Rolling your Own Flatpak

Once Flatpak is installed and the flatpack program is available, the developer creates an empty directory for a new flatpak:

flatpak build-init helloworld org.example. HelloWorld org.gnome.Sdk org.gnome.Platform 3.22

The individual parameters behind build-init each have a special significance: helloworld specifies the directory name within which the flatpak is created; org.example.HelloWorld defines the D-bus name that the flatpak uses later to communicate with D-Bus. The SDK and the runtime, which the flatpak uses, are defined by org.gnome.Sdk and org.gnome.Platform, and 3.22 is the version number.

This example uses Gnome's SDK and runtime, which have been extensively tested and are fine for most everyday applications, but other options are also available.

The next step adds the program you wish to include in the flatpak. The flatpak build command is very useful; the package builder uses it to execute arbitrary commands in the flatpak build environment. For example,

flatpak build helloworld touch /app/test

creates a test file in the flatpak app directory. You can also compile a program within Flatpak. First, download the sources [11] and unzip them. Next, type cd to change to the folder with the unpacked sources; enter the following commands when you get there:

flatpak build flatpak_folder_name gcc $(pkg-config --cflags dbus-1 glib-2.0) -o send-hello dbus-send-hello.c $(pkg-config --libs dbus-1 glib-2.0)

flatpak_folder_name is followed by the relative path to the directory just created by flatpak build-init.

In the next step, you create the actual flatpak:

flatpak build-finish helloworld --socket=x11 --share=network --command=send-hello --socket=system-bus --socket=session-bus

The parameters --socket=x11 and --share=network give the flatpak app access to X11 and the network, which is not necessary for this example but is for many applications.

In the next step, you can execute the flatpak with the help of the commands from Listing 1.

The four commands create a Flatpak repository based on the local flatpak, add it to the Flatpak repository configuration, install the Flatpak from the repo, and finally execute it. The reward for all this effort is a Hello World message in the D-Bus logfile on the host system.

Powerful Tool

This example demonstrates only a tiny part of Flatpak's capability. The Flatpak website offers examples of how the package system can accommodate far more complex applications in a flatpak [12]. The granularity with which the developers have organized rights assignments is worthy of note. For each flatpak, the package maker determines whether the software needs access to X11, Wayland, D-Bus, or individual folders within the host filesystem. You can even control access to audio devices.

If you experiment with more complex Flatpak examples, you will also stumble across flatpak-builder, which sounds a lot like flatpak-build but is a separate tool. Flatpak-builder accesses information from a JSON file that you created up front and builds a flatpak automatically.

The JSON file essentially contains the same information that you previously passed into flatpak-build at the command line. The tool thus extends the principle to include a kind of batch mode, which proves to be very useful in practice (Figure 2).

Figure 2: You can generate a Flatpak using a JSON file.

Exploring Snap

Unlike Flatpak, Snap is not targeted at desktop users. Canonical instead envisions Snap as a tool for revolutionizing the way enterprise applications reach servers. Like Flatpak, Snap relies on the kernel's various isolation techniques, combining them so applications run in their own isolated environments.

The structure of a snap is not fundamentally different from that of a flatpak, but the terms have different names. Instead of a runtime, Snap refers to a framework, which is a basic set of libraries on which Snap can build.

Unlike Flatpack, Snap is not aimed at general-purpose systems. Canonical is developing a basic system known as the Snappy Ubuntu Core [13] that offers different frameworks designed to work with the Snap package system (Figure  3).

Figure 3: Snap system is designed to work with Snappy Ubuntu Core.