How to create a package slice for Chisel

If your package doesn’t yet have the slice definitions you actually need to create your own slice definition (which you can, later on, propose to be merged upstream for everyone else to use How to publish a slice definition ).

Let’s assume you are trying to create a slice definition for installing the OpenSSL binary into your rock!

Make sure the slice definition doesn’t exist yet

To avoid re-creating a slice, check the following to see if something that fits your needs already exists:

  1. Look into the upstream chisel-releases repository

  2. Switch to the branch corresponding to the desired Ubuntu release for your rock

  3. Search your package name within the list of slice definitions files

    • if you find it, open it and try to find a slice name containing the bits and pieces you need from that package

Structure of a slice definitions file

There can be only one slice definitions file for each Ubuntu package. All of the slice definitions files follow the same structure:

# (req) Name of the package.
# The slice definition file should be named accordingly (eg. "openssl.yaml")

package: <package-name>

# (req) List of slices

    # (req) Name of the slice

        # (opt) Optional list of slices that this slice depends on
          - <pkgA_slice-name>
          - ...

        # (req) The list of files, from the package, that this slice will install
            </path/to/moved/content>: {copy: /bin/original}
            </path/to/link>: {symlink: /bin/mybin}
            </path/to/new/dir>: {make: true}
            </path/to/file/with/text>: {text: "Some text"}
            </path/to/mutable/file/with/default/text>: {text: FIXME, mutable: true}
            </path/to/temporary/content>: {until: mutate}

        # (opt) Mutation scripts, to allow for the reproduction of maintainer scripts,
        # based on Starlark (
        mutate: |

Find the dependencies of your package

Find the dependencies of the package for which you want to create a new slice definition (openssl in this guide) with this command:

apt show openssl

The output will be similar to:

package: openssl
Version: 3.0.2-0ubuntu1.7
Origin: Ubuntu
Maintainer: Ubuntu Developers <[email protected]>
Original-Maintainer: Debian OpenSSL Team <[email protected]>
Depends: libc6 (>= 2.34), libssl3 (>= 3.0.2-0ubuntu1.2)

From the above output, you can confirm that openssl depends on libc6 and libssl3. So when creating your slice definitions file for OpenSSL, you will need to remember to include those packages’ slices as a dependency as well, whenever needed. Let’s do that in the following section.

Create your slice definition

You now have everything needed to actually define the OpenSSL slice that will install the content you are looking to have in the final rock. Since you are looking to install just the OpenSSL binaries from the openssl package, what about naming this new slice bins? Let’s go for it:

  1. What is the name of your slice definitions file? It is a YAML file called openssl.yaml

  2. What package name should be defined inside this file? The package name is openssl

  3. What is your slice name? It should be called bins

  4. What contents do you need from the OpenSSL package? Just the binaries - /usr/bin/c_rehash and /usr/bin/openssl

  5. Does your slice depend on any other package slice? Yes, OpenSSL depends on libc6 and libssl3

    • Do these two packages have slice definitions files upstream? Yes, there is already a slice definitions file for libc6 and another one for libssl3. If these dependencies were not present in the upstream Chisel release, you would also need to create their corresponding slice definitions

    • Which slices do you depend on then? Since you only want the OpenSSL binaries, you might only need the libraries from libc6 and libssl3, as well as the configuration files from libc6 and openssl themselves.

Create a new YAML file named openssl.yaml, with the following content:

package: openssl
          - libc6_libs
          - libc6_config
          - libssl3_libs
          - openssl_config


Notice the unforeseen new slice config. Because your OpenSSL binaries depend on the OpenSSL configuration files, and those were not yet present anywhere in the Chisel releases upstream, you also need to create that slice! You may also ask “why not put those configuration files inside the “bins” slice”? You could! But we recommend, as a best practice, to separate and group contents according to their nature, as you may tomorrow need to create a new slice definition that only needs the OpenSSL configurations and not the binaries.

And that’s it. This is your brand new slice definitions file, which will allow Chisel to install just the OpenSSL binaries (and their dependencies) into your rock! To learn about how to actually use this new slice definition file and publish it upstream for others to use, please check the following guides.