UC-prepare plugin¶

The UC-prepare plugin creates the seed directory for Ubuntu Core images with the snap prepare-image command. This downloads snaps and their assertions from the Snap Store and prepares them to be installed during the initial boot.

The plugin can also install snaps and run their hooks when the image is being packed. This process, known as preseeding, reduces the initial boot time.

Keys¶

This plugin provides the following unique keys.

uc-prepare-model-assert¶

Type: string

Required

The path to the model assertion file that defines the target device and its required snaps.

The Ubuntu Core documentation details model assertions and their fields in the model.

uc-prepare-snaps¶

Type: list of strings

Snaps to seed into the image in addition to those required by the model assertion. Valid entries are:

a snap name
a snap name and channel in the format <snap-name> @ <channel> (spaces around @ are optional)
a path to a local snap within the project directory

If the model assertion has a grade of signed or secured, only snaps declared in the model assertion can be specified, and they can’t be referenced by a local path. This is commonly used to include optional snaps from the model assertion.

uc-prepare-channel¶

Type: string

The default store channel to fetch snaps from, overriding any channels in the model assertion. If this key is set, the model assertion’s grade must be set to dangerous.

This is overridden by snaps listed with a channel in uc-prepare-snaps.

uc-prepare-validation¶

Type: string

Default: enforce

Controls whether validation set constraints are enforced. Valid values are ignore and enforce.

uc-prepare-assertions¶

Type: list of strings

Additional assertion files to include in the image.

The Ubuntu Core documentation lists the available assertion types in Assertions.

uc-prepare-revisions¶

Type: string

Path to a manifest file specifying snap revisions to use.

Each line in a manifest file identifies a snap by name and revision number, like so:

core24 1587
snapd 26865

uc-prepare-write-revisions¶

Type: string or boolean

Default: False

If set to true, the plugin writes the resolved snap revisions to the seed.manifest file. If set to a file path, the revisions are written there instead.

uc-prepare-preseed¶

Type: boolean

Default: false

If set to true, the plugin preseeds snaps to improve the initial boot speed.

uc-prepare-preseed-sign-key¶

Type: string

The signing key for the preseed assertion. Requires uc-prepare-preseed to be set to true.

uc-prepare-apparmor-features-dir¶

Type: string

Path to the AppArmor features directory to use during preseeding.

This directory should be a snapshot of the /sys/kernel/security/apparmor/features directory from the target system. If this key is unset and uc-prepare-preseed is set to true, the directory from the host system is used.

uc-prepare-sysfs-overlay¶

Type: string

Path to a sysfs overlay directory for preseeding. Requires uc-prepare-preseed to be set to true.

Output¶

The seed content for the image is placed under system-seed. Place it in the desired partition with the organize key.

Example¶

The following snippet prepares snaps for an Ubuntu Core image as described by a model assertion file named model.assert.

parts:
  uc-seed:
    plugin: uc-prepare
    uc-prepare-model-assert: model.assert
    organize:
      "system-seed/*": (volume/disk/ubuntu-seed)/