mkosi — Build Bespoke OS Images

A fancy wrapper around dnf --installroot, apt, pacman and zypper that generates customized disk images with a number of bells and whistles.

For a longer description and available features and options, see the man page or run mkosi documentation. When getting started see the EXAMPLES section.

Installation

You can install mkosi from your distribution using its package manager or install the development version from git. If you install mkosi using your distribution's package manager, make sure it installs at least mkosi v16 or newer (Use mkosi --version to check). If your distribution only packages an older version of mkosi, it is recommended to install mkosi using one of the alternative installation methods listed below instead.

Running mkosi from the repository

To run mkosi straight from its git repository, you can invoke the shim bin/mkosi. The MKOSI_INTERPRETER environment variable can be set when using the bin/mkosi shim to configure the python interpreter used to execute mkosi. The shim can be symlinked to e.g. ~/.local/bin to make it accessible from the PATH. Note that to make this work you might have to add ~/.local/bin to your user's PATH.

git clone https://github.com/systemd/mkosi
ln -s $PWD/mkosi/bin/mkosi ~/.local/bin/mkosi
mkosi --version

mkosi also provides other companion tools that can be enabled in a similar manner.

ln -s $PWD/mkosi/bin/mkosi-addon ~/.local/bin/mkosi-addon
ln -s $PWD/mkosi/bin/mkosi-initrd ~/.local/bin/mkosi-initrd
ln -s $PWD/mkosi/bin/mkosi-sandbox ~/.local/bin/mkosi-sandbox

Python installation methods

mkosi can also be installed straight from the git repository url using pipx:

pipx install git+https://github.com/systemd/mkosi.git
mkosi --version

which will transparently install mkosi into a Python virtual environment and a mkosi binary to ~/.local/bin. This is, up to the path of the virtual environment and the mkosi binary, equivalent to

python3 -m venv mkosivenv
mkosivenv/bin/pip install git+https://github.com/systemd/mkosi.git
mkosivenv/bin/mkosi --version

You can also package mkosi as a zipapp that you can deploy anywhere in your PATH. Running this will leave a mkosi binary in builddir/

git clone https://github.com/systemd/mkosi
cd mkosi
tools/generate-zipapp.sh
builddir/mkosi --version

Besides the mkosi binary, you can also call mkosi via

python3 -m mkosi

when not installed as a zipapp.

Please note, that the python module exists solely for the usage of the mkosi binary and is not to be considered a public API.

Installing packages from upstream repositories

mkosi packages for Debian, Ubuntu, Fedora and SUSE are built from latest main and published as repositories for the respective distributions on OBS. Follow these instructions to enable the appropriate repository.

kernel-install plugins

mkosi can also be used as a kernel-install plugin to build initrds and addons. It is recommended to use only one of these two plugins at a given time.

UKI plugin

To enable this feature, install kernel-install/50-mkosi.install into /usr/lib/kernel/install.d. Extra distro configuration for the initrd can be configured in /usr/lib/mkosi-initrd. Users can add their own customizations in /etc/mkosi-initrd. A full self-contained UKI will be built and installed.

Once installed, the mkosi plugin can be enabled by writing initrd_generator=mkosi-initrd and layout=uki to /usr/lib/kernel/install.conf or to /etc/kernel/install.conf.

Addon plugin

To enable this feature, install kernel-install/51-mkosi-addon.install into /usr/lib/kernel/install.d. Extra distro configuration for the addon can be configured in /usr/lib/mkosi-addon. Users can add their own customizations in /etc/mkosi-addon and /run/mkosi-addon. Note that unless at least one of the last two directories are present, the plugin will not operate.

This plugin is useful to enhance a vendor-provided UKI with local-only modifications.

Hacking on mkosi

To hack on mkosi itself, you can run the full test suite locally, just like CI does. The tests include linting, type checking, and unit tests, all runnable via pytest.

All linters such as ruff or mypy are run inside mkosi box (i.e. from inside mkosi.tools/) for a consistent environment. Build that with:

bin/mkosi -f box -- true

Then run the full test suite inside the tools tree:

bin/mkosi box -- pytest

You can use pytest options to only run a subset, for example only run the linters:

bin/mkosi box -- pytest -k test_linters

Installation tests (venv, editable and zipapp installs) are marked with the install marker and are skipped by default, as they create virtual environments and install packages from the network. Run them explicitly with:

bin/mkosi box -- pytest -m install

When a tool that mkosi runs inside its sandbox fails, see Debugging failing sandboxed commands for how to replay the command by hand.

Integration tests

Integration tests build and boot full images. They are marked with the integration marker, and are skipped by default. They need a tools tree and an image to be built first. tools/integration-test-setup.sh writes a local configuration for the given image and tools tree distribution and builds both:

tools/integration-test-setup.sh $DISTRIBUTION $TOOLS_TREE_DISTRIBUTION

For example, to test an Arch image built with a Fedora tools tree, then run a single integration test:

tools/integration-test-setup.sh arch fedora

bin/mkosi box -- pytest -m integration --distribution arch --capture=no --verbose \
    'tests/test_boot.py::test_bootloader[systemd-boot]'

The integration tests require KVM and are skipped (or very slow) without /dev/kvm.

References

• Primary mkosi git repository on GitHub
• A re-introduction to mkosi — A Tool for Generating OS Images
• The mkosi OS generation tool story on LWN (2017)
• systemd-repart: Building Discoverable Disk Images and mkosi: Building Bespoke Operating System Images talks at All Systems Go! 2023
• Building RHEL and RHEL UBI images with mkosi an article in Fedora Magazine (2023)
• Building USIs with mkosi
• Constellation 💖 mkosi — Minimal TCB, tailor-made for measured boot
• Streamlining kernel hacking with mkosi-kernel
• mkosi-initrd: Building initrds out of distribution packages
• Running systemd integration tests with mkosi
• Arch Linux rescue image with mkosi
• Building vagrant images with mkosi
• Building images with mkosi on the Open Build Service (OBS)

Community

Find us on Matrix at #mkosi:matrix.org.