Pre-install snaps¶

When crafting your image, you may want to install and configure a default set of snaps. Instead of installing these snaps during the initial boot, it’s best to build the scaffolding for them directly into the image. How you do this depends on the type of image you’re crafting.

Because Ubuntu Core images are composed entirely of snaps, pre-installation is strongly recommended and dramatically reduces the initial boot time. If you’re crafting an Ubuntu Core image, read on.

For classic images, default snaps are an optional customization. If you’re crafting an Ubuntu Desktop or Ubuntu Server image and want to ship it with default snaps, skip ahead to Classic images.

Core images¶

Prepare your signing key¶

The snap daemon needs a signing key to pre-install snaps. If you haven’t registered a key or signed your model, complete the Sign the model tutorial in the Ubuntu Core documentation before continuing.

In your project directory, run the following command, replacing <key-name> with the name of the signing key you wish to use:

gpg --homedir ~/.snap/gnupg --export-secret-keys <key-name> sign.key

To make the signing key available in the build environment, copy the following part into your project file:

imagecraft.yaml¶

import-key:
  plugin: nil
  source: .
  override-build: |
    mkdir -p /home/ubuntu/.snap/gnupg
    chmod 700 /home/ubuntu/.snap/gnupg
    gpg --homedir /home/ubuntu/.snap/gnupg --import $CRAFT_PART_SRC/sign.key

Required interaction during packing

When packing, you’ll be prompted to unlock this signing key twice. If these prompts aren’t acknowledged within a few minutes, the packing process will time out and fail.

Pre-install model assertion snaps¶

If you haven’t already, copy your image’s model assertion into the directory holding your project file.

Next, copy the following part into your project file, replacing <assertion-name> and <key-name> with the name of your model assertion and signing key, respectively:

imagecraft.yaml¶

uc-seed:
  after: [import-key]
  plugin: uc-prepare
  source: .
  uc-prepare-model-assert: <assertion-name>
  uc-prepare-preseed: true
  uc-prepare-preseed-sign-key: <key-name>
  organize:
    "system-seed/*": (volume/disk/ubuntu-seed)/
  prime:
    - -kernel
    - -gadget
    - -resolved-content
    - -system-seed

This part prepares the scaffolding for the snaps in a system-seed/ directory, copies them to your image’s ubuntu-seed partition, and cleans up the original copies so they don’t cause conflicts.

If your model assertion contains optional snaps that you wish to install, list them with the uc-prepare-snaps key:

imagecraft.yaml¶

uc-seed:
  after: [import-key]
  plugin: uc-prepare
  source: .
  uc-prepare-model-assert: <assertion-name>
  uc-prepare-preseed: true
  uc-prepare-preseed-sign-key: <key-name>
  uc-prepare-snaps:
    - core22
    - hello-world@latest/edge
  organize:
    "system-seed/*": (volume/disk/ubuntu-seed)/
  prime:
    - -kernel
    - -gadget
    - -resolved-content
    - -system-seed

To tweak the snaps’ channels or revisions further, refer to the additional keys in the UC-prepare plugin reference. Configuration that deviates from the model assertion requires the model assertion’s grade to be dangerous.

Classic images¶

To pre-install snaps into your classic image, declare a part that uses the Snap-preseed plugin. This plugin prepares the scaffolding for the snaps in the part’s var/lib/snapd/seed/ directory, which you’ll need to copy into your image with the organize key.

For example, if you wanted to pre-install the core24 snap from the latest/stable channel and the hello-world snap from the latest/edge channel, your part would be declared as:

imagecraft.yaml¶

seed-snaps:
  plugin: snap-preseed
  snap-preseed-snaps:
    - core24
    - hello-world @ latest/edge
  organize:
    "var/*": (overlay)/var/

To tweak the snap channels or revisions further, refer to the additional keys in the Snap-preseed plugin reference.