Base snaps

A base snap is a special kind of snap that provides a run-time environment with a minimal 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.

Supported base snaps

There are currently five supported bases:

• core22: the newest base, built from Ubuntu 22.04 LTS.
• core20: built from Ubuntu 20.04 LTS.
• core18: the previous standard base for snap building, based on Ubuntu 18.04 LTS.
• core: based on Ubuntu 16.04 ESM, not to be confused with core16 (see below).
• bare: an empty base that’s useful with fully statically linked snaps and when testing.

Older releases of core were occasionally referred to as core 16, but core and core16 are now two distinct packages.

In most Ubuntu bases (except core), snapd and its associated tools are provided by their relevant snaps.

Defining a base

Bases are defined by adding the base keyword to a snap’s snapcraft.yaml followed by the base name.

For example, to specify core18, use the following:

base: core18

To specify core22, use the following

base: core22

Snapcraft no longer supports building snaps with the older core base. Snapcraft 4 needs to be used instead. Snapcraft 4 is still being supported and can be installed from Snapcraft’s 4.x track:

$ snap install snapcraft --channel 4.x

For snaps using core, we highly recommend reading Snapcraft and Extended Security Maintenance for essential support details.

The base snap mounts itself as the root filesystem within your snap such that when your application runs, the base’s library paths are searched directly after the paths for your specific snap.

Choosing a base

core22 is the currently recommended base for the majority of snaps. But 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 with core22 then core20 is a valid and supported alternative, as is the older core18.

Snapcraft extensions are a great way to easily bundle a set of common requirements into a snap, such as for running KDE Plasma or GNOME applications, but you need to select a base that’s supported by the extension you require. See Supported extensions for a list of which extensions support which bases.

Base support was added with the release of Snapcraft 3. As noted above, snaps created before this, and snaps not using the base: keyword, can still be built but they cannot use specific new features. Instead, snaps built without bases inherit attributes from their respective build environments.

Snaps that don’t use bases can often migrate to one without too much difficulty. See Upgrading snapcraft for more details on potential differences.

Building a base snap

While it is possible to build your own base snap, its publisher needs to take responsibility for its 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 pro-active

Base snaps can be either bootable or non-bootable. The former needs to include systemd while the latter can be leaner.

build-base

The base keyword on its own does not not take into account the creation of bases. Instead, with older versions of snapcraft, the name keyword was arbitrarily used to determine the build environment:

name: core18
type: base
# base: is not set elsewhere 

The above example uses name to specify the creation of an Ubuntu 18.04 (core18) based build environment.

But the above fails if a base has yet to be bootstrapped, or is otherwise unavailable. For example, the following will currently generate a `launch failed: Unable to find an image matching “futurecore” error:

name: futurecore
type: base
# base: is not set elsewhere 

In cases like the above, where the base has not yet been bootstrapped, the build-base keyword should be used to explicitly define the base to use for the build environment.

To solve the above issue, for example, use the following:

name: futurecore
type: base
build-base: core18
# base: is not set elsewhere 

Last updated 4 months ago.