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Image generation is based entirely on configuration files. These configuration files define the image format, the layout and size of disks, the boot configurations, the package composition, and other details.
To learn more about the image configuration file format, see formats/imageconfig.md
The toolkit includes several image configurations in ./imageconfigs/ which can be used as a starting point.
The most straightforward, and default, config is ./imageconfigs/core-efi.json. This config will create core.vhdx which can be booted on Gen-2 Hyper-V VMs. A legacy version is also available which creates a core.vhd image for Gen-1 VMs.
The imageconfigvalidator tool is used to validate configuration files before they are used anywhere in the build.
The first stage of image generation is to create the desired filesystem locally. This can be either in the form of a raw disk image (*.raw) or a directory tree. If it is a raw disk image the *.raw file is mounted as a loopback device.
The imager tool uses a chroot environment (see Chroot Worker) to install all the required packages into the filesystem.
The roast tool bakes the raw disk image into its final format (*.ext4, *.vhd, *.vhdx, etc.).
ISOs are slightly different than simple images. They require a stand-alone installer which is responsible for taking the configured image, and applying it to a target computer.
The core of the ISO installer is an initrd image, (aka ramdisk,initramfs) which can be booted from memory. Two versions are available, a calamares based GUI installer and a terminal based installer (see Building ISOs).
The initrd image is based on a static configuration included with the toolkit. Building the initrd image requires a recursive call to Make. Since a number of components in the build system have a dependency on the CONFIG_FILE variable (see Config Tracking) rebuilding the initrd will invalidate a number of components and cause a brief rebuild.
Once the initrd image is available it is combined with the requested images in the config file using the isomaker tool.