In Debian and derivative systems, there are many ways to build images.
The simplest tool of choice is often debootstrap. It works by downloading
the .deb files from a mirror and unpacking them into a directory which can
eventually be chrooted into.
More often than not, we want to make some customization on this image, install some extra packages, run a script, add some files, etc
debos is a tool to make this kind of trivial tasks easier. debos works using recipe files in YAML listing the actions you want to perform in your image sequentially and finally, choosing the output formats.
As opposite to debootstrap and other tools, debos doesn't need to be run
as root for making actions that require root privileges in the images.
debos uses fakemachine a library that setups qemu-system allowing you
to work in the image with root privileges and to create images for all
the architectures supported by qemu user. However, for this to work,
make sure your user has permission to use /dev/kvm.
Let's see how debos works with a simple example. If we wanted to create an arm64 image for Debian Stretch customized, we would follow these steps:
- debootstrap the image
- install the packages we need
- create a user
- setup our preferred hostname
- run a script creating a user
- copy a file adding the user to sudoers
- creating a tarball with the final image
This would translate into a debos recipe like this one:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 | {{- $architecture := or .architecture "arm64" -}}
{{- $suite := or .suite "stretch" -}}
{{ $image := or .image (printf "debian-%s-%s.tgz" $suite $architecture) }}
architecture: {{ $architecture }}
actions:
- action: debootstrap
suite: {{ $suite }}
components:
- main
mirror: http://deb.debian.org/debian
variant: minbase
- action: apt
recommends: false
packages:
- adduser
- sudo
- action: run
description: Set hostname
chroot: true
command: echo debian-{{ $suite }}-{{ $architecture }} > /etc/hostname
- action: run
chroot: true
script: scripts/setup-user.sh
- action: overlay
description: Add sudo configuration
source: overlays/sudo
- action: pack
file: {{ $image }}
compression: gz
|
(The files used in this example are available from this git repository)
We run debos on the recipe file:
$ debos simple.yaml
The result will be a tarball named debian-stretch-arm64.tar.gz. If you
check the top two lines of the recipe, you can see that the recipe
defaults to architecture arm64 and Debian stretch. We can override these
defaults when running debos:
$ debos -t suite:"buster" -t architecture:"amd64" simple.yaml
This time the result will be a tarball debian-buster-amd64.tar.gz.
The recipe allows some customization depending on the parameters.
We could install packages depending on the target architecture, for
example, installing python-libsoc in armhf and arm64:
1 2 3 4 5 6 7 8 | - action: apt
recommends: false
packages:
- adduser
- sudo
{{- if eq $architecture "armhf" "arm64" }}
- python-libsoc
{{- end }}
|
What happens if in addition to a tarball we would like to create a filesystem image? This could be done adding two more actions to our example, a first action creating the image partition with the selected filesystem and a second one deploying the image in the filesystem:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | - action: image-partition
imagename: {{ $ext4 }}
imagesize: 1GB
partitiontype: msdos
mountpoints:
- mountpoint: /
partition: root
partitions:
- name: root
fs: ext4
start: 0%
end: 100%
flags: [ boot ]
- action: filesystem-deploy
description: Deploying filesystem onto image
|
{{ $ext4 }} should be defined in the top of the file as follows:
1 | {{ $ext4 := or .image (printf "debian-%s-%s.ext4" $suite $architecture) }}
|
We could even make this step optional and make the recipe by default to only create the tarball and add the filesystem image only adding an option to debos:
$ debos -t type:"full" full.yaml
The final debos recipe will look like this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 | {{- $architecture := or .architecture "arm64" -}}
{{- $suite := or .suite "stretch" -}}
{{ $type := or .type "min" }}
{{ $image := or .image (printf "debian-%s-%s.tgz" $suite $architecture) }}
{{ $ext4 := or .image (printf "debian-%s-%s.ext4" $suite $architecture) }}
architecture: {{ $architecture }}
actions:
- action: debootstrap
suite: {{ $suite }}
components:
- main
mirror: http://deb.debian.org/debian
variant: minbase
- action: apt
recommends: false
packages:
- adduser
- sudo
{{- if eq $architecture "armhf" "arm64" }}
- python-libsoc
{{- end }}
- action: run
description: Set hostname
chroot: true
command: echo debian-{{ $suite }}-{{ $architecture }} > /etc/hostname
- action: run
chroot: true
script: scripts/setup-user.sh
- action: overlay
description: Add sudo configuration
source: overlays/sudo
- action: pack
file: {{ $image }}
compression: gz
{{ if eq $type "full" }}
- action: image-partition
imagename: {{ $ext4 }}
imagesize: 1GB
partitiontype: msdos
mountpoints:
- mountpoint: /
partition: root
partitions:
- name: root
fs: ext4
start: 0%
end: 100%
flags: [ boot ]
- action: filesystem-deploy
description: Deploying filesystem onto image
{{end}}
|
debos also provides some other actions that haven't been covered in the example above:
- download allows to download a single file from the internet
- raw can directly write a file to the output image at a given offset
- unpack can be used to unpack files from archive in the filesystem
- ostree-commit create an OSTree commit from rootfs
- ostree-deploy deploy an OSTree branch to the image
The example in this blog post is simple and short on purpose. Combining the actions presented above, you could also include a kernel and install a bootloader to make a bootable image. Upstream is planning to add more examples soon to the debos recipes repository.
debos is a project from Sjoerd Simons at Collabora, it's still missing some features but it's actively being developed and there are big plans for the future!