Requirements & Conventions

Flatpak deliberately imposes very few requirements on applications. 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 additional desktop integration options can be found in Desktop Integration.

Expected Standards

Applications using 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 meet these requirements.

Application IDs

As described in Using Flatpak, Flatpak requires each application to have a unique identifier, which follows a format such as org.example.app.

The format is in reverse-DNS style, meaning the first section should be a domain controlled by the project, while the trailing section represents the specific project. There are exceptions to this, such as extensions, which use 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 also the Appstream specification.

Here are some practical examples of poor ID choices:

• org.example.desktop

  This ID is problematic because the Appstream standard, for legacy reasons, treats IDs ending with .desktop as a special case, leading to inconsistency. For the same reason, .Desktop suffixes should not be used for newly named applications. Don’t hesitate to repeat the application name even if it is already part of the domain name section of the identifier (e.g. 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. For instance, if another project creates io.github.foo-bar, issues can arise since the two IDs would then be treated as similar by flatpak (even though they deserve to be treated as different namespaces). A better ID would be io.github.foo.foo, even if the second foo seems redundant.

• org.example-site.foo

  This ID is not valid according to the D-Bus specification, as the dash - isn’t allowed except in the last component. You should replace the dash with an underscore _, and thus 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 control over the github.com or gitlab.com domains. Instead, you should use io.github or io.gitlab as shown above.

• Org.Example.App

  The domain portion of the ID must be in lowercase, and while not required, the application portion is recommended to be in lowercase as well. Therefore, you should use org.example.app.

MetaInfo files

MetaInfo 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 MetaInfo. You can use the online AppStream MetaInfo Creator to generate a basic file.

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

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

A legacy convention allows for placing the .appdata.xml file in /app/share/appdata. flatpak-builder will check either directory for its respective extension.

The appstreamcli validate --explain command can be used to check MetaInfo 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 must be named using 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, meaning 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

Flatpak will export the following icon name patterns: $FLATPAK_ID, $FLATPAK_ID.foo, $FLATPAK_ID-foo. They may end with an extension suffix like .png or .svg. Exported icons can be found in the icons subfolder of either $HOME/.local/share/flatpak/exports/share or /var/lib/flatpak/exports/share, depending on whether it’s a system or user install.

The distribution usually appends those paths to $XDG_DATA_DIRS on the host when installing the flatpak package. Unless an icon is exported by Flatpak, host applications cannot access it.

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.

Desktop files must be named using the application’s ID, followed by the .desktop file extension, and must 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=gnome-dictionary
Type=Application
Icon=org.gnome.Dictionary
Categories=Office;Dictionary;

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

The Exec key of the desktop files is rewritten by Flatpak when installing an app. The original value of the key becomes the value of the --command argument like so:

Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=gnome-dictionary org.gnome.Dictionary

Flatpak will export the following desktop filename patterns: $FLATPAK_ID.desktop, $FLATPAK_ID.foo.desktop, $FLATPAK_ID-foo.desktop. Exported desktop files can be found in the applications subfolder of either $HOME/.local/share/flatpak/exports/share or /var/lib/flatpak/exports/share, depending on whether it’s a system or user install.

The distribution usually appends those paths to $XDG_DATA_DIRS on the host when installing the flatpak package. Unless a desktop file is exported by Flatpak, host applications cannot access it.

The metainfo file along with the other metadata such as icon and desktop files is composed into a catalogue by appstream. Since 1.3.4, Flatpak Builder by default uses appstreamcli from libappstream to compose this and executes the following command:

$ appstreamcli compose --no-partial-urls --prefix=/ --origin=${FLATPAK_ID} --media-baseurl=<repo-media-url> --media-dir=${FLATPAK_DEST}/share/app-info/media --result-root=${FLATPAK_DEST} --data-dir=${FLATPAK_DEST}/share/app-info/xmls --icons-dir=${FLATPAK_DEST}/share/app-info/icons/flatpak --components=${FLATPAK_ID} ${FLATPAK_DEST}

${FLATPAK_ID} is the Flatpak application ID and ${FLATPAK_DEST} is set to /app for apps and /usr for runtimes. The media-baseurl controls the base URL used for mirroring screenshots. Usually each app store will have their own base URL for this. On Flathub this is set to https://dl.flathub.org/media/.

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 these, 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: XDG base directories can be accessed through the g_get_user_cache_dir (), g_get_user_data_dir (), g_get_user_config_dir () functions.

• Qt: XDG base directories can be accessed 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

These environment variables are always set by Flatpak and override any host values. However, if access to directories on the host is needed, the $HOST_XDG_CONFIG_HOME, $HOST_XDG_DATA_HOME, $HOST_XDG_CACHE_HOME, and $HOST_XDG_STATE_HOME environment variables will be set if they are also available on the host.

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

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