I’m building an armbian image and want to add some partitions to the result disk. I use the post_create_partitions hook to do that. The documentation doesn’t show how to use it, so I looked at the armbian’s sources:
https://github.com/armbian/build/blob/99815c0a8880d4e04fc64742412632e1d41c1334/lib/functions/image/partitioning.sh#L206
Here they create a string and pass it to sfdisk. In my hook I do a similar row to add two partitions: