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The output might be written to the pipe by `flatpak --help` and/or read
from the pipe by `head -2` in more than one batch. If `head -2` reads
the first two lines before `flatpak --help` has written everything, it
will exit, causing the pipe to have no process at the read end. This
results in `flatpak --help` being killed by `SIGPIPE` next time it tries
to write to the pipe, because it has not opted out of this behaviour
(as shell tools usually shouldn't).

We're running under `set -o pipefail`, so this causes a nonzero exit
status that makes the test fail. Worse, this failure is intermittent,
because `head -2` *usually* doesn't exit until `flatpak --help` has
already written out everything it is going to write - it depends on
the precise behaviour of read(), write() and kernel scheduling.

We know that `flatpak --help` output is not *that* long, so it's OK
for `flatpak --help` not to be terminated early: we can send it all
into an intermediate file, and then run `head` on the file.

Signed-off-by: Simon McVittie <>

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Flatpak icon

Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux.

See for more information.

Community discussion happens in #flatpak on Freenode, on the mailing list, and on the Flathub Discourse.

Read documentation for Flatpak here.


Flatpak welcomes contributions from anyone! Here are some ways you can help:


Flatpak uses a traditional autoconf-style build mechanism. To build just do

 ./configure [args]
 make install

To automatically install dependencies on apt-based distributions you can try running apt build-dep flatpak and on dnf ones try dnf builddep flatpak. Dependencies you will need include: autoconf, automake, libtool, bison, gettext, gtk-doc, gobject-introspection, libcap, libarchive, libxml2, libsoup, gpgme, polkit, libXau, ostree, json-glib, appstream, libseccomp (or their devel packages).

Most configure arguments are documented in ./configure --help. However, there are some options that are a bit more complicated.

Flatpak relies on a project called Bubblewrap for the low-level sandboxing. By default, an in-tree copy of this is built (distributed in the tarball or using git submodules in the git tree). This will build a helper called flatpak-bwrap. If your system has a recent enough version of Bubblewrap already, you can use --with-system-bubblewrap to use that instead.

Bubblewrap can run in two modes, either using unprivileged user namespaces or setuid mode. This requires that the kernel supports this, which some distributions disable. For instance, Debian and Arch (linux kernel v4.14.5 or later), support user namespaces with the kernel.unprivileged_userns_clone sysctl enabled.

If unprivileged user namespaces are not available, then Bubblewrap must be built as setuid root. This is believed to be safe, as it is designed to do this. Any build of Bubblewrap supports both unprivileged and setuid mode, you just need to set the setuid bit for it to change mode.

However, this does complicate the installation a bit. If you pass --with-priv-mode=setuid to configure (of Flatpak or Bubblewrap) then make install will try to set the setuid bit. However that means you have to run make install as root. Alternatively, you can pass --enable-sudo to configure and it will call sudo when setting the setuid bit. Alternatively you can enable setuid completely outside of the installation, which is common for example when packaging Bubblewrap in a .deb or .rpm.

There are some complications when building Flatpak to a different prefix than the system-installed version. First of all, the newly built Flatpak will look for system-installed flatpaks in $PREFIX/var/lib/flatpak, which will not match existing installations. You can use --with-system-install-dir=/var/lib/flatpak to make both installations use the same location.

Secondly, Flatpak ships with a root-privileged PolicyKit helper for system-wide installation, called flatpak-system-helper. It is D-Bus activated (on the system bus) and if you install in a non-standard location it is likely that D-Bus will not find it and PolicyKit integration will not work. However, if the system installation is synchronized, you can often use the system installed helper instead— at least if the two versions are close enough.

This repository

The Flatpak project consists of multiple pieces, and it can be a bit challenging to find your way around at first. Here is a quick intro to the major components of the flatpak repo:

  • common: contains the library, libflatpak. It also contains various pieces of code that are shared between the library, the client and the services. Non-public code can be recognized by having a -private.h header file.
  • app: the commandline client. Each command has a flatpak-builtins- source file
  • data: D-Bus interface definition files
  • session-helper: The flatpak-session-helper service, which provides various helpers for the sandbox setup at runtime
  • system-helper: The flatpak-system-helper service, which runs as root on the system bus and allows non-root users to modify system installations
  • portal: The Flatpak portal service, which lets sandboxed apps request the creation of new sandboxes
  • doc: The sources for the documentation, both man pages and library documentation
  • tests: The testsuite
  • bubblewrap: Flatpak's unprivileged sandboxing tool which is developed separately and exists here as a submodule
  • libglnx: a small utility library for projects that use GLib on Linux, as a submodule
  • dbus-proxy: a filtering proxy for D-Bus connections, as a submodule
  • icon-validator: A small utility that is used to validate icons
  • revokefs: A fuse filesystem that is used to transfer files to the system-helper without copying