nocaps turns Caps Lock into a configurable keyboard layer on Windows, macOS, and Linux. Hold Caps Lock and press a bound physical key to invoke an action such as an arrow key, Control, volume control, or media playback.
Right-click the struck-through graduation-cap icon and choose Configure. The window shows every available action in the same interface on each OS. Click an action's binding, press the physical key that should trigger it, and the change is compiled, applied, and saved immediately. Assigning an already-used key moves that key to the new action.
Remapping defaults to enabled. The tray menu shows Enabled or Disabled and toggles the state when clicked. The window uses the same state with a No Caps or Caps button.
The configuration window is implemented in src/app.rs. src/config.rs contains the JSON model, validation, and compiled runtime lookup; it does not create a window.
Configuration is loaded at startup and saved as formatted JSON:
- Windows:
%APPDATA%\nocaps\config.json - macOS:
~/Library/Application Support/nocaps/config.json - Linux:
~/.config/nocaps/config.json
The repository's config/default.json is compiled into the executable. It is not a runtime sidecar file. If the user configuration does not exist, nocaps loads the embedded original bindings, writes an editable user copy, and uses them immediately. Restore defaults uses the same embedded data.
Example:
{
"version": 1,
"enabled": true,
"bindings": {
"left_control": "a",
"left_shift": "s",
"arrow_up": "i",
"arrow_down": "k",
"volume_up": "w",
"volume_down": "q",
"volume_mute": "tab"
}
}Each action can have one key and each key can have one action. Unknown versions and duplicate key assignments are rejected with an explicit error.
JSON and validation run only at startup or when a binding changes. Valid bindings are compiled into a fixed-size array indexed by physical key and published with an atomic pointer swap. Keyboard hooks perform no JSON parsing, hash lookups, linear searches, allocations, or configuration locks. Platform modules only translate native input codes to the shared physical-key enum and shared actions back to native output codes.
Remapping input processing never runs on the renderer thread. Windows installs the low-level hook on a dedicated THREAD_PRIORITY_HIGHEST Win32 message-loop thread. Linux keyboard-device workers and the macOS capture worker request realtime/high scheduling priority and continue at normal priority with a warning if the OS denies that request. Linux realtime priority generally requires CAP_SYS_NICE or an equivalent service limit.
The configuration window is event-driven and does not poll. The egui loop repaints only in response to window input or a tray activation: tray menu clicks are delivered through a handler that wakes the UI thread, so while the window sits hidden in the tray the process stays idle instead of repainting on a timer. Linux is the one exception — its AppIndicator menu only emits events while the GTK main loop is iterated, so the window keeps a low-frequency tick to drive that pump.
No additional runtime setup is required. To launch at sign-in, place nocaps.exe in the Startup folder (Win+R, then shell:startup).
Grant nocaps access in System Settings → Privacy & Security → Accessibility.
The Linux backend uses evdev and uinput, independently of X11 or Wayland. The user needs read access to keyboard devices under /dev/input and write access to /dev/uinput. Be aware that membership in the input group grants access to raw keyboard input.
Ubuntu/Debian build dependencies:
sudo apt install libgtk-3-dev libayatana-appindicator3-devTypical device setup:
sudo modprobe uinput
sudo usermod -aG input "$USER"Log out and back in after changing group membership. A newly connected keyboard currently requires restarting nocaps.
cargo test
cargo build --releaseThe executable is target/release/nocaps (nocaps.exe on Windows). CI builds and tests Windows, macOS, and Linux.