Developing Elements

Conform to the following conventions:

  • Use the environment for overridable defaults, prefixing environment variable names with DIB_. For example:


    If you do not use the DIB prefix you may find that your overrides are discarded as the build environment is sanitised.

  • Consider that your element co-exists with many others and try to guard against undefined behaviours. Some examples:

    • Two elements use the source-repositories element, but use the same filename for the source-repositories config file. Files such as these (and indeed the scripts in the various .d directories listed below) should be named such that they are unique. If they are not unique, when the combined tree is created by disk-image-builder for injecting into the build environment, one of the files will be overwritten.
    • Two elements copy different scripts into /usr/local/bin with the same name. If they both use set -e and cp -n then the conflict will be caught and cause the build to fail.
  • If your element mounts anything into the image build tree ($TMP_BUILD_DIR) then it will be automatically unmounted when the build tree is unmounted - and not remounted into the filesystem image - if the mount point is needed again, your element will need to remount it at that point.

  • If caching is required, elements should use a location under $DIB_IMAGE_CACHE.

  • Elements should allow for remote data to be cached. When $DIB_OFFLINE is set, this cached data should be used if possible. See the Global image-build variables section of this document for more information.

  • Elements in the upstream diskimage-builder elements should not create executables which run before 10- or after 90- in any of the phases if possible. This is to give downstream elements the ability to easily make executables which run after our upstream ones.

Phase Subdirectories

Make as many of the following subdirectories as you need, depending on what part of the process you need to customise. The subdirectories are executed in the order given here. Scripts within the subdirectories should be named with a two-digit numeric prefix, and are executed in numeric order.

Only files which are marked executable (+x) will be run, so other files can be stored in these directories if needed. As a convention, we try to only store executable scripts in the phase subdirectories and store data files elsewhere in the element.

The phases are:

  1. root.d
  2. extra-data.d
  3. pre-install.d
  4. install.d
  5. post-install.d
  6. block-device.d
  7. finalise.d
  8. cleanup.d

Create or adapt the initial root filesystem content. This is where alternative distribution support is added, or customisations such as building on an existing image.

Only one element can use this at a time unless particular care is taken not to blindly overwrite but instead to adapt the context extracted by other elements.

  • runs: outside chroot
  • inputs:
    • $ARCH=i386|amd64|armhf|arm64
    • $TARGET_ROOT=/path/to/target/workarea

Pull in extra data from the host environment that hooks may need during image creation. This should copy any data (such as SSH keys, http proxy settings and the like) somewhere under $TMP_HOOKS_PATH.

  • runs: outside chroot
  • inputs: $TMP_HOOKS_PATH
  • outputs: None

Run code in the chroot before customisation or packages are installed. A good place to add apt repositories.

  • runs: in chroot

Runs after pre-install.d in the chroot. This is a good place to install packages, chain into configuration management tools or do other image specific operations.

  • runs: in chroot

Run code in the chroot. This is a good place to perform tasks you want to handle after the OS/application install but before the first boot of the image. Some examples of use would be:

Run chkconfig to disable unneeded services

Clean the cache left by the package manager to reduce the size of the image.

  • runs: in chroot

Customise the block device that the image will be made on (for example to make partitions). Runs after the target tree has been fully populated but before the cleanup.d phase runs.

  • runs: outside chroot
  • inputs:
    • $IMAGE_BLOCK_DEVICE={path}
    • $TARGET_ROOT={path}
  • outputs: $IMAGE_BLOCK_DEVICE={path}

Perform final tuning of the root filesystem. Runs in a chroot after the root filesystem content has been copied into the mounted filesystem: this is an appropriate place to reset SELinux metadata, install grub bootloaders and so on.

Because this happens inside the final image, it is important to limit operations here to only those necessary to affect the filesystem metadata and image itself. For most operations, post-install.d is preferred.

  • runs: in chroot

Perform cleanup of the root filesystem content. For instance, temporary settings to use the image build environment HTTP proxy are removed here in the dpkg element.

  • runs: outside chroot
  • inputs:
    • $ARCH=i386|amd64|armhf|arm64
    • $TARGET_ROOT=/path/to/target/workarea

Other Subdirectories

Elements may have other subdirectories that are processed by specific elements rather than the diskimage-builder tools themselves.

One example of this is the bin directory. The rpm-distro, dpkg and opensuse elements install all files found in the bin directory into /usr/local/bin within the image as executable files.

Environment Variables

To set environment variables for other hooks, add a file to your element environment.d. This directory contains bash script snippets that are sourced before running scripts in each phase. Note that because environment includes are sourced together, they should not set global flags like set -x because they will affect all preceeding imports.


Each element can use the following files to define or affect dependencies:

A plain text, newline separated list of elements which will be added to the list of elements built into the image at image creation time.

A plain text, newline separated list of elements which are provided by this element. These elements will be excluded from elements built into the image at image creation time.

For example if element A depends on element B and element C includes element B in its element-provides file and A and C are included when building an image, then B is not used.

Operating system elements

Some elements define the base structure for an operating system – for example, the opensuse element builds a base openSUSE system. Such elements have more requirements than the other elements:

  • they must have operating-system in their element-provides, so this indicates they are an “operating system”.
  • they must export the DISTRO_NAME environment variable with the name of the distribution built, using an environment.d script. For example, the opensuse element exports DISTRO_NAME=opensuse.

Ramdisk Elements

Ramdisk elements support the following files in their element directories:

Text files listing executables required to be fed into the ramdisk. These need to be present in $PATH in the build chroot (i.e. need to be installed by your elements as described above).
POSIX shell script fragments that will be appended to the default script executed as the ramdisk is booted (/init).
Called to copy files into the ramdisk. The variable $TMP_MOUNT_PATH points to the root of the tree that will be packed into the ramdisk.
udev rules files that will be copied into the ramdisk.

Element coding standard

  • lines should not include trailing whitespace.
  • there should be no hard tabs in the file.
  • indents are 4 spaces, and all indentation should be some multiple of them.
  • do and then keywords should be on the same line as the if, while or for conditions.

Global image-build variables

This is always set. When not empty, any operations that perform remote data access should avoid it if possible. If not possible the operation should still be attempted as the user may have an external cache able to keep the operation functional.
This contains the UUID of the root filesystem, when diskimage-builder is building a disk image. This works only for ext filesystems.
Path to where cached inputs to the build process are stored. Defaults to ~/.cache/image_create.

Structure of an element

The above-mentioned global content can be further broken down in a way that encourages composition of elements and reusability of their components. One possible approach to this would be to label elements as either a “driver”, “service”, or “config” element. Below are some examples.

  • Driver-specific elements should only contain the necessary bits for that driver:

          init           - modprobe line
             10-mlx      - package installation
  • An element that installs and configures Nova might be a bit more complex, containing several scripts across several phases:

          source-repository-nova - register a source repository
             50-my-ppa           - add a PPA
             10-user             - common Nova user accts
             50-my-pack          - install packages from my PPA
             60-nova             - install nova and some dependencies
  • In the general case, configuration should probably be handled either by the meta-data service (eg, o-r-c) or via normal CM tools (eg, salt). That being said, it may occasionally be desirable to create a set of elements which express a distinct configuration of the same software components.

In this way, depending on the hardware and in which availability zone it is to be deployed, an image would be composed of:

  • zero or more driver-elements
  • one or more service-elements
  • zero or more config-elements

It should be noted that this is merely a naming convention to assist in managing elements. Diskimage-builder is not, and should not be, functionally dependent upon specific element names.

diskimage-builder has the ability to retrieve source code for an element and place it into a directory on the target image during the extra-data phase. The default location/branch can then be overridden by the process running diskimage-builder, making it possible to use the same element to track more then one branch of a git repository or to get source for a local cache. See source-repositories for more information.

Finding other elements

DIB exposes an internal $IMAGE_ELEMENT_YAML variable which provides elements access to the full set of included elements and their paths. This can be used to process local in-element files across all the elements (pkg-map for example).

import os
import yaml

elements = yaml.load(os.getenv('IMAGE_ELEMENT_YAML'))
for element, path in elements:

For elements written in Bash, there is a function get_image_element_array that can be used to instantiate an associative-array of elements and paths (note arrays can not be exported in bash).

# note eval to expand the result of the get function
eval declare -A image_elements=($(get_image_element_array))
for i in ${!image_elements[$i]}; do

Debugging elements

Export break to drop to a shell during the image build. Break points can be set either before or after any of the hook points by exporting “break=[before|after]-hook-name”. Multiple break points can be specified as a comma-delimited string. Some examples:

  • break=before-block-device-size will break before the block device size hooks are called.
  • break=before-pre-install will break before the pre-install hooks.
  • break=after-error will break after an error during an in target hookpoint.

The manifests element will make a range of manifest information generated by other elements available for inspection inside and outside the built image. Environment and command line arguments are captured as described in the documentation and can be useful for debugging.

Images are built such that the Linux kernel is instructed not to switch into graphical consoles (i.e. it will not activate KMS). This maximises compatibility with remote console interception hardware, such as HP’s iLO. However, you will typically only see kernel messages on the console - init daemons (e.g. upstart) will usually be instructed to output to a serial console so nova’s console-log command can function. There is an element in the tripleo-image-elements repository called “remove-serial-console” which will force all boot messages to appear on the main console.

Ramdisk images can be debugged at run-time by passing troubleshoot as a kernel command line argument, or by pressing “t” when an error is reached. This will spawn a shell on the console (this can be extremely useful when network interfaces or disks are not detected correctly).

Testing Elements

An element can have functional tests encapsulated inside the element itself. The tests can be written either as shell or python unit tests.


In order to create a test case, follow these steps:

  • Create a directory called test-elements inside your element.

  • Inside the test-elements directory, create a directory with the name of your test case. The test case directory should have the same structure as an element. For example:

  • Assert state during each of the element build phases you would like to test. You can exit 1 to indicate a failure.

  • To exit early and indicate a success, touch a file /tmp/dib-test-should-fail in the image chroot, then exit 1.

Tests are run with tools/ Running -l will show available tests (the example above would be called apt-sources/test-case-1, for example). Specify your test (or a series of tests as separate arguments) on the command line to run it. If it should not be run as part of the default CI run, you can submit a change with it added to DEFAULT_SKIP_TESTS in that file.

Running the functional tests is time consuming. Multiple parallel jobs can be started by specifying -j <job count>. Each of the jobs uses a lot resources (CPU, disk space, RAM) - therefore the job count must carefully be chosen.


To run functional tests locally, install and start docker, then use the following tox command:

tox -efunc

Note that running functional tests requires sudo rights, thus you may be asked for your password.

To run functional tests for one element, append its name to the command:

tox -efunc ironic-agent

Additionally, elements can be tested using python unittests. To create a a python test:

  • Create a directory called tests in the element directory.
  • Create an empty file called to make it into a python package.
  • Create your test files as test\, using regular python test code.

To run all the tests use testr - testr run. To run just some tests provide one or more regex filters - tests matching any of them are run - testr run apt-proxy.

Third party elements

Additional elements can be incorporated by setting ELEMENTS_PATH, for example if one were building tripleo-images, the variable would be set like:

export ELEMENTS_PATH=tripleo-image-elements/elements
disk-image-create rhel7 cinder-api


You should always run bin/dib-lint over your elements. It will warn you of common issues.


Using sudo outside the chroot environment can cause breakout issues where you accidentally modify parts of the host system. dib-lint will warn if it sees sudo calls that do not use the path arguments given to elements running outside the chroot.

To disable the error for a call you know is safe, add

# dib-lint: safe_sudo

to the end of the sudo command line. To disable the check for an entire file, add

# dib-lint: disable=safe_sudo