Provided by: mkosi_5-1ubuntu0.20.04.1_all 
NAME
mkosi - Build Legacy-Free OS Images
SYNOPSIS
mkosi [options...] build
mkosi [options...] clean
mkosi [options...] summary
mkosi [options...] shell [command line...]
mkosi [options...] boot [command line...]
mkosi [options...] qemu
DESCRIPTION
mkosi is a tool for easily building legacy-free OS images. It’s a fancy wrapper around dnf --install‐
root, debootstrap, pacstrap and zypper that may generate disk images with a number of bells and whistles.
Supported output formats
The following output formats are supported:
• Raw GPT disk image, with ext4 as root (gpt_ext4)
• Raw GPT disk image, with xfs as root (gpt_xfs)
• Raw GPT disk image, with btrfs as root (gpt_btrfs)
• Raw GPT disk image, with squashfs as read-only root (gpt_squashfs)
• Plain squashfs image, without partition table, as read-only root (plain_squashfs)
• Plain directory, containing the OS tree (directory)
• btrfs subvolume, with separate subvolumes for /var, /home, /srv, /var/tmp (subvolume)
• Tarball (tar)
When a GPT disk image is created, the following additional options are available:
• A swap partition may be added in
• The image may be made bootable on EFI and BIOS systems
• Separate partitions for /srv and /home may be added in
• The root, /srv and /home partitions may optionally be encrypted with LUKS.
• A dm-verity partition may be added in that adds runtime integrity data for the root partition
Other features
• Optionally, create an SHA256SUMS checksum file for the result, possibly even signed via gpg.
• Optionally, place a specific .nspawn settings file along with the result.
• Optionally, build a local project’s source tree in the image and add the result to the generated image
(see below).
• Optionally, share RPM/DEB package cache between multiple runs, in order to optimize build speeds.
• Optionally, the resulting image may be compressed with XZ.
• Optionally, the resulting image may be converted into a QCOW2 file suitable for qemu storage.
• Optionally, btrfs’ read-only flag for the root subvolume may be set.
• Optionally, btrfs’ compression may be enabled for all created subvolumes.
• By default images are created without all files marked as documentation in the packages, on distribu‐
tions where the package manager supports this. Use the --with-docs flag to build an image with docs
added.
Command Line Verbs
The following command line verbs are known:
build This builds the image, based on the settings passed in on the command line or read from a
mkosi.default file, see below. This verb is the default if no verb is explicitly specified. This
command must be executed as root.
clean Remove build artifacts generated on a previous build. If combined with -f, also removes incremen‐
tal build cache images. If -f is specified twice, also removes any package cache.
summary
Outputs a human-readable summary of all options used for building an image. This will parse the
command line and mkosi.default file as it would do on build, but only output what it is configured
for and not actually build anything.`
shell This builds the image if it is not build yet, and then invokes systemd-nspawn to acquire an inter‐
active shell prompt in it. If this verb is used an optional command line may be specified which
is then invoked in place of the shell in the container. Combine this with -f in order to rebuild
the image unconditionally before acquiring the shell, see below. This command must be executed as
root.
boot Similar to shell but boots the image up using systemd-nspawn. If this verb is used an optional
command line may be specified which is passed as “kernel command line” to the init system in the
image.
qemu Similar to boot but uses qemu to boot up the image, i.e. instead of container virtualization VM
virtualization is used. This verb is only supported on images that contain a boot loader,
i.e. those built with --bootable (see below). This command must be executed as root unless the
image already exists and -f is not specified.
help This verb is equivalent to the --help switch documented below: it shows a brief usage explanation.
Command Line Parameters
The following command line parameters are understood. Note that many of these parameters can also be set
in the mkosi.default file, for details see the table below.
--distribution=, -d
The distribution to install in the image. Takes one of the following arguments: fedora, debian,
ubuntu, arch, opensuse, mageia, centos, clear. If not specified, defaults to the distribution of
the host.
--release=, -r
The release of the distribution to install in the image. The precise syntax of the argument this
takes depends on the distribution used, and is either a numeric string (in case of Fedora, CentOS,
..., e.g. 29), or a distribution version name (in case of Debian, Ubuntu, ..., e.g. artful). If
neither this option, not --distribution= is specified, defaults to the distribution version of the
host. If the distribution is specified, defaults to a recent version of it.
--mirror=, -m
The mirror to use for downloading the distribution packages. Expects a mirror URL as argument.
--repositories=
Additional package repositories to use during installation. Expects one or more URLs as argument,
separated by commas. This option may be used multiple times, in which case the list of reposito‐
ries to use is combined.
--architecture=
The architecture to build the image for. Note that this currently only works for architectures
compatible with the host’s architecture.
--format=, -t
The image format type to generate. One of directory (for generating OS images inside a local di‐
rectory), subvolume (similar, but as a btrfs subvolume), tar (similar, but a tarball of the image
is generated), gpt_ext4 (a block device image with an ext4 file system inside a GPT partition ta‐
ble), gpt_xfs (similar, but with an xfs file system), gpt_btrfs (similar, but with an btrfs file
system), gpt_squashfs (similar, but with a squashfs file system), plain_squashfs (a plain squashfs
file system without a partition table).
--output=, -o
Path for the output image file to generate. Takes a relative or absolute path where the generated
image will be placed. If neither this option nor --output-dir= is used (see below), the image is
generated under the name image, but its name suffixed with an appropriate file suffix (e.g. im‐
age.raw.xz in case gpt_ext4 is used in combination with --xz).
--output-dir=, -O
Path to a directory where to place all generated artifacts (i.e. the SHA256SUMS file and similar).
If this is not specified and a directory mkosi.output/ exists in the local directory it is auto‐
matically used for this purpose. If this is not specified and such a directory does not exist,
all output artifacts are placed adjacent to the output image file.
--force, -f
Replace the output file if it already exists, when building an image. By default when building an
image and an output artifact already exists mkosi will refuse operation. Specify -f to delete all
build artifacts from a previous run before re-building the image. If incremental builds are en‐
abled (see below), specifying this option twice will ensure the intermediary cache files are re‐
moved, too, before the re-build is initiated. If a package cache is used (see below), specifying
this option thrice will ensure the package cache is removed too, before the re-build is initiated.
For the clean operation -f has a slightly different effect: by default the verb will only remove
build artifacts from a previous run, when specified once the incremental cache files are deleted
too, and when specified twice the package cache is also removed.
--bootable, -b
Generate a bootable image. By default this will generate an image bootable on UEFI systems. Use
--boot-protocols= to select support for a different boot protocol.
--boot-protocols=
Pick one or more boot protocols to support when generating a bootable image, as enabled with
--bootable above. Takes a comma-separated list of uefi or bios. May be specified more than once
in which case the specified lists are merged. If uefi is specified the sd-boot UEFI boot loader
is used, if bios is specified the GNU Grub boot loader is used.
--kernel-command-line=
Use the specified kernel command line for when building bootable images.
--secure-boot
Sign the resulting kernel/initrd image for UEFI SecureBoot
--secure-boot-key=
Path to the PEM file containing the secret key for signing the UEFI kernel image, if --secure-boot
is used.
--secure-boot-certificate=
Path to the X.509 file containing the certificate for the signed UEFI kernel image, if --se‐
cure-boot is used.
--read-only
Make root file system read-only. Only applies to gpt_ext4, gpt_xfs, gpt-btrfs, subvolume output
formats, and implied on gpt_squashfs and plain_squashfs.
--encrypt
Encrypt all partitions in the file system or just the root file system. Takes either all or data
as argument. If all the root, /home and /srv file systems will be encrypted using dm-crypt/LUKS
(with its default settings). If data the root file system will be left unencrypted, but /home and
/srv will be encrypted. The passphrase to use is read from the mkosi.passphrase file in the cur‐
rent working directory (see below). Note that the UEFI System Partition (ESP) containing the boot
loader and kernel to boot is never encrypted since it needs to be accessible by the firmware.
--verity
Add an “Verity” integrity partition to the image. If enabled, the root partition is protected
with dm-verity against off-line modification, the verification data is placed in an additional GPT
partition. Implies --read-only.
--compress=
Compress the generated file systems. Only applies to gpt_btrfs, subvolume, gpt_squashfs,
plain_squashfs. Takes one of zlib, lzo, zstd, lz4, xz or a boolean value as argument. If the
latter is used compression is enabled/disabled and the default algorithm is used. In case of the
squashfs output formats compression is implied, however this option may be used to select the al‐
gorithm.
--mksquashfs=
Set the path to the mksquashfs executable to use. This is useful in case the parameters for the
tool shall be augmented, as the tool may be replaced by a script invoking it with the right para‐
meters, this way.
--xz Compress the resulting image with xz. This only applies to gpt_ext4, gpt_xfs, gpt_btrfs,
gpt_squashfs and is implied for tar. Note that when applied to the block device image types this
means the image cannot be started directly but needs to be decompressed first. This also means
that the shell, boot, qemu verbs are not available when this option is used.
--qcow2
Encode the resulting image as QEMU QCOW2 image. This only applies to gpt_ext4, gpt_xfs,
gpt_btrfs, gpt_squashfs. QCOW2 images can be read natively by qemu, but not by the Linux kernel.
This means the shell and boot verbs are not available when this option is used, however qemu will
work.
--hostname=
Set the image’s hostname to the specified name.
--no-chown
By default, if mkosi is run inside a sudo environment all generated artifacts have their UNIX
user/group ownership changed to the user which invoked sudo. With this option this may be turned
off and all generated files are owned by root.
--incremental, -i
Enable incremental build mode. This only applies if the two-phase mkosi.build build script logic
is used. In this mode, a copy of the OS image is created immediately after all OS packages are
unpacked but before the mkosi.build script is invoked in the development container. Similar a
copy of the final image is created immediately before the build artifacts from the mkosi.build
script are copied in. On subsequent invocations of mkosi with the -i switch these cached images
may be used to skip the OS package unpacking, thus drastically speeding up repetitive build times.
Note that when this is used and a pair of cached incremental images exists they are not automati‐
cally regenerated, even if options such as --packages= are modified. In order to force rebuilding
of these cached images, combined -i with -ff, which ensures the cached images are removed first,
and then re-created.
--package=, -p
Install the specified distribution packages (i.e. RPM, DEB, ...) in the image. Takes a comma sep‐
arated list of packages. This option may be used multiple times in which case the specified pack‐
age list is combined. Packaged specified this way will be installed both in the development and
the final image (see below). Use --build-package= (see below) to specify packages that shall only
be used for the image generated in the build image, but that shall not appear in the final image.
--with-docs
Include documentation in the image built. By default if the underlying distribution package man‐
ager supports it documentation is not included in the image built. The $WITH_DOCS environment
variable passed to the mkosi.build script indicates whether this option was used or not, see be‐
low.
--without-tests, -T
If set the $WITH_TESTS environment variable is set to 0 when the mkosi.build script is invoked.
This is supposed to be used by the build script to bypass any unit or integration tests that are
normally run during the source build process. Note that this option has no effect unless the
mkosi.build build script honors it.
--cache=
Takes a path to a directory to use as package cache for the distribution package manager used. If
this option is not used, but a mkosi.cache/ directory is found in the local directory it is auto‐
matically used for this purpose (also see below). The directory configured this way is mounted
into both the development and the final image while the package manager is running.
--extra-tree=
Takes a path to a directory to copy on top of the OS tree the package manager generated. Use this
to override any default configuration files shipped with the distribution. If this option is not
used, but the mkosi.extra/ directory is found in the local directory it is automatically used for
this purpose (also see below). Instead of a directory a tar file may be specified too. In this
case it is unpacked into the OS tree before the package manager is invoked. This mode of opera‐
tion allows setting permissions and file ownership explicitly, in particular for projects stored
in a version control system such as git which does retain full file ownership and access mode
metadata for committed files. If a tar file mkosi.extra.tar is found in the local directory it
automatically used for this purpose.
--skeleton-tree=
Takes a path to a directory to copy into the OS tree before invoking the package manager. Use
this to insert files and directories into the OS tree before the package manager installs any
packages. If this option is not used, but the mkosi.skeleton/ directory is found in the local di‐
rectory it is automatically used for this purpose (also see below). As with the extra tree logic
above, instead of a directory a tar file may be used too, and mkosi.skeleton.tar is automatically
used.
--build-script=
Takes a path to an executable that is used as build script for this image. If this option is used
the build process will be two-phased instead of single-phased (see below). The specified script
is copied onto the development image and executed inside an systemd-nspawn container environment.
If this option is not used, but the mkosi.build file found in the local directory it is automati‐
cally used for this purpose (also see below).
--build-sources=
Takes a path of a source tree to copy into the development image, if a build script is used. This
only applies if a build script is used, and defaults to the local directory. Use
--source-file-transfer= to configure how the files are transferred from the host to the container
image.
--build-dir=
Takes a path of a directory to use as build directory for build systems that support out-of-tree
builds (such as Meson). The directory used this way is shared between repeated builds, and allows
the build system to reuse artifacts (such as object files, executable, ...) generated on previous
invocations. This directory is mounted into the development image when the build script is in‐
voked. The build script can find the path to this directory in the $BUILDDIR environment vari‐
able. If this option is not specified, but a directory mkosi.builddir/ exists in the local direc‐
tory it is automatically used for this purpose (also see below).
--build-package=
Similar to --package=, but configures packages to install only in the first phase of the build,
into the development image. This option should be used to list packages containing header files,
compilers, build systems, linkers and other build tools the mkosi.build script requires to oper‐
ate. Note that packages listed here are only included in the image created during the first phase
of the build, and are absent in the final image. use --package= to list packages that shall be
included in both.
--postinst-script=
Takes a path to an executable that is invoked inside the final image right after copying in the
build artifacts generated in the first phase of the build. This script is invoked inside a sys‐
temd-nspawn container environment, and thus does not have access to host resources. If this op‐
tion is not used, but an executable mkosi.postinst is found in the local directory, it is automat‐
ically used for this purpose (also see below).
--finalize-script=
Takes a path to an executable that is invoked outside the final image right after copying in the
build artifacts generated in the first phase of the build, and after having executed the
mkosi.postinst script (see above). This script is invoked directly in the host environment, and
hence has full access to the host’s resources. If this option is not used, but an executable
mkosi.finalize is found in the local directory, it is automatically used for this purpose (also
see below).
--source-file-transfer=
Configures how the source file tree (as configured with --build-sources=) is transferred into the
container image during the first phase of the build. Takes one of copy-all (to copy all files
from the source tree), copy-git-cached (to copy only those files git-ls-files --cached lists),
copy-git-others (to copy only those files git-ls-files --others lists), mount to bind mount the
source tree directly. Defaults to copy-git-cached if a git source tree is detected, otherwise
copy-all.
--with-network
Enables network connectivity while the build script mkosi.build is invoked. By default, the build
script runs with networking turned off. The $WITH_NETWORK environment variable is passed to the
mkosi.build build script indicating whether the build is done with or without this option.
--settings=
Specifies a .nspawn settings file for systemd-nspawn to use in the boot and shell verbs, and to
place next to the generated image file. This is useful to configure the systemd-nspawn environ‐
ment when the image is run. If this setting is not used but an mkosi.nspawn file found in the lo‐
cal directory it is automatically used for this purpose (also see below).
--root-size=
Takes a size in bytes for the root file system. The specified numeric value may be suffixed with
K, M, G to indicate kilo-, mega- and gigabytes (all to the base of 1024). This applies to output
formats gpt_ext4, gpt_xfs, gpt_btrfs. Defaults to 1G, except for gpt_xfs where it defaults to
1.3G.
--esp-size=
Similar, and configures the size of the UEFI System Partition (ESP). This is only relevant if the
--bootable option is used to generate a bootable image. Defaults to 256M.
--swap-size=
Similar, and configures the size of a swap partition on the image. If omitted no swap partition
is created.
--home-size=
Similar, and configures the size of the /home partition. If omitted no separate /home partition
is created.
--srv-size=
Similar, and configures the size of the /srv partition. If omitted no separate /srv partition is
created.
--checksum
Generate a SHA256SUMS file of all generated artifacts after the build is complete.
--sign Sign the generated SHA256SUMS using gpg after completion.
--key= Select the gpg key to use for signing SHA256SUMS. This key is required to exist in the gpg
keyring already.
--bmap Generate a bmap file for usage with bmaptool from the generated image file.
--password=
Set the password of the root user. By default the root account is locked. If this option is not
used but a file mkosi.rootpw exists in the local directory the root password is automatically read
from it.
--extra-search-paths=
List of colon-separated paths to look for tools in, before using the regular $PATH search path.
--directory=, -C
Takes a path to a directory. mkosi switches to this directory before doing anything. Note that
the various mkosi.* files are searched for only after changing to this directory, hence using this
option is an effective way to build a project located in a specific directory.
--default=
Loads additional settings from the specified settings file. Most command line options may also be
configured in a settings file. See the table below to see which command line options match which
settings file option. If this option is not used, but a file mkosi.default is found in the local
directory it is automatically used for this purpose. If a setting is configured both on the com‐
mand line and in the settings file, the command line generally wins, except for options taking
lists in which case both lists are combined.
--all, -a
Iterate through all files mkosi.* in the mkosi.files/ subdirectory, and build each as if --de‐
fault=mkosi.files/mkosi.... was invoked. This is a quick way to build a large number of images in
one go. Any additional specified command line arguments override the relevant options in all
files processed this way.
--all-directory=
If specified, overrides the directory the --all logic described above looks for settings files in.
If unspecified, defaults to mkosi.files/ in the current working directory (see above).
--version
Show package version.
--help, -h
Show brief usage information.
Command Line Parameters and their Settings File Counterparts
Most command line parameters may also be placed in an mkosi.default settings file (or any other file
--default= is used on). The following table shows which command lines parameters correspond with which
settings file options.
Command Line Parameter mkosi.default section mkosi.default setting
─────────────────────────────────────────────────────────────────────────────
--distribution=, -d [Distribution] Distribution=
--release=, -r [Distribution] Release=
--repositories= [Distribution] Repositories=
--mirror=, -m [Distribution] Mirror=
--architecture= [Distribution] Architecture=
--format=, -t [Output] Format=
--output=, -o [Output] Output=
--output-dir=, -O [Output] OutputDirectory=
--force, -f [Output] Force=
--bootable, -b [Output] Bootable=
--boot-protocols= [Output] BootProtocols=
--kernel-command-line= [Output] KernelCommandLine=
--secure-boot [Output] SecureBoot=
--secure-boot-key= [Output] SecureBootKey=
--secure-boot-certificate= [Output] SecureBootCertificate=
--read-only [Output] ReadOnly=
--encrypt= [Output] Encrypt=
--verity= [Output] Verity=
--compress= [Output] Compress=
--mksquashfs= [Output] Mksquashfs=
--xz [Output] XZ=
--qcow2 [Output] QCow2=
--hostname= [Output] Hostname=
--package= [Packages] Packages=
--with-docs [Packages] WithDocs=
--without-tests, -T [Packages] WithTests=
--cache= [Packages] Cache=
--extra-tree= [Packages] ExtraTrees=
--skeleton-tree= [Packages] SkeletonTrees=
--build-script= [Packages] BuildScript=
--build-sources= [Packages] BuildSources=
--source-file-transfer= [Packages] SourceFileTransfer=
--build-directory= [Packages] BuildDirectory=
--build-packages= [Packages] BuildPackages=
--postinst-script= [Packages] PostInstallationScript=
--finalize-script= [Packages] FinalizeScript=
--with-network [Packages] WithNetwork=
--settings= [Packages] NSpawnSettings=
--root-size= [Partitions] RootSize=
--esp-size= [Partitions] ESPSize=
--swap-size= [Partitions] SwapSize=
--home-size= [Partitions] HomeSize=
--srv-size= [Partitions] SrvSize=
--checksum [Validation] CheckSum=
--sign [Validation] Sign=
--key= [Validation] Key=
--bmap [Validation] BMap=
--password= [Validation] Password=
--extra-search-paths= [Host] ExtraSearchPaths=
Command line options that take no argument are not suffixed with a = in their long version in the table
above. In the mkosi.default file they are modeled as boolean option that take either 1, yes, true for
enabling, and 0, no, false for disabling.
Supported distributions
Images may be created containing installations of the following OSes.
• Fedora
• Debian
• Ubuntu
• Arch Linux
• openSUSE
• Mageia
• CentOS
• Clear Linux
In theory, any distribution may be used on the host for building images containing any other distribu‐
tion, as long as the necessary tools are available. Specifically, any distribution that packages deboot‐
strap may be used to build Debian or Ubuntu images. Any distribution that packages dnf may be used to
build Fedora or Mageia images. Any distro that packages pacstrap may be used to build Arch Linux images.
Any distribution that packages zypper may be used to build openSUSE images. Any distribution that pack‐
ages yum (or the newer replacement dnf) may be used to build CentOS images.
Currently, Fedora packages all relevant tools as of Fedora 28.
Compatibility
Generated images are legacy-free. This means only GPT disk labels (and no MBR disk labels) are support‐
ed, and only systemd based images may be generated.
All generated GPT disk images may be booted in a local container directly with:
systemd-nspawn -bi image.raw
Additionally, bootable GPT disk images (as created with the --bootable flag) work when booted directly by
EFI and BIOS systems, for example in KVM via:
qemu-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -drive format=raw,file=image.raw
EFI bootable GPT images are larger than plain GPT images, as they additionally carry an EFI system parti‐
tion containing a boot loader, as well as a kernel, kernel modules, udev and more.
All directory or btrfs subvolume images may be booted directly with:
systemd-nspawn -bD image
FILES
To make it easy to build images for development versions of your projects, mkosi can read configuration
data from the local directory, under the assumption that it is invoked from a source tree. Specifically,
the following files are used if they exist in the local directory:
• mkosi.default may be used to configure mkosi’s image building process. For example, you may configure
the distribution to use (fedora, ubuntu, debian, arch, opensuse, mageia) for the image, or additional
distribution packages to install. Note that all options encoded in this configuration file may also be
set on the command line, and this file is hence little more than a way to make sure simply typing mkosi
without further parameters in your source tree is enough to get the right image of your choice set up.
Additionally if a mkosi.default.d directory exists, each file in it is loaded in the same manner
adding/overriding the values specified in mkosi.default. The file format is inspired by Windows .ini
files and supports multi-line assignments: any line with initial whitespace is considered a continua‐
tion line of the line before. Command-line arguments, as shown in the help description, have to be in‐
cluded in a configuration block (e.g. “[Packages]”) corresponding to the argument group (e.g. “Pack‐
ages”), and the argument gets converted as follows: “–with-network” becomes “WithNetwork=yes”. For
further details see the table above.
• mkosi.extra/ or mkosi.extra.tar may be respectively a directory or archive. If any exist all files
contained in it are copied over the directory tree of the image after the OS was installed. This may
be used to add in additional files to an image, on top of what the distribution includes in its pack‐
ages. When using a directory file ownership is not preserved: all files copied will be owned by root.
To preserve ownership use a tar archive.
• mkosi.skeleton/ or mkosi.skeleton.tar may be respectively a directory or archive, and they work in the
same way as mkosi.extra/mkosi.skeleton.tar. However the files are copied before anything else so to
have a skeleton tree for the OS. This allows to change the package manager and create files that need
to be there before anything is installed. When using a directory file ownership is not preserved: all
files copied will be owned by root. To preserve ownership use a tar archive.
• mkosi.build may be an executable script. If it exists the image will be built twice: the first itera‐
tion will be the development image, the second iteration will be the final image. The development im‐
age is used to build the project in the current working directory (the source tree). For that the
whole directory is copied into the image, along with the mkosi.build build script. The script is then
invoked inside the image (via systemd-nspawn), with $SRCDIR pointing to the source tree. $DESTDIR
points to a directory where the script should place any files generated it would like to end up in the
final image. Note that make/automake/meson based build systems generally honor $DESTDIR, thus making
it very natural to build source trees from the build script. After the development image was built and
the build script ran inside of it, it is removed again. After that the final image is built, without
any source tree or build script copied in. However, this time the contents of $DESTDIR are added into
the image.
When the source tree is copied into the build image, all files are copied, except for mkosi.builddir/,
mkosi.cache/ and mkosi.output/. That said, .gitignore is respected if the source tree is a git check‐
out. If multiple different images shall be built from the same source tree it’s essential to exclude
their output files from this copy operation, as otherwise a version of an image built earlier might be
included in a later build, which is usually not intended. An alternative to excluding these built im‐
ages via .gitignore entries is making use of the mkosi.output/ directory (see below), which is an easy
way to exclude all build artifacts.
• mkosi.postinst may be an executable script. If it exists it is invoked as the penultimate step of
preparing an image, from within the image context. It is once called for the development image (if
this is enabled, see above) with the “build” command line parameter, right before invoking the build
script. It is called a second time for the final image with the “final” command line parameter, right
before the image is considered complete. This script may be used to alter the images without any re‐
strictions, after all software packages and built sources have been installed. Note that this script
is executed directly in the image context with the final root directory in place, without any $SR‐
CDIR/$DESTDIR setup.
• mkosi.finalize may be an executable script. If it exists it is invoked as last step of preparing an
image, from the host system. It is once called for the development image (if this is enabled, see
above) with the “build” command line parameter, as the last step before invoking the build script, af‐
ter the mkosi.postinst script is invoked. It is called the second time with the “final” command line
parameter as the last step before the image is considered complete. The environment variable $BUILD‐
ROOT points to the root directory of the installation image. Additional verbs may be added in the fu‐
ture, the script should be prepared for that. This script may be used to alter the images without any
restrictions, after all software packages and built sources have been installed. This script is more
flexible than mkosi.postinst in two regards: it has access to the host file system so it’s easier to
copy in additional files or to modify the image based on external configuration, and the script is run
in the host, so it can be used even without emulation even if the image has a foreign architecture.
• mkosi.mksquashfs-tool may be an executable script. If it exists is is called instead of mksquashfs.
• mkosi.nspawn may be an nspawn settings file. If this exists it will be copied into the same place as
the output image file. This is useful since nspawn looks for settings files next to image files it
boots, for additional container runtime settings.
• mkosi.cache/ may be a directory. If so, it is automatically used as package download cache, in order
to speed repeated runs of the tool.
• mkosi.builddir/ may be a directory. If so, it is automatically used as out-of-tree build directory, if
the build commands in the mkosi.build script support it. Specifically, this directory will be mounted
into the build container, and the $BUILDDIR environment variable will be set to it when the build
script is invoked. The build script may then use this directory as build directory, for automake-style
or ninja-style out-of-tree builds. This speeds up builds considerably, in particular when mkosi is
used in incremental mode (-i): not only the disk images but also the build tree is reused between sub‐
sequent invocations. Note that if this directory does not exist the $BUILDDIR environment variable is
not set, and it is up to build script to decide whether to do in in-tree or an out-of-tree build, and
which build directory to use.
• mkosi.rootpw may be a file containing the password for the root user of the image to set. The password
may optionally be followed by a newline character which is implicitly removed. The file must have an
access mode of 0600 or less. If this file does not exist the distribution’s default root password is
set (which usually means access to the root user is blocked).
• mkosi.passphrase may be a passphrase file to use when LUKS encryption is selected. It should contain
the passphrase literally, and not end in a newline character (i.e. in the same format as cryptsetup and
/etc/crypttab expect the passphrase files). The file must have an access mode of 0600 or less. If
this file does not exist and encryption is requested the user is queried instead.
• mkosi.secure-boot.crt and mkosi.secure-boot.key may contain an X.509 certificate and PEM private key to
use when UEFI SecureBoot support is enabled. All EFI binaries included in the image’s ESP are signed
with this key, as a late step in the build process.
• mkosi.output/ may be a directory. If it exists, and the image output path is not configured (i.e. no
--output= setting specified), or configured to a filename (i.e. a path containing no / character) all
build artifacts (that is: the image itself, the root hash file in case Verity is used, the checksum and
its signature if that’s enabled, and the nspawn settings file if there is any) are placed in this di‐
rectory. Note that this directory is not used if the image output path contains at least one slash,
and has no effect in that case. This setting is particularly useful if multiple different images shall
be built from the same working directory, as otherwise the build result of a preceding run might be
copied into a build image as part of the source tree (see above).
All these files are optional.
Note that the location of all these files may also be configured during invocation via command line
switches, and as settings in mkosi.default, in case the default settings are not acceptable for a
project.
BUILD PHASES
If no build script mkosi.build (see above) is used the build consists of a single phase only: the final
image is generated as the combination of mkosi.skeleton/ (see above), the unpacked distribution packages
and mkosi.extra/.
If a build script mkosi.build is used the build consists of two phases: in the the first development
phase an image that includes necessary build tools (i.e. the combination of Packages= and BuildPackages=
is installed) is generated (i.e. the combination of mkosi.skeleton/ and unpacked distribution packages).
Into this image the source tree is copied and mkosi.build executed. The artifacts the mkosi.build gener‐
ates are saved. Then, the second final phase starts: an image that excludes the build tools (i.e. only
Packages= is installed, BuildPackages= is not) is generated. This time the build artifacts saved from
the first phase are copied in, and mkosi.extra copied on top, thus generating the final image.
The two-phased approach ensures that source tree is executed in a clean and comprehensive environment,
while at the same the final image remains minimal and contains only those packages necessary at runtime,
but avoiding those necessary at build-time.
Note that only the package cache mkosi.cache/ (see below) is shared between the two phases. The distrib‐
ution package manager is executed exactly once in each phase, always starting from a directory tree that
is populated with mkosi.skeleton but nothing else.
CACHING
mkosi supports three different caches for speeding up repetitive re-building of images. Specifically:
1. The package cache of the distribution package manager may be cached between builds. This is config‐
ured with the --cache= option or the mkosi.cache/ directory. This form of caching relies on the dis‐
tribution’s package manager, and caches distribution packages (RPM, DEB, ...) after they are down‐
loaded, but before they are unpacked.
2. If an mkosi.build script is used, by enabling incremental build mode with --incremental (see above) a
cached copy of the development and final images can be made immediately before the build sources are
copied in (for the development image) or the artifacts generated by mkosi.build are copied in (in case
of the final image). This form of caching allows bypassing the time-consuming package unpacking step
of the distribution package managers, but is only effective if the list of packages to use remains
stable, but the build sources and its scripts change regularly. Note that this cache requires manual
flushing: whenever the package list is modified the cached images need to be explicitly removed before
the next re-build, using the -f switch.
3. Finally, between multiple builds the build artifact directory may be shared, using the mkosi.builddir/
directory. This directory allows build systems such as Meson to reuse already compiled sources from a
previous built, thus speeding up the build process of the mkosi.build build script.
The package cache (i.e. the first item above) is unconditionally useful. The latter two caches only ap‐
ply to uses of mkosi with a source tree and build script. When all three are enabled together
turn-around times for complete image builds are minimal, as only changed source files need to be recom‐
piled: an OS image rebuilt will be almost as quick to build the source tree only.
ENVIRONMENT VARIABLES
The build script mkosi.build receives the following environment variables:
• $SRCDIR contains the path to the sources to build.
• $DESTDIR is a directory into which any artifacts generated by the build script shall be placed.
• $BUILDDIR is only defined if mkosi.builddir and points to the build directory to use. This is useful
for all build systems that support out-of-tree builds to reuse already built artifacts from previous
runs.
• $WITH_DOCS is either 0 or 1 depending on whether a build without or with installed documentation was
requested (see --with-docs above). The build script should suppress installation of any package docu‐
mentation to $DESTDIR in case $WITH_DOCS is set to 0.
• $WITH_TESTS is either 0or 1 depending on whether a build without or with running the test suite was re‐
quested (see --without-tests above). The build script should avoid running any unit or integration
tests in case $WITH_TESTS is 0.
• $WITH_NETWORK is either 0or 1 depending on whether a build without or with networking is being executed
(see --with-network above). The build script should avoid any network communication in case $WITH_NET‐
WORK is 0.
EXAMPLES
Create and run a raw GPT image with ext4, as image.raw:
# mkosi
# systemd-nspawn -b -i image.raw
Create and run a bootable btrfs GPT image, as foobar.raw:
# mkosi -t gpt_btrfs --bootable -o foobar.raw
# systemd-nspawn -b -i foobar.raw
# qemu-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -drive format=raw,file=foobar.raw
Create and run a Fedora image into a plain directory:
# mkosi -d fedora -t directory -o quux
# systemd-nspawn -b -D quux
Create a compressed image image.raw.xz and add a checksum file, and install SSH into it:
# mkosi -d fedora -t gpt_squashfs --checksum --xz --package=openssh-clients
Inside the source directory of an automake-based project, configure mkosi so that simply invoking mkosi
without any parameters builds an OS image containing a built version of the project in its current state:
# cat > mkosi.default <<EOF
[Distribution]
Distribution=fedora
Release=24
[Output]
Format=gpt_btrfs
Bootable=yes
[Packages]
Packages=openssh-clients httpd
BuildPackages=make gcc libcurl-devel
EOF
# cat > mkosi.build <<EOF
#!/bin/sh
cd $SRCDIR
./autogen.sh
./configure --prefix=/usr
make -j `nproc`
make install
EOF
# chmod +x mkosi.build
# mkosi
# systemd-nspawn -bi image.raw
To create a Fedora image with hostname:
# mkosi -d fedora --hostname image
Also you could set hostname in configuration file:
# cat mkosi.default
...
[Output]
Hostname=image
...
REQUIREMENTS
mkosi is packaged for various distributions: Debian, Ubuntu, Arch (in AUR), Fedora. It is usually easi‐
est to use the distribution package.
The current version requires systemd 233 (or actually, systemd-nspawn of it).
When not using distribution packages make sure to install the necessary dependencies. For example, on
Fedora you need:
dnf install arch-install-scripts btrfs-progs debootstrap dosfstools edk2-ovmf e2fsprogs squashfs-tools gnupg python3 tar veritysetup xfsprogs xz zypper sbsigntools
On Debian/Ubuntu it might be necessary to install the ubuntu-keyring, ubuntu-archive-keyring and/or de‐
bian-archive-keyring packages explicitly, in addition to debootstrap, depending on what kind of distribu‐
tion images you want to build. debootstrap on Debian only pulls in the Debian keyring on its own, and
the version on Ubuntu only the one from Ubuntu.
Note that the minimum required Python version is 3.6.
REFERENCES
• Primary mkosi git repository on GitHub (https://github.com/systemd/mkosi/)
• mkosi — A Tool for Generating OS Images (http://0pointer.net/blog/mkosi-a-tool-for-generating-os-im‐
ages.html) introductory blog post by Lennart Poettering
• The mkosi OS generation tool (https://lwn.net/Articles/726655/) story on LWN
SEE ALSO
systemd-nspawn(1), dnf(8), debootstrap(8)
AUTHORS
The mkosi Authors.
2016- mkosi(1)