Rex

Rex is a lightweight application launcher for X11, developed as part of the Monteverde Linux distribution.

Status: early development. Rex works, but it is missing features and has not been hardened for daily use yet.

What it does

Rex opens a small undecorated window and fuzzy-matches what you type against two sources:

• Installed applications, by their display name (e.g. "Firefox"), read from .desktop files in $XDG_DATA_HOME/applications (default ~/.local/share/applications) and each $XDG_DATA_DIRS entry (default /usr/local/share:/usr/share). Entries marked NoDisplay or Hidden are skipped, and a file in your home directory overrides a system file with the same name.
• Executables in every $PATH directory.

Pick a result and press Enter to launch it; Rex exits right after.

Matching ignores case and is fuzzy: the letters you type must appear in the program name in order, but not necessarily next to each other, so ffx matches firefox. Results are ranked exact match first, then names starting with what you typed, then names containing it, then fuzzy matches; shorter names come first within each group, and applications come before executables.

Keys

Key	Action
Type	Filter the list of programs
Up / Down	Move the selection (wraps around)
Enter	Launch the selected program and exit; if nothing matches, run what you typed
Backspace	Delete the last character
Escape	Quit without launching anything

Arguments

Everything after the first word is passed to the program, so typing firefox --private-window and pressing Enter launches the selected match with that argument. The exception is input that matches a name containing spaces: visual studio selects "Visual Studio Code" rather than running visual with the argument studio. Arguments are split like a shell would: quotes, ~ and $VAR work, but pipes, redirections and $(...) are not supported.

Current limitations

• Input is limited to printable ASCII.
• X11 only (works under XWayland, but there is no native Wayland support).

Building

Dependencies

• CMake 3.16 or newer
• A C++17 compiler (GCC or Clang)
• pkg-config
• XCB: xcb, xcb-keysyms
• Cairo (with XCB support)
• Pango
• X11 protocol headers (for X11/keysym.h)

Arch Linux

sudo pacman -S cmake gcc pkgconf libxcb xcb-util-keysyms xorgproto cairo pango

Debian / Ubuntu

sudo apt install cmake g++ pkg-config libxcb1-dev libxcb-keysyms1-dev x11proto-dev libcairo2-dev libpango1.0-dev

Fedora

sudo dnf install cmake gcc-c++ pkgconf-pkg-config libxcb-devel xcb-util-keysyms-devel xorg-x11-proto-devel cairo-devel pango-devel

Build and run

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
./build/src/Rex

The build type defaults to Debug if you don't pass one.

To install (to /usr/local/bin by default):

sudo cmake --install build

Rex is meant to be bound to a hotkey in your window manager, for example in i3:

bindsym $mod+d exec Rex

Project layout

include/            Public headers
src/
  launcher.cpp      Entry point
  rex.cpp           Owns the X connection and runs the event loop
  ui.cpp            Window creation and drawing (XCB + Cairo + Pango)
  inputhandler.cpp  Keyboard handling and selection state
  suggestion.cpp    Collecting launchable programs and fuzzy matching
  desktopentry.cpp  Reading .desktop files
  executionengine.cpp  Launching the selected program

License

Rex is licensed under the BSD 3-Clause License. See LICENSE for details.