVNODE | caps2esc | uinput -d DEVNODE
  DEVICE:
    LINK: /dev/input/by-path/platform-i8042-serio-0-event-kbd

The LINK configuration will match a device with a specific name, but it will accept also a regex option. This can be combined with multiple job specifications to create a default behavior, in each case only the first matching job is going to be executed:

- JOB: intercept -g DEVNODE | caps2esc -m 2 | uinput -d DEVNODE
  DEVICE:
    LINK: /dev/input/by-id/usb-SEMITEK_USB-HID_Gaming_Keyboard_SN0000000001-event-kbd
- JOB: intercept -g DEVNODE | caps2esc | uinput -d DEVNODE
  DEVICE:
    EVENTS:
      EV_KEY: [[KEY_CAPSLOCK, KEY_ESC]]
    LINK: .*-event-kbd

Combine devices

Beside input emulation, the uinput tool also serves purpose to print a device's description in YAML format:

$ uinput -p -d /dev/input/by-id/my-kbd

which itself can be fed back to uinput as:

$ uinput -c my-kbd.yaml

It can also merge device and YAML characteristics, which can be used for instance to combine events coming from keyboard and mouse:

e.g. instance CapsLock+Click as Ctrl+Click

$ uinput -p -d /dev/input/by-id/my-kbd -d /dev/input/by-id/my-mouse -c my-extra.yaml

Handle multiple jobs

The mux is used to combine multiple pipelines into a single one. A muxer needs to be created first, and it can later be used as the input or the output of a given pipeline. In a YAML specification file, the muxer is created using the CMD key:

- CMD: mux -c caps2esc
- JOB: mux -i caps2esc | caps2esc | uinput -c /etc/interception/gaming-keyboard.yaml
- JOB: intercept -g DEVNODE | mux -o caps2esc
  DEVICE:
    LINK: /dev/input/by-id/my-kbd
- JOB: intercept DEVNODE | mux -o caps2esc
  DEVICE:
    LINK: /dev/input/by-id/my-mouse

In the example above, when the keyboard is connected, it's grabbed and its input events are sent to the caps2esc muxer that was initially created. Observed input (not grabbed) from mouse is also sent to the same muxer. The buttons of the mouse generate EV_KEY events, so caps2esc will accept them.