Parts and Steps

Parts and steps are the basic data types craft-parts will work with. Together, they define the lifecycle of a project (i.e. how to process each step of each part in order to obtain the final primed result).


When the LifecycleManager is invoked, parts are defined in a dictionary under the parts key. If the dictionary contains other keys, they will be ignored.


Parts can declare read/write/execute permissions and ownership for the files they produce. This is achieved by adding a permissions subkey in the specific part:

# ...
    # ...
      - path: bin/my-binary
        owner: 1111
        group: 2222
        mode: "755"

The permissions subkey is a list of permissions definitions, each with the following keys:

  • path: a string describing the file(s) and dir(s) that this definition applies to. The path should be relative, and supports wildcards. This field is optional and its absence is equivalent to "*", meaning that the definition applies to all files produced by the part;

  • owner: an integer describing the numerical id of the owner of the files. This field is optional in the general case but mandatory if group is specified;

  • group: an integer describing the numerical id of the group for the files. The semantics are otherwise the same as owner, including being optional in the general case and mandatory if owner is specified;

  • mode: string describing the desired permissions for the files as a number in base 8. This field is optional.


Steps are used to establish plan targets and in informational data structures such as StepInfo. They are defined by the Step enumeration, containing entries for the lifecycle steps PULL, OVERLAY, BUILD, STAGE, and PRIME.

Step execution environment

Craft-parts defines the following environment for use during step processing and execution of user-defined scriptlets:

  • CRAFT_ARCH_TRIPLET: The the machine-vendor-os platform triplet definition.

  • CRAFT_TARGET_ARCH: The architecture we’re building for.

  • CRAFT_PARALLEL_BUILD_COUNT: The maximum number of concurrent build jobs to execute.

  • CRAFT_PROJECT_DIR: The path to the current project’s subtree in the filesystem.

  • CRAFT_PART_NAME: The name of the part currently being processed.

  • CRAFT_PART_SRC: The path to the part source directory. This is where sources are located after the PULL step.

  • CRAFT_PART_SRC_WORK: The path to the part source subdirectory, if any. Defaults to the part source directory.

  • CRAFT_PART_BUILD: The path to the part build directory. This is where parts are built during the BUILD step.

  • CRAFT_PART_BUILD_WORK: The path to the part build subdirectory in case of out-of-tree builds. Defaults to the part source directory.

  • CRAFT_PART_INSTALL: The path to the part install directory. This is where built artefacts are installed after the BUILD step.

  • CRAFT_OVERLAY: The path to the part’s layer directory during the OVERLAY step if overlays are enabled.

  • CRAFT_STAGE: The path to the project’s staging directory. This is where installed artefacts are migrated after the STAGE step.

  • CRAFT_PRIME: The path to the final primed payload directory after the PRIME step.

Step output directories

Some of the environment variables above reference directories that are the output locations for specific steps. These are repeated below for fast reference:

  • PULL:
    • CRAFT_PART_SRC locates the source of the part.

    • CRAFT_PART_SRC_WORK locates the source subdirectory if overridden.

    • CRAFT_OVERLAY locates the combined overlay output from all parts.

  • BUILD:
    • CRAFT_PART_INSTALL contains the location of the build output step. This directory is the expected location of CARGO_INSTALL_ROOT for Rust, GOBIN for go or DESTDIR for make.

  • STAGE:
    • CRAFT_STAGE contains the expected location of all staged outputs.

  • PRIME:
    • CRAFT_PRIME contains the path of the primed payload directory. This directory is shared by all parts.