Parts, alongside plugins, are a key component in any Snapcraft project.
See Adding parts for a general overview of what parts are and how to use them and Scriptlets for details on how they can be scripted outside of snapcraft.yaml.
All parts within a project, by means of the logic encoded the plugins they’re using, all go through the same series of steps. Knowing these steps, and which directories are used for each step, can help when creating more advanced snaps, and when troubleshooting build issues.
The steps a part goes through are as follows:
source-*
keywords of a part to specify which components to retrieve. If source
points to a git repository, for example, the pull step will clone that repository.plugin
of a part specifies how it is constructed. The meson
plugin, for example, executes meson
and ninja
to compile source code. Each part is built in a separate directory, but it can use the contents of the staging area if it specifies a dependency on other parts using the after
keyword. See Step dependencies for more information.stage
keyword to enable or block files coming from the part. You can also use this keyword to filter out files that are not required in the snap itself, for example build files specific to a single part.prime
step exists because the staging area might still contain files that are required for the build but not for the snap. For example, if you have a part that downloads and installs a compiler, then you stage this part so other parts can use the compiler during building. You can then use the prime
filter keyword to make sure that it doesn’t get copied to the priming area, so it’s not taking up space in the snap. Some extra checks are also run during this step to ensure that all dependencies are satisfied for a proper run time. If confinement was set to classic
, then files will be scanned and, if needed, patched to work with this confinement mode.Finally, pack takes the entire contents of the prime
directory and packs it into a snap.
Each of these lifecycle steps can be run from the command line, and the command can be part specific or apply to all parts in a project.
snapcraft pull [<part-name>]
snapcraft build [<part-name>]
snapcraft stage [<part-name>]
snapcraft prime [<part-name>]
snapcraft pack
or snapcraft
Note that each command also executes the previous lifecycle steps, so snapcraft
executes all the lifecycle steps chained together.
To access the part environment at any stage, add the --shell
argument. For example, snapcraft prime --shell
will run up to the prime step and open a shell. See Iterating over a build for more details.
Each lifecycle step depends on the completion of the previous step for that part, so to reach a desired step, all prior steps need to have successfully run. By default, snapcraft
runs the same lifecycle step of all parts before moving to the next step. However, you can change this behavior using the after
keyword in the definition of a part in snapcraft.yaml
. This creates a dependency chain from one part to another.
grv:
plugin: go
go-channel: 1.11/stable
after:
- libgit2
In the above example, the part named grv
will be built after the part named libgit2
has been successfully built and staged.
Each plugin defines the default actions that happen during a step. This behavior can be changed in two ways.
override-<step-name>
in snapcraft.yaml
. See Overriding steps for more details.See Parts environment variables for a list of part-specific environment variables that can be accessed to help build a part.
When running through its build steps, a part will use different working directories. These closely follow the step names for the lifecycle.
The directories are specified by their environment variables:
CRAFT_
SNAPCRAFT_
but are otherwise identicalSee Parts environment variables for more details.
Environment variable | Directory | Purpose |
---|---|---|
CRAFT_PART_SRC |
parts/<part-name>/src |
the location of the source during the pull step |
CRAFT_PART_BUILD |
parts/<part-name>/build |
the working directory during the build step |
CRAFT_PART_INSTALL |
parts/<part-name>/install |
contains the results of the build step and the stage packages. |
CRAFT_STAGE |
stage |
shared by all parts, this directory contains the development libraries, headers, and other components (e.g.; pkgconfig files) that need to be accessible from other parts |
CRAFT_PRIME |
prime |
shared by all parts, this directory holds the final components for the resulting snap. |
The following table gives an overview of which directories each step uses.
Step | Explanation |
---|---|
pull | downloads and retrieves the sources specified by the source key and puts them in CRAFT_PART_SRC
|
build | builds the sources in CRAFT_PART_BUILD and places the result in CRAFT_PART_INSTALL |
organize | renames built files in CRAFT_PART_INSTALL |
stage | copies built files from CRAFT_PART_INSTALL to the shared CRAFT_STAGE |
prime | copies the staged files from the shared CRAFT_STAGE to the shared CRAFT_PRIME |
pack | packs contents of CRAFT_PRIME into a snap and puts the snap in CRAFT_PROJECT_DIR |
Last updated 1 year, 15 days ago.