Application Bundles

helloSystem uses application bundles to manage native applications. Whenever possible, application bundles are used. XDG-style .desktop files are considered legacy and should be avoided.

Benefits

Applications shipped as application bundles can

  • Easily be moved around in the filesystem (relocation)

  • Easily be copied to e.g., another machine or a network share

  • Easily be copied on the local machine to get a second instance of the same application (e.g., to make changes to it while keeping the original around)

  • Easily be deleted by moving to the Trash

  • Easily be managed without the need for a package manager

  • Easily be modified (e.g., changing the icon) without any side effects to other parts of the system

  • Easily be distributed (e.g., in archives like zip files or in disk images) without the need for packaging

  • Easily be understood by switchers coming from other operating systems with similar application distribution formats

Application bundle formats

.app bundles

helloSystem supports simplified GNUstep-style .app bundles.

Minimal requirements:

./Application.app
./Application.app/Application <-- (link to) the executable to be launched when Application.app is double clicked
./Application.app/Resources/Application.png <-- the application icon (png format)
./Application.app/Resources/can-open <-- optional, MIME types the application can open, separated by `;`

In order to realize the full intended functionality, additional metadata may be required (such as localized application name, version, supported file formats, etc.). Possibly the GNUstep metadata format is sufficient for this.

.AppDir bundles

helloSystem supports simplified ROX-style .AppDir bundles.

Minimal requirements:

./Application.AppDir
./Application.AppDir/AppRun <-- (link to) the executable to be launched when Application.app is double clicked
./Application.AppDir/.DirIcon <-- the application icon (png format)

In order to realize the full intended functionality, additional metadata may be required (such as localized application name, version, supported file formats, etc.). Possibly the ROX AppDir specification is not sufficient for this and may need to be amended (to be determined).

Wrappers for legacy packages

helloSystem supports applications that are not shipped in bundle formats yet. These can be bridged by wrapper bundles, application bundles that merely launch (but do not contain) the payload application (which may be installed in traditional ways).

A desktop2app tool ships with helloSystem that automates the creation of such wrappers.

Please note that the use of wrapper bundles is discouraged and is only available as a bridge technology for backward compatibility with existing application packages.

Building application bundles

Here is a real-world example on how to build an application bundle for a Qt application written in C++ using GitHub and Cirrus CI:

https://github.com/helloSystem/QHexEdit

The application gets compiled and uploaded in a fully automated process.

The resulting application bundle is provided for download in zipped form on GitHub Releases.

Please see the .cirrus.yml file for details.

Making an application load privately bundled libraries

If an application is supposed to load privately bundled libraries, one must patch it so that it loads privately bundled libraries from a path relative to itself ($ORIGIN) rather than from /usr/local/lib:

sed -i -e 's|/usr/local/lib|$ORIGIN/../lib|g' usr/local/bin/falkon
ln -s usr/local/lib .
rm usr/local/bin/falkon-e

This works because by coincidence the string /usr/local/lib has the exact same length as $ORIGIN/../lib. If this was not the case, one would need to either specify the rpath at compilation time, or use a tool such as patchelf.

Avoiding absolute paths

For an application to be fully relocatable in the filesystem, one must take care that no absolute paths to data files (e.g., those in /usr/share/<APPNAME> get compiled in.

In Qt applications, void QStringList QStandardPaths::standardLocations(QStandardPaths::AppDataLocation);. According to the Qt documentation, this resolves to "~/.local/share/<APPNAME>", "/usr/local/share/<APPNAME>", "/usr/share/<APPNAME>" but clearly /usr is not where these things are located in an .app bundle.

Instead, use QString QCoreApplication::applicationDirPath() and construct a relative path to ../share/<APPNAME> from there.

For an example, see: https://github.com/KaidanIM/Kaidan/commit/da38011b55a1aa5d17764647ecd699deb4be437f

Getting application metadata for running applications

Many desktop environments use XDG-style .desktop files to figure out which application a given window belongs to but since helloSystem is using .app bundles and .AppDir application directories this approach is not sufficient since such applications normally do not install XDG-style .desktop files in central locations.

Docks (and other similar applications) can find the icon that belongs to a window on the screen by the following procedure:

  1. Get the Window IDs of the windows on the screen from Xorg (to simulate this, you can use the xprop tool)

  2. Get the process ID (PID) that has launched the window with this ID from the _NET_WM_PID property of the Xorg window

  3. Get the path of the executable and its arguments that launched this PID from the operating system, e.g., using /proc/$PID/cmdline (Linux) or the procstat command (FreeBSD)

  4. From the path and the arguments figure out which element might be the relevant .app bundle or .AppDir. For example, a process might have been invoked with sudo -E launch python3 /Applications/Some.app/Some --arguments 123. In this case the relevant information is in a “random” location within the long list of arguments. Also consider the case python3 /Applications/Some.app/Resources/some-executable. In this case some-executable is in a subdirectory of the application bundle

A rudimentary implementation of this logic is in place in Dock in the Utils::readInfoFromPid(quint32 pid) and Utils::examinePotentialBundlePath(QString path) methods, but improvements are highly welcome.

Note

To display the correct icons in Dock for processes running as root, users must be able to see information on processes running as root. Hence security.bsd.see_other_uids=0 must not be set in sysctl.conf for this to work.

Credits

Application directories and application bundles have been used by many desktop-oriented operating systems, including RISC OS, NeXTStep/OPENSTEP, Mac OS X, and various other systems. Classic Macintosh System used single-file applications that kept resources in the Resource Fork.

  • https://en.wikipedia.org/wiki/Application_directory

  • http://rox.sourceforge.net/desktop/AppDirs.html

  • https://en.wikipedia.org/wiki/Resource_fork