Autoinstall

By using autoinstall we could automate the subiquity installer like what preseed is used for debian-installer, a.k.a. d-i.

Usage

Upload your user-data to a www server. You may use basic.yaml as an example. Rename basic.yaml to be user-data and put it into a www server to serve it. By using basic.yaml the default username and password will be both ubuntu. You can change the default username and password by following the instruction in the comment block of basic.yaml.
touch <your www server>/meta-data to create an empty but necessary file required by cloud-init.
Include the necessary kernel parameters autoinstall "ds=nocloud-net;s=http://<www-server-url>/". For UEFI, it is usually the line of linux in your grub.cfg. If you are working on pxelinux.cfg for legacy mode, please do something similar in your APPEND line. For example,
- as is quiet splash root=/dev/ram0 ramdisk_size=1500000 ip=dhcp url=http://<server-provides-image>/focal-live-server-arm64.iso ---
- to be quiet splash root=/dev/ram0 ramdisk_size=1500000 ip=dhcp url=http://<server-provides-image>/focal-live-server-arm64.iso autoinstall "ds=nocloud-net;s=http://<server-provides-user-data>/" ---

The format of the first line "shebang", a.k.a. #cloud-config matters. Do not change it or input extra spaces unless you know what you are doing.
The double quotes matter when injecting user-data in the kernel parameters.
The file name user-data is a must. Its the file name that cloud-init will look for.
The file meta-data is also a must, even it is an empty file in our example.
The trailing slash of the url serving user-data matters in the current autoinstall version.