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The stacker.yaml file

During a stacker build, the behavior of stacker is specified by yaml directives in a stacker.yaml file, as described in this document.

Substitutions and variables

In the yaml directives, stacker allows variable substitions of several forms. For example, these three substition statements:

$ONE ${{TWO}} ${{THREE:3}}

when run with stacker build --substitute ONE=1 --substitute TWO=2 are processed by stacker as:

1 2 3

In order to avoid conflict with bash or POSIX shells in the run section, only placeholders with two braces are supported, e.g. ${{FOO}}. Placeholders with a default value like ${{FOO:default}} will evaluate to their default if not specified on the command line or in a substitution file.

Using a ${{FOO}} placeholder without a default will result in an error if there is no substitution provided. If you want an empty string in that case, use an empty default: ${{FOO:}}.

In order to avoid confusion, it is also an error if a placeholder in the shell style ($FOO or ${FOO}) is found when the same key has been provided as a substitution either via the command line (for example, --substitute FOO=bar) or in a substitution file. An error will be printed that explains how to rewrite it. For example:

error "A=B" was provided as a substitution and unsupported placeholder "${A}" was found. Replace "${A}" with "${{A}}" to use the substitution.

Substitutions can also be specified in a yaml file given with the argument --substitute-file, with any number of key: value pairs:

ONE: 1
TWO: 2
FOO: bar
BAZ: bat

In addition to substitutions provided on the command line or in a file, the following variables are also available with their values from either command line flags or stacker-config file.

STACKER_STACKER_DIR config name 'stacker_dir', cli flag '--stacker-dir'-
STACKER_ROOTFS_DIR  config name 'rootfs_dir', cli flag '--roots-dir'
STACKER_OCI_DIR     config name 'oci_dir', cli flag '--oci-dir'

The stacker build environment has the following environment variables available for reference:

  • STACKER_LAYER_NAME: the name of the layer being built. STACKER_LAYER_NAME will be my-build when the run section below is executed.

      run: echo "Your layer is ${STACKER_LAYER_NAME}"

YAML directives


The from directive describes the base image that stacker will start from. It takes the form:

    type: $type
    url: $url
    tag: $tag
    insecure: true

Some directives are irrelevant depending on the type. Supported types are:

docker: url is required, insecure is optional. When insecure is specified, stacker attempts to connect via http instead of https to the Docker Hub.

tar: url is required, everything else is ignored.

oci: url is required, and must be a local OCI layout URI of the form oci:/local/path/image:tag

built: tag is required, everything else is ignored. built bases this layer on a previously specified layer in the stacker file.

scratch: which is an empty rootfs and can be used to host statically built binaries.


The imports directive describes what files should be made available in /stacker/imports during the run phase. There are three forms of importing supported today:


Will import a file or directory from the local filesystem. If the file or directory changes between stacker builds, it will be hashed and the new file will be imported on subsequent builds.

Will import foo.tar.gz and make it available in /stacker. Note that stacker will NOT update this file unless the cache is cleared, to avoid excess network usage. This means that updates after the first time stacker downloads the file will not be reflected.


Will grab /path/to/file from the previously built image $name.

import hash

Each entry in the imports directive also supports specifying the hash (sha256sum) of import source, for all the three forms presented above. For example:

  - path: config.json
    hash: f55af805b012017bc....
  - path:
    hash: b458dfd63e7883a64....
  - path: stacker://$name/path/to/file
    hash: f805b012017bc769a....
Before copying or downloading the file, stacker will check that the file's hash matches the given value. For file imports, the source file is hashed at build time. For HTTP imports, the value returned by the server in the X-Checksum-Sha256 HTTP header is checked first. If that value matches, the file is downloaded and then hashed and compared again.

stacker build supports the flag --require-hash, which will cause a build error if any HTTP(S) remote imports do not have a hash specified, in all transitively included stacker YAMLs.

If --require-hash is not passed, this import mode can be combined with unchecked imports, and only files that have the hash specified will be checked.

  - path: "config.json
    hash: "BEEFcafeaaaaAAAA...."
  - /path/to/file

import dest

The import directive also supports specifying the destination path (specified by dest) in the resulting container image, where the source file (specified by path) will be copied to. For example:

  - path: config.json
    dest: /

import (Deprecated)

The deprecated import directive operates like imports except that the entries in the import array will be placed into /stacker instead of /stacker/imports.

See for timeline and migration info.


This directive works only with OverlayFS backend storage.

The overlay_dirs directive describes the directories (content) from the host that should be available in the container's filesystem. It preserves all file/dirs attributes but not owner or group.

  - source: /path/to/directory
    dest: /usr/local/          ## optional arg, default is '/'
  - source: /path/to/directory2
This example will result in all the files/dirs from the host's /path/to/directory to be available under container's /usr/local/ and all the files/dirs from the host's /path/to/directory2 to be available under container's /

environment, labels, working_dir, volumes, cmd, entrypoint, user

These correspond exactly to the similarly named bits in the OCI image config spec, and are available for users to pass variables through to the runtime environment of the image.


The generate_labels entry is similar to run in that it contains a list of commands to run inside the generated rootfs. It runs after the run section is done, and its mutations to the filesystem are not recorded, except in one case: /oci-labels. /oci-labels is a special directory where this code can write a file, and the name of the file will be the OCI label name, and the content will be the label content.

build_env and build_env_passthrough

By default, environment variables do not pass through (pollute) the build environment.

build_env: A dictionary of environment variable definitions. The values will be present in the build's environment.

build_env_passthrough: A list of regular expressions that work as a filter on which environment variables should be passed through from the current env into the container. To let all variables through, simply set build_env_passthrough: [".*"]

If build_env_passthrough is not set, the default behavior is to allow through proxy variables HTTP_PROXY, HTTPS_PROXY, FTP_PROXY, http_proxy, https_proxy, ftp_proxy.

Values in the build_env override values passed through via build_env_passthrough.


Because of the odd behavior of cmd and entrypoint, and the inherited nature of these from previous stacker layers, full_command provides a way to set the full command that will be executed in the image, clearing out any previous cmd and entrypoint values that were set in the image.


build_only: indicates whether or not to include this layer in the final OCI image. This can be useful in conjunction with an import from this layer in another image, if you want to isolate the build environment for a binary but not include all of its build dependencies.


binds: specifies bind mounts from the host to the container. There are two formats:

    - /foo/bar -> /bar/baz
      - /zomg

The first format binds /foo/bar to /bar/baz, and the second binds host /zomg to container /zomg.

At this time there is no awareness of change for any of these bind mounts, so --no-cache should be used to re-build if the content of the bind mount has changed.


The config key is a special type of entry in the root of the stacker.yaml file. It cannot contain an image definition; its purpose is to provide configuration applicable for building all the images defined in this file. For example:

        - ../folder2/stacker.yaml
        - ../folder3/stacker.yaml


If the prerequisites list is present under the config key, stacker will make sure to build all the images in the stacker.yaml files found at the paths contained in the list. In this way, stacker supports building multiple stacker.yaml files in the correct order.

In this particular case, the parent folder of the current folder (let's call it parent) has 3 subfolders (folder1, folder2, and folder3), each containing a stacker.yaml file. The config example above is in parent/folder1/stacker.yaml.

When stacker build -f parent/folder1/stacker.yaml is invoked, stacker searches for the other two stacker.yaml files and builds them first before building the stacker.yaml file specified in the command line.


annotations is a user-specified key value map that will be included in the final OCI image. Note that these annotations are included in the image manifest itself and not as part of the index.json.

  a.b.c.key: abc_val
  p.q.r.key: pqr_val

While the config section supports a similar labels, it is more pertinent to the image runtime. On the other hand, annotations is intended to be image-specific metadata aligned with the annotations in the image spec.


The os key value is a user-specified string indicating which operating system this image is being built for. Examples include linux and darwin. This directive is optional and defaults to the host operating system if not specified.


The arch key value is a user-specified string indicating which machine architecture this image is being built for. Examples include amd64 and arm64. This directive is optional and defaults to the host machine architecture if not specified.

Last update: April 12, 2024