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Requirements & Conventions

Flatpak deliberately makes as few requirements of applications as possible. However, some adherence to some conventions are necessary to enable integration with Linux desktops and app centers.

Those who have previously targeted the Linux desktop will typically be familiar with these conventions.

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

Desktop files

Desktop files are another Freedesktop standard which is used to provide information about each application the desktop environment where it is being run. The Freedesktop specification provides a complete reference for writing desktop files, and other useful information about them are available.

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 and icon name:

[Desktop Entry]
Name=Gnome Dictionary
Exec=org.gnome.Dictionary
Type=Application
Icon=org.gnome.Dictionary

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

AppData files

AppData files provide metadata about applications, which is used by application stores (such as Flathub, GNOME Software and KDE Discover). It can contain a description of the application, screenshots.

The Freedesktop specification provides a complete reference for providing AppData.

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

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

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

XDG base directories

XDG base directories are a Freedesktop standard, which defines standard locations where user-specific application data and configuration should be stored.

By default, Flatpak sets three XDG base directories that should be used by applications for user-specific storage. These are:

Base directory Usage Default location
XDG_CONFIG_HOME User-specific configuration files ~/.var/<app-id>/config
XDG_DATA_HOME User-specific data ~/.var/<app-id>/data
XDG_CACHE_HOME Non-essential user-specific data ~/.var/<app-id>/cache

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

~/.var/org.gnome.Dictionary/data

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

Filesystem layout

Each application sandbox contains the filesystem of the application's runtime, which follows standard Linux filesystem conventions. For example:

  • /bin - binaries
  • /dev - device files
  • /lib - libraries
  • /opt - application software packages
  • /usr - 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.