ktrl

TL;DR

ktrl is a Linux keyboard programming daemon. It aims to aid you in the never-ending quest of achieving the ultimate keybinding setup.

You can dip your toes by remapping a few modifier keys (e.g CapLock to Ctrl). Or you can go all-in by creating a sophisticated layering setup with dual-function keys, tap-dancing, etc...

ktrl is heavily inspired by the amazing open-source keyboard firmware project QMK. You can think of ktrl as an attempt to re-implement QMK as a Linux daemon.

This is an alpha state project. If you find any bugs or quirks please reach out to me.

Table of Contents

• ktrl
  • Intro
  • Features Overview
    • Layers
    • Tap-Hold (Dual Function Keys)
    • Tap-Dancing (Multi-Tap)
    • Meh and Hyper
    • Audible Feedback
  • Installation
  • Configuration
    • Primitives
      • Input Event Codes
      • Actions
      • Effects
    • Configuration File Format
  • Examples
    • Remapping Ctrl to Capslock
    • Home-row Modifiers
  • Limitations

Intro

ktrl sits right in the middle of the human-interface software stack. It lives in userspace, between the kernel and your display server (a.k.a X11 / Wayland).

This position allows ktrl complete control over the events your keyboard generates. These events are either transparently passed-on or transformed into ktrl's "Effects" (more on that later).

Features Overview

Aside from the obvious key remapping capability, here's a taste of some of the major things you can do with ktrl -

Layers

Although "layers" might seem like a foreign idea, it's something you're already very familiar with. After all, you apply "layers" all the time by using modifier and function keys :)

QMK takes this mechanism and generalizes it. Letting you design your own custom keyboard's layers!

If that sounds confusing, I encourage you to head over to QMK's documentation about layers.

Tap-Hold (Dual Function Keys)

Tap-Hold keys let you do one thing when the key is pressed, and another thing when it is held. For example, you can make your Spacebar act as normal when tapping, but serve as Ctrl when held.

Tap-Dancing (Multi-Tap)

Tap-dancing is quite similar to Tap-Hold. The key will act in one way if tapped once, and will act differently if tapped multiple times.

Meh and Hyper

Again, both of these were shamelessly taken from QMK. Meh and Hyper are special modifiers for creating keybindings that'll probably never conflict with existing ones. That's possible since Hyper is equal to pressing Ctrl+Alt+Shift+Win and Meh is the same as pressing Ctrl+Alt+Shift.

Audible Feedback

Ever wanted to bind your favorite 8bit tunes to your key-presses? Well, now you can! Though, aside from making your hacking session more musical, this feature as some very practical uses as well.

For example, it can help you build new muscle-memory connections using audible feedback. See the Capslock <-> Ctrl example below for more on that.

Installation

Getting the Executable

Start off by grabbing the main ktrl executable. Here's how you do that -

sudo cargo install --root /usr/local ktrl

Note: you may need to install alsa development bindings, autoconf and libtool. For Debian/Ubuntu distributions this can be done with

# apt install libalsa-ocaml-dev autoconf libtool libtool-bin

Setting up ktrl's User and Groups

Although a bit cumbersome, this step makes sure we can run ktrl without root privileges. Instead of running it as root, we'll make a new user for ktrl. Then, we'll add the new user to the input and audio groups. Let's get started -

sudo useradd -r -s /bin/false ktrl
sudo groupadd uinput
sudo usermod -aG input ktrl
sudo usermod -aG uinput ktrl

# If you're using the sound effects
sudo usermod -aG audio ktrl

Now, let's add a new udev rule that'll allow ktrl to write to /dev/uinput. /dev/uinput is ktrl's output device. Your keyboard being the input device.

git clone https://github.com/itaygarin/ktrl
cd ktrl
sudo cp ./etc/99-uinput.rules /etc/udev/rules.d/

Note that'll need to reboot your machine for the changes to take effect...

Setting up ktrl's Assets and Config

Now, it's time to decide where you'd like ktrl's assets and config to live.

By default, ktrl will assume you've placed these under /opt/ktrl. Specifically, /opt/ktrl/cfg.ron and /opt/ktrl/assets. Though, you can override these defaults with the --cfg and --assets cli arguments.

To set-up the defaults, you can follow these steps -

# Asumming you've already cloned and cd`d into the ktrl project

sudo mkdir /opt/ktrl
sudo cp -r ./assets /opt/ktrl
sudo cp examples/cfg.ron /opt/ktrl

sudo chown -R ktrl:$USER /opt/ktrl
sudo chmod -R 0770 /opt/ktrl

Locating your Keyboard's input device

The easy and obvious way

If you want ktrl to process events from all input devices, just run it without arguments. If you also want ktrl to watch for new devices in /dev/input, you can pass --watch as an argument. This could be useful if you use bluetooth devices that connect and reconnect all the time.

Specifying devices manually

If you want to process events only from selected devices, you have to supply it with a path to your keyboard's input device. Input devices reside in the /dev/input directory.

Linux provides two convenient symlinks-filled directories to make the search process easier. These directories are /dev/input/by-id and /dev/input/by-path.

Within these two directories keyboard devices usually have a -kbd suffix. For example, in my laptop, the path is /dev/input/by-path/platform-i8042-serio-0-event-kbd.

If you provide both --watch and --device arguments, then ktrl will watch for file events only on those devices.

Bluetooth keyboard location

If you are using a bluetooth keyboard, you will need to locate the associated /dev/input/event<#> as /dev/input/by-* only lists usb connected inputs. The best way to do this is to run cat /proc/bus/input/devices and search for the associated bluetooth keyboard (the Phys output will have the MAC address). Here is an example:

I: Bus=0005 Vendor=05ac Product=0239 Version=0050
N: Name="My Keyboard"
P: Phys=SO:ME:MA:CA:DD:RS
S: Sysfs=/devices/pci0000:00/0000:00:01.2/0000:02:00.0/0000:03:08.0/0000:07:00.3/usb3/3-5/3-5:1.0/bluetooth/hci0/hci0:256/0005:05AC:0239.0009/input/input40
U: Uniq=SO:ME:MA:CA:DD:RS
H: Handlers=sysrq kbd event25 leds
<snip>

Notice in Handlers, it tells us the event # we need. So our path to our device is /dev/input/event25.

Listening on all input devices

If you still can not figure out which device to use, or want to map events from several devices, you can pass all (or chosen) devices as an arg:

--device /dev/input/event*
# or
--device /dev/input/event2 /dev/input/event10

Setting up ktrl as a Service (Optional)

ktrl is a daemon that's designed to run as a background process. Therefore, you might want to set it up as a service. Feel free to skip this step if you just want to experiment with it.

Creating a service will vary from distro to distro, though, here are some basic steps that'll get you started on systemd based systems -

# Again, asumming you've cloned and cd`d into the ktrl project

edit ./etc/ktrl.service # change your device path
sudo cp ./etc/ktrl.service /etc/systemd/system
sudo systemctl daemon-reload
sudo systemctl start ktrl.service

Configuration

Finally, we get to the cool part! Though, let's briefly go over ktrl's config primitives before assembling our first config file

Primitives

Input Event Codes

ktrl uses Linux's input-event-codes everywhere. The full list can be found either in Linux's codebase here or under ktrl's KeyCode enum.

Specifically, ktrl uses a subset of the event codes that describe keyboard keys. E.g KEY_A and KEY_LEFTCTRL describe the 'A' and Left-Ctrl keys.

Actions

Within a layer, we map a source key (ex: KEY_A) into an Action. Actions describe the physical input movements you'll apply to the source key. E.g A TapHold describes a tap and a hold.

Actions List

• Tap: This is the default keyboard action. Use for simple key remappings.
• TapHold: Attach different Effects for a tap and hold.
• TapDance: Attach different Effects for a tap and multiple taps.

Effects

An Action will contain one or more Effects. These are the virtual output effects that'll manifest following the action. E.g Playing a sound, toggling a layer, sending a key sequence, etc...

Effects List

• NoOp: As the name suggests. This won't do anything.
• Key: This is the default effect you're "used to".
• KeySticky: Once pressed, the key will remain active until pressed again (like Capslock).
• KeySeq: Outputs multiple keys at once. E.g Meh and Hyper are KeySeqs
• Meh: A shorthand for KeySeq(KEY_LEFTCTRL, KEY_LEFTALT, KEY_LEFTSHIFT)
• Hyper: A shorthand for KeySeq(KEY_LEFTCTRL, KEY_LEFTALT, KEY_LEFTSHIFT, KEY_LEFTMETA)
• ActivateProfile: Activates a user-defined profile
• DeactivateProfile: Deactivates a user-defined profile
• DeactivateAllProfiles: Deactivates all user-defined profiles
• ToggleLayerAlias: When pressed, either turns on or off a named layer.
• ToggleLayer: When pressed, either turns on or off a layer.
• MomentaryLayer: While pressed, the relevant layer will remain active
• Sound: Plays one of the pre-built sounds
• SoundEx: Plays a custom sound provided by you.
• Multi: Lets you combine all the above effects. E.g Multi([Sound(Sticky), KeySticky(KEY_LEFTCTRL)])

Configuration File Format

ktrl uses the wonderful ron (Rust Object Notation) to make serializing configuration much easier. The format should be pretty intuitive, though please refer to the supplied cfg.ron for a practical example.

Examples

Remapping Ctrl to Capslock

This is probably one of the most effective yet simple changes you can make right now. You're left pinky will greatly appreciate this change in the long-run.

Doing this with ktrl is easy. In one of your layers, add the following -

KEY_CAPSLOCK:  Tap(Key(KEY_LEFTCTRL)),
KEY_LEFTCTRL:  Tap(Key(KEY_LEFTCTRL))),

Though, let's make this more interesting, shall we?

To make the transition smoother, let's add an error sound effect to the left Ctrl. This'll remind you you're doing something wrong -

KEY_CAPSLOCK:  Tap(Key(KEY_LEFTCTRL)),
KEY_LEFTCTRL:  Tap(Multi([Key(KEY_LEFTCTRL), Sound(Error)])),

Ah, much better!

Of course, you can also go cold turkey and only leave the sound effect. Like so -

KEY_CAPSLOCK:  Tap(Key(KEY_LEFTCTRL)),
KEY_LEFTCTRL:  Tap(Sound(Error)),

Home-row Modifiers

Another change I've been experimenting with is mapping modifiers to home row keys Though, due note you'll have to calibrate the tap_hold_wait_time config value to avoid false-positives.

Here's an example setup -

KEY_A:  TapHold(Key(KEY_A), Key(KEY_LEFTCTRL)),
KEY_S:  TapHold(Key(KEY_S), Key(KEY_LEFTSHIFT)),
KEY_D:  TapHold(Key(KEY_D), Key(KEY_LEFTALT)),

This will make A, S and D act as usual on taps and as modifiers when held.

Remap mouse button to control musical player

If you have a mouse with a side buttons, you can remap a button to act as a media button. This example shows how to map tap to play/pause and long tap to next song:

BTN_SIDE: TapHold(Key(KEY_PLAYPAUSE), Key(KEY_NEXTSONG))

Limitations

• TapHold and TapDance require calibration and tinkering. as stated above, you'll have to tweak the wait times for both of these to minimize false-positives.

Similar Projects

• alt: An Event Aggregator that connects to ktrl
• QMK: An open source keyboard firmware (ktrl's inspiration)
• kmonad: Very similar to ktrl (written in Haskell)
• xcape: Implements tap-hold only for modifiers (Linux)
• Space2Ctrl: Similar to xcape
• interception tools: A framework for implementing tools like ktrl
• karabiner-elements: A mature keyboard customizer for Mac