Tap

Press and release LS within TAP_MILLIS (default 200ms) for DE.

By default, until the tap is complete, we get LS. See below for other options.

                <---------200ms--------->     <---------200ms--------->
keyboard:       LS↓      LS↑                  LS↓                          LS↑
computer sees:  LS↓      LS↑ DE↓ DE↑          LS↓                          LS↑

Double Tap

Tap then press again with DOUBLE_TAP_MILLIS (default 150ms) to hold DE.

                             <-------150ms------->
                <---------200ms--------->
keyboard:       LS↓         LS↑             LS↓               LS↑
computer sees:  LS↓         LS↑ DE↓ DE↑     DE↓ ..(repeats).. DE↑

You can continue double tapping so long as it is within the DOUBLE_TAP_MILLIS window.

Consumption

Press or release another key during the TAP_MILLIS window and the tap will not occur.

This is especially useful for modifiers, for instance a quick ctrl-C. In this example we press the a key during the window.

Double taps do not apply after consumption; you will need to tap first.

Mouse and touchpad events (EV_REL and EV_ABS) can also consume taps, however you will need to use a Multiple Devices configuration.

                                                               <-------150ms------->
                                                 <---------200ms--------->
                                 <-------150ms------->
                <---------200ms--------->
keyboard:       LS↓      a↓  a↑  LS↑             LS↓          LS↑           LS↓
computer sees:  LS↓      a↓  a↑  LS↑             LS↓          LS↑ DE↓ DE↑   DE↓ ..(repeats)..

Package Manager

[IMAGE: Packaging status (https://repology.org/badge/vertical-allrepos/interception-dual-function-keys.svg)] (https://repology.org/project/interception-dual-function-keys/versions)

From Source

See runtime dependencies (https://gitlab.com/interception/linux/tools#runtime-dependencies).

Install Interception Tools (https://gitlab.com/interception/linux/tools) first.

git clone https://gitlab.com/interception/linux/plugins/dual-function-keys.git
cd dual-function-keys
make && sudo make install

Installation prefix defaults to /usr/local. This can be overridden in config.mk.

dual-function-keys

This yaml file conventionally resides in /etc/interception/dual-function-keys.

You can use raw (integer) keycodes, however it is easier to use the #defined strings from input-event-codes.h (https://github.com/torvalds/linux/blob/master/include/uapi/linux/input-event-codes.h).

# optional
TIMING:
  TAP_MILLISEC: <integer>
  DOUBLE_TAP_MILLISEC: <integer>
  SYNTHETIC_KEYS_PAUSE_MILLISEC: <integer>
# necessary
MAPPINGS:
  - KEY: <integer | string>
    TAP: [ <integer | string>, ... ]
    HOLD: [ <integer | string>, ... ]
    # optional
    HOLD_START: [ AFTER_PRESS | AFTER_RELEASE | BEFORE_CONSUME | BEFORE_CONSUME_OR_RELEASE ]
  - KEY: ...

Our example from the previous section looks like:

TIMING:
  TAP_MILLISEC: 200
  DOUBLE_TAP_MILLISEC: 150
MAPPINGS:
  - KEY: KEY_LEFTSHIFT
    TAP: KEY_DELETE
    HOLD: KEY_LEFTSHIFT

Combo Keys

You can configure the TAP as a “combo”, which will press then release multiple keys in order e.g. space cadet (:

MAPPINGS:
  - KEY: KEY_LEFTSHIFT
    TAP: [ KEY_LEFTSHIFT, KEY_9, ]
    HOLD: KEY_LEFTSHIFT

You can configure the HOLD as a “combo”, which will press then release multiple keys in order e.g. hyper modifier:

MAPPINGS:
  - KEY: KEY_TAB
    TAP: KEY_TAB
    HOLD: [ KEY_LEFTCTRL, KEY_LEFTMETA, KEY_LEFTALT, ]

By default, there will be a pause of 20ms between keys in the “combo”. This may be changed:

TIMING:
    SYNTHETIC_KEYS_PAUSE_MILLISEC: 10

Changing the Behavior of HOLD Keys

You can optionally use HOLD_START to configure the behavior of HOLD keys.

HOLD_START: AFTER_PRESS

If HOLD_START is unspecified, AFTER_PRESS or an unrecognized value, the default behaviour will apply.

HOLD_START: BEFORE_CONSUME

HOLD keys are pressed before KEY is consumed, and released when KEY is released. Therefore no extra keys beside TAP keys are sent when KEY is tapped, while HOLD keys can still be used as modifiers.

MAPPINGS:
  - KEY: KEY_LEFTSHIFT
    TAP: KEY_DELETE
    HOLD: KEY_LEFTSHIFT
    HOLD_START: BEFORE_CONSUME

                <---------200ms--------->     <---------200ms--------->
keyboard:       LS↓      LS↑                  LS↓                          LS↑
computer sees:           DE↓ DE↑

                                                               <-------150ms------->
                                                 <---------200ms--------->
                                 <-------150ms------->
                <---------200ms--------->
keyboard:       LS↓      a↓  a↑   LS↑             LS↓          LS↑           LS↓
computer sees:       LS↓ a↓  a↑   LS↑                          DE↓ DE↑       DE↓ ..(repeats)..

HOLD_START: BEFORE_CONSUME_OR_RELEASE

The behavior is like BEFORE_CONSUME except that when KEY is released and is neither tapped nor consumed before, HOLD keys are pressed in order and then released in order.

MAPPINGS:
  - KEY: KEY_LEFTSHIFT
    TAP: KEY_DELETE
    HOLD: KEY_LEFTSHIFT
    HOLD_START: BEFORE_CONSUME_OR_RELEASE

                <---------200ms--------->     <---------200ms--------->
keyboard:       LS↓      LS↑                  LS↓                          LS↑
computer sees:           DE↓ DE↑                                           LS↓ LS↑

HOLD_START: AFTER_RELEASE

Hold will only start after key release if the TAP_MILLISEC time has been exceded. This hold start is not affected by any kind of consumption

MAPPINGS:
  - KEY: KEY_A
    TAP: KEY_A
    HOLD: [KEY_LEFTSHIFT, KEY_A]
    HOLD_START: AFTER_RELEASE

                <---------200ms--------->     <---------200ms--------->
keyboard:       a↓       a↑                   a↓                           a↑
computer sees:           a↓  a↑                                            A↓ A↑

Warning

Do not assign the same modifier to two keys that you intend to press at the same time, as they will interfere with each other. Use left and right versions of the modifiers e.g. alt-tab with space-caps:

MAPPINGS:
  - KEY: KEY_CAPSLOCK
    TAP: KEY_TAB
    HOLD: KEY_LEFTALT
  - KEY: KEY_SPACE
    TAP: KEY_SPACE
    HOLD: KEY_RIGHTALT

Alternatively, you can use HOLD_START: BEFORE_CONSUME or HOLD_START: BEFORE_CONSUME_OR_RELEASE and then assigning the same modifier will be fine:

MAPPINGS:
  - KEY: KEY_CAPSLOCK
    TAP: KEY_TAB
    HOLD: KEY_LEFTALT
    HOLD_START: BEFORE_CONSUME_OR_RELEASE
  - KEY: KEY_SPACE
    TAP: KEY_SPACE
    HOLD: KEY_LEFTALT
    HOLD_START: BEFORE_CONSUME_OR_RELEASE

udevmon

udevmon needs to be informed that we desire Dual Function Keys. See How It Works (https://gitlab.com/interception/linux/tools#how-it-works) for the full story.

- JOB: "intercept -g $DEVNODE | dual-function-keys -c </path/to/dual-function-keys.yaml> | uinput -d $DEVNODE"
  DEVICE:
    NAME: <keyboard name>

The name may be determined by executing:

sudo uinput -p -d /dev/input/by-id/X

where X is the device with the name that looks like your keyboard. Ensure that all EV_KEYs are present under EVENTS. If you can’t find your keyboard under /dev/input/by-id, look at devices directly under /dev/input.

See Interception Tools: How It Works (https://gitlab.com/interception/linux/tools#how-it-works) for more information on uinput -p.

Usually the name is sufficient to uniquely identify the keyboard, however some keyboards register many devices such as a virtal mouse. You can run dual-function-keys for all the devices, however I prefer to run it only for the actual keyboard.

My /etc/interception/udevmon.d/my-keyboards.yaml:

- JOB: "intercept -g $DEVNODE | dual-function-keys -c /etc/interception/dual-function-keys/home-row-modifiers.yaml | uinput -d $DEVNODE"
  DEVICE:
    NAME: "Minimalist Keyboard ABC"
    EVENTS:
      EV_KEY: [ KEY_LEFTSHIFT ]
- JOB: "intercept -g $DEVNODE | dual-function-keys -c /etc/interception/dual-function-keys/thumb-cluster.yaml | uinput -d $DEVNODE"
  DEVICE:
    NAME: "Split Keyboard XYZ"
    EVENTS:
      EV_KEY: [ KEY_LEFTSHIFT ]

Multiple Devices

When using inputs from multiple devices e.g. ctrl-scroll it may be necessary to mux (https://gitlab.com/interception/linux/tools#mux) those devices for dual-function-keys to work across these devices e.g. scroll consuming ctrl.

Example udevmon configuration for a mouse and keyboard:

- CMD: mux -c dfk -c my-keyboard -c my-mouse
- JOB:
  - mux -i dfk | dual-function-keys -c /etc/interception/dual-function-keys/my-cfg.yaml | mux -o my-keyboard -o my-mouse
  - mux -i my-keyboard | uinput -c /etc/interception/udevmon.d/my-keyboard.yaml
  - mux -i my-mouse | uinput -c /etc/interception/udevmon.d/my-mouse.yaml
- JOB: intercept -g $DEVNODE | mux -o dfk
  DEVICE:
    NAME: AT Translated Set 2 keyboard
    EVENTS:
      EV_KEY: [ KEY_LEFTCTRL ]
- JOB: intercept -g $DEVNODE | mux -o dfk
  DEVICE:
    NAME: Razer Razer Naga Trinity
    EVENTS:
      EV_REL: [REL_WHEEL]
      EV_KEY: [BTN_LEFT]

In the above example, my-keyboard.yaml and my-mouse.yaml represent the virtual devices that udevmon will create to output events. They are generated once from the device itself e.g.

sudo uinput -p -d /dev/input/by-id/usb-my-keyboard-kbd > my-keyboard.yaml

An alternative, if you want to live dangerously (https://gitlab.com/interception/linux/plugins/dual-function-keys/-/issues/31#note_725722450), is to generate the virtual device configuration on the fly e.g.:

- CMD: mux -c dfk -c my-keyboard -c my-mouse
- JOB:
  - mux -i dfk | dual-function-keys -c /etc/interception/dual-function-keys/my-cfg.yaml | mux -o my-keyboard -o my-mouse
  - mux -i my-keyboard | uinput -d /dev/input/by-path/my-keyboard-event-kbd
  - mux -i my-mouse | uinput -d /dev/input/by-id/usb-my-mouse-event-mouse
- JOB: intercept -g $DEVNODE | mux -o dfk
  DEVICE:
    LINK: /dev/input/by-path/my-keyboard-event-kbd
- JOB: intercept -g $DEVNODE | mux -o dfk
  DEVICE:
    LINK: /dev/input/by-id/usb-my-mouse-event-mouse

I have a new use case. Can you support it?

Please raise an issue.

dual-function-keys has been built for my needs. I will be intrigued to hear your ideas and help you make them happen.

As usual, PRs are very welcome.

I see you are using q.m.k HHKB mod Keyboard in your udevmon. It uses QMK Firmware (https://qmk.fm/). Why not just use Tap-Hold (https://docs.qmk.fm/#/tap_hold)?

Good catch! That does indeed provide the same functionality as dual-function-keys. Unfortunately there are some drawbacks:

1.

Few keyboards run QMK Firmware.

2.

There are some issues with that functionality, as noted in the documentation Tap-Hold (https://docs.qmk.fm/).

3.

It requires a fast processor in the keyboard. My unscientific testing with an Ergodox (~800 scans/sec) and HHKB (~140) revealed that the slower keyboard is mushy and unuseably inaccurate.

Why not use xcape (https://github.com/alols/xcape)?

Xcape only provides simple tap/hold functionality. It appears difficult (impossible?) to add the remaining functionality using its XTestFakeKeyEvent mechanisms.

My Key Combination Isn’t Working

Ensure that your window manager is not intercepting that key combination.

I Don’t Want Double Tap Functionality

Set DOUBLE_TAP_MILLISEC to 0. See Key Combinations, No Double Tap (https://gitlab.com/interception/linux/plugins/dual-function-keys/-/blob/master/doc/examples.md#key-combinations-no-double-tap).