Select Default Application (Qt6)

A powerful, modern GUI tool to manage default applications on Linux. Built with Qt 6 and C++17, it strictly adheres to the XDG MIME Apps Specification, ensuring your file associations work correctly across all desktop environments (KDE Plasma, GNOME, XFCE, Hyprland, i3, etc.).

Main Interface	Granular Conflict Resolution

Features

Core Functionality

• XDG Compliant: Fully implements the XDG MIME Apps Specification for both reading and writing associations
• Precedence Handling: Correctly respects desktop-specific overrides ($desktop-mimeapps.list) and system-wide defaults
• Two-Way Management:
  • Add Associations: Set an application as the default for specific file types
  • Remove Associations: Remove explicit user overrides to fall back to system defaults
• Application-Centric UI: Select an application to see everything it supports and what it currently handles
• Visual Feedback: Full icon support via QIcon::fromTheme for both applications and MIME types

User Experience

• Granular Conflict Resolution: When setting associations that conflict with existing defaults, a checkbox dialog allows you to selectively choose which specific MIME types to overwrite
• Search & Filtering: Search applications by name and filter MIME types by category (e.g., audio, video, image)
• Symmetrical Layout: Clean, balanced three-panel interface with consistent spacing and alignment
• Default Window Size: Opens at a comfortable 1000×600 pixels
• Rich Help Dialog: HTML-formatted help text with clear instructions

Developer Features

• Verbose Logging: Use -V flag with custom sda.log category to see only relevant application logs (parsing, associations) without Qt internal noise
• Clean CLI Exit: Proper resource cleanup when using --help or --version flags (no QThreadStorage warnings)
• Modern C++17: Uses range-based for loops, structured bindings, and Qt 6 best practices

Installation

Prerequisites

You need Qt 6, CMake, and a C++17 compatible compiler.

Fedora / RHEL

sudo dnf install qt6-qtbase-devel cmake gcc-c++

Ubuntu / Debian

sudo apt install qt6-base-dev cmake g++

Arch Linux

sudo pacman -S qt6-base cmake gcc

Nix / NixOS

This project provides first-class support for Nix and NixOS.

Optional: Setup Cachix (Faster Installation)

To avoid building from source, use the sda-qt6 binary cache:

# Install cachix if not already installed
nix-env -iA cachix -f https://cachix.org/api/v1/install

# Add the sda-qt6 cache
cachix use sda-qt6

Or add it directly to your NixOS configuration:

{
  nix.settings = {
    substituters = [ "https://sda-qt6.cachix.org" ];
    trusted-public-keys = [ "sda-qt6.cachix.org-1:f+mZPJwYQKpzaAdGWj2DfdHV5uEjnYx1ZoAWfZfEGoA=" ];
  };
}

Using Nix Flakes (Recommended)

1. Ad-hoc Usage

# Run directly without installing
nix run github:THShafi170/selectdefaultapplication-Qt6

2. Permanent Installation

nix profile install github:THShafi170/selectdefaultapplication-Qt6

3. Declarative (NixOS / Home Manager) Add to your flake.nix inputs:

inputs.sda-qt6.url = "github:THShafi170/selectdefaultapplication-Qt6";

Then add it to your packages:

environment.systemPackages = [ inputs.sda-qt6.packages.${system}.default ];

4. Development Shell

nix develop

Using Legacy Nix

1. Permanent Installation

nix-env -i -f .

2. Declarative (NixOS) Add to your configuration.nix:

{ pkgs, ... }:
let
  sda-qt6 = import (fetchTarball "https://github.com/THShafi170/selectdefaultapplication-Qt6/archive/master.tar.gz") { inherit pkgs; };
in {
  environment.systemPackages = [ sda-qt6 ];
}

3. Development Environment

nix-shell

Building from Source

1. Clone the repository:

   git clone https://github.com/THShafi170/selectdefaultapplication-Qt6.git
   cd selectdefaultapplication-Qt6

2. Configure and Build:

   cmake -S . -B build
   cmake --build build

3. Run:

   ./build/sda-qt6

4. Optional: Install (installs to /usr/local/bin by default):

   sudo cmake --install build

Usage

GUI Workflow

1. Left Panel: Browse or search for an application
2. Center Panel: View MIME types the selected application can handle
   • Bold entries = already set as default for this app
   • Select types you want to associate → click "Add association(s)"
3. Right Panel: View current default associations for the selected app
   • Select types → click "Remove association(s)" to revert to system defaults

Conflict Resolution

When setting associations that conflict with existing defaults, a dialog will appear showing:

• Each conflicting MIME type
• Its current default application
• Checkboxes (checked by default) to selectively choose which to overwrite

Simply uncheck any associations you want to preserve.

Command Line Arguments

• -h, --help: Display help information
• -v, --version: Display application version (2.0)
• -V, --verbose: Enable verbose logging (shows XDG parsing, association writes/removals)

Example:

# Run with verbose logging to debug MIME type parsing
./build/sda-qt6 -V

Technical Details

Architecture

This project uses a clean separation between UI and backend logic:

• XdgMimeApps Class: Handles all XDG specification logic

  • Parses ~/.config/mimeapps.list, $XDG_CONFIG_DIRS, and desktop-specific overrides
  • Discovers .desktop files from all XDG application directories
  • Handles [Default Applications], [Added Associations], and [Removed Associations] groups
  • Writes user overrides to ~/.config/mimeapps.list only (never modifies system files)

• SelectDefaultApplication Class: Qt widget for the UI

  • Three-panel layout with application list, MIME type list, and current defaults
  • Delegates all file I/O and parsing to XdgMimeApps

Custom Logging

Uses QLoggingCategory("sda.log") for all debug output. When -V is set:

sda.log: XdgMimeApps: Parsing "/home/user/.config/mimeapps.list"
sda.log: XdgMimeApps: Writing setting: "image/png" = "gwenview.desktop"
sda.log: SelectDefaultApplication: Sync-ed 235 associations to UI

Cross-Desktop Compatibility

Works seamlessly across all freedesktop.org-compliant desktops:

• KDE Plasma, GNOME, XFCE, Cinnamon (traditional DEs)
• Hyprland, Sway, i3, bspwm (Wayland/X11 window managers)
• No KDE Frameworks dependency—only pure Qt 6 Core/Gui/Widgets

File Structure

Main source files:

• main.cpp - Application entry point
• selectdefaultapplication.{h,cpp} - UI implementation
• xdgmimeapps.{h,cpp} - XDG MIME specification backend
• CMakeLists.txt - Build configuration

License

This project is licensed under the GNU General Public License v3.0 (GPLv3). See the LICENSE file for details.

Credits

This is a Qt 6 port and modernization of the original selectdefaultapplication by magnus-ISU. This fork includes significant enhancements:

• Ported from Qt 5 to Qt 6
• Modern C++17 codebase
• Granular conflict resolution dialog
• Improved UI symmetry and layout
• Custom logging infrastructure
• Clean command-line handling
• Enhanced XDG specification compliance

Special thanks to the original author for creating this useful tool!

Contributing

Contributions are welcome! Please ensure your changes:

• Follow the existing code style (see .clang-format)
• Maintain XDG specification compliance
• Include appropriate error handling
• Update documentation as needed