Bases
=====

.. include:: /reuse/bases-intro.rst

``base`` and ``build-base``
---------------------------

If ``build-base`` is defined , then the ``build-base`` will determine the
environment where the snap is built.

If ``build-base`` is not defined, the ``base`` will determine the
environment where the snap is built.

For example, ``base: core24`` will build a snap in a Ubuntu 24.04 environment
with the ``core24`` snap installed. Stage packages will be installed from the
24.04 repository.

``build-base: devel`` will build a snap using the upcoming Ubuntu release in
development. This is defined as the Ubuntu image with the ``devel`` alias in
the `Ubuntu buildd image server`_.

For ``base: bare`` snaps, a ``build-base`` is required to determine the feature
set, build environment, and ``snapcraft.yaml`` schema.

Base snaps
----------

A base snap is a special kind of snap that provides a run-time environment with
a cardinal set of libraries that are common to most applications. They’re
transparent to users, but they need to be considered and specified when building
a snap.

Mounting
--------

For strictly confined snaps, the base snap mounts itself as the root filesystem
within a snap's runtime environment. When an application runs, the base's
library paths are searched directly after the paths for that snap.

For classic confined snaps, the base snap is not mounted as the root
filesystem. The base snap is mounted as ``/snap/<base>/``, so the snap can
still load libraries from the base snap. For more information, see the
documentation for `classic confinement`_.

Choosing a base
---------------

``core24`` is the recommended base for most snaps. Much like choosing a
distribution base for a project or server, the best base for an application is
dependent on an application’s requirements and which plugins or extensions a
base supports. If there are specific dependencies that cannot be easily met
then the next newest base ``core22`` is a valid and supported alternative.

Snapcraft `extensions`_ enable bundling a set of common requirements into a
snap, such as for running KDE Plasma or GNOME applications. Extensions support
specific bases. See `supported extensions`_ for a list of which extensions
support which bases.

``bare`` is the recommended base for fully statically linked snaps because they
will not have access to a base snap when running. The snap will have a smaller
footprint at runtime because it does not require a base snap to be downloaded,
installed, and mounted.

.. _base-snap-explanation:

Building a base snap
--------------------

While uncommon, developers can build their own base snap. They are responsible
for maintenance and updates, in particular:

* bases need to be built from stable packages
* ABI compatibility cannot broken (ie. never replace symbols or libraries, and
  be strict)
* security updates must be proactive

`Ubuntu Core`_ systems need a base snap. These base snaps must be bootable and
include ``systemd``.


.. _`Ubuntu buildd image server`: https://cloud-images.ubuntu.com/buildd/daily/
.. _`classic confinement`: https://snapcraft.io/docs/classic-confinement
.. _`extensions`: https://snapcraft.io/docs/snapcraft-extensions
.. _`supported extensions`: https://snapcraft.io/docs/supported-extensions