Requirements & Conventions

Flatpak deliberately makes as few requirements of applications as possible. However, a small number of standard Linux desktop conventions are expected, primarily to ensure that applications integrate with Linux desktops and app centers. Developers might also encounter a small number of Linux technical conventions.

Information on further desktop integration options can be found in Desktop Integration.

Expected Standards

Applications that use Flatpak are generally expected to comply with the following standards. Applications that have previously targeted the Linux desktop will typically need to make very few (if any) changes to do this.

Application IDs

As described in Using Flatpak, Flatpak requires each application to have a unique identifier, which has a form such as org.gnome.Dictionary.

The format is in reverse-DNS style so the first section should be a domain controlled by the project and the trailing section represents the specific project. There are some exceptions to this, such as extensions using the base application-id of the project they extend rather than their own.

As will be seen below and in future sections, this ID is expected to be used in a number of places. Developers must follow the standard D-Bus naming conventions for bus names when creating their own IDs. This format is already recommended by the Desktop File specification and Appstream specification also.

For some practical examples of bad IDs

• org.example.desktop

  This is a bad ID because the Appstream standard for legacy reasons treats IDs ending with .desktop as a special case causing inconsistency. For this same reason, .Desktop suffixes should not be used for newly named applications. Don’t hesitate to repeat the application name even if it already is part of the domain name section of the identifier (eg. org.example.Example).

• io.github.Foo

  This is problematic because while foo.github.io may be unique to your project, it does not include a project-specific identifier. This may cause issues if another project creates io.github.Foo-Bar which should be its own namespace but areas of flatpak may treat them similar. A better ID would be io.github.foo.Foo even if it is redundant.

• org.example-site.Foo

  This ID is not valid according to the DBus specification as a dash - isn’t allowed except on the last component. You should replace - with an undercore _ and therefore, use org.example_site.Foo instead.

• com.github.foo.Bar or com.gitlab.foo.Bar

  While a project may be hosted on GitHub or GitLab it does not have any control over the github.com or gitlab.com domain. Instead you should use io.github or io.gitlab as shown above.

AppData files

AppData files provide metadata about applications, which is used by application stores (such as Flathub, GNOME Software and KDE Discover).

The Freedesktop AppStream specification provides a complete reference for providing AppData. You can use the online AppStream MetaInfo Creator to generate a basic file.

AppData files should be named with the application ID and the .metainfo.xml file extension, and should be placed in /app/share/metainfo/. For example:

/app/share/metainfo/org.gnome.Dictionary.metainfo.xml

A legacy convention of having the .appdata.xml installed in /app/share/appdata is also accepted as well, and flatpak-builder will check either directory with either extension.

The appstream-util validate-relax command can be used to check AppData files for errors.

Application icons

Applications are expected to provide an application icon, which is used for their application launcher. These icons should be provided in accordance with the Freedesktop icon specification.

Icons should be named with the application’s ID, be in either PNG or SVG format, and must be placed in the standard location:

/app/share/icons/hicolor/$size/apps/

For example, the path to the 128✕128px version of GNOME Dictionary’s icon is:

/app/share/icons/hicolor/128x128/apps/org.gnome.Dictionary.png

Icons must be square shaped, ie their width and height must be the same. The maximum size allowed by the specification is 512x512px. SVG icons are of size scalable:

/app/share/icons/hicolor/scalable/apps/org.gnome.Dictionary.svg

Desktop files

Desktop files are used to provide the desktop environment with information about each application. The Freedesktop specification provides a complete reference for writing desktop files, and additional information about them is available online.

Desktop files should be named with the application’s ID, followed by the .desktop file extension, and should be placed in /app/share/applications/. For example:

/app/share/applications/org.gnome.Dictionary.desktop

A minimal desktop file should contain at least the application’s name, exec command, type, icon name and categories:

[Desktop Entry]
Name=Gnome Dictionary
Exec=org.gnome.Dictionary
Type=Application
Icon=org.gnome.Dictionary
Categories=GNOME;GTK;Office;Dictionary;

The desktop-file-validate command can be used to check for errors in desktop files.

Exporting through extra-data

Files downloaded through extra-data are only downloaded when installing, as such they aren’t yet available for flatpak-builder to automatically export during the build process.

When using extra-data, place any files that must be exported under this location:

/app/extra/export/share/

For example, if GNOME Dictionary used extra-data to download a 96x96 icon this would be its path:

/app/extra/export/share/icons/hicolor/96x96/apps/org.gnome.Dictionary.png

Technical conventions

The following are standard technical conventions used by Flatpak and Linux desktops. Those with Linux experience will likely already be aware of them. However, developers who are new to Linux might find some of this information useful.

D-Bus

D-Bus is the standard IPC framework used on Linux desktops. A lot of applications won’t need to use it, but it is supported by Flatpak should it be required.

D-Bus can be used for application launching and communicating with some system services. Applications can also provide their own D-Bus services (when doing this, the D-Bus service name is expected to be the same as the application ID).

Filesystem layout

Each Flatpak sandbox, which is the environment in which an application is run, contains the filesystem of the application’s runtime. This follows standard Linux filesystem conventions.

For example, the root of the sandbox contains the /etc directory for configuration files and /usr for multi-user utilities and applications. In addition to this, each sandbox contains a top-level /app directory, which is where the application’s own files are located.

XDG base directories

XDG base directories are standard locations for user-specific application data. Popular toolkits provide convenience functions for accessing XDG base directories. These include:

• Electron: XDG base directories can be accessed with app.getPath

• Glib: provides access to the XDG base directories through the g_get_user_cache_dir (), g_get_user_data_dir (), g_get_user_config_dir () functions

• Qt: provides access to XDG base directories with the QStandardPaths Class

However, applications that aren’t using one of these toolkits can expect to find their XDG base directories in the following locations:

Base directory	Usage	Default location
XDG_CONFIG_HOME	User-specific configuration files	~/.var/app/<app-id>/config
XDG_DATA_HOME	User-specific data	~/.var/app/<app-id>/data
XDG_CACHE_HOME	Non-essential user-specific data	~/.var/app/<app-id>/cache
XDG_STATE_HOME	State data such as undo history	~/.var/app/<app-id>/.local/state

For example, GNOME Dictionary will store user-specific data in:

~/.var/app/org.gnome.Dictionary/data/gnome-dictionary

Note that applications can be configured to use non-default base directory locations (see Sandbox Permissions).

Note that $XDG_STATE_HOME is only supported by Flatpak 1.13 and later. If your app needs to work on earlier versions of Flatpak, you can use the --persist=.local/state and --unset-env=XDG_STATE_HOME finish args so the app will use the correct directory, even after Flatpak is later upgraded to >1.13.