$ nix-build -A rootfs /nix/store/apxsq3b0cy62y6c0cwy0x0v08qv95n3v-squashfs.img
The rootfs derivation, defined as
config.system.build.squashfs, creates a squashfs image. It contains the closures of toplevel and a registration file.
The rootfs is also available as an ext4 image.
To explore the result, we can extract the content of the rootfs in a temporary directory as follow:
$ mkdir tmp ; cd tmp $ unsquashfs $(readlink -fn ../result) $ find -maxdepth 2 . ./squashfs-root ./squashfs-root/5f18ah2yzyf4mmnn8jqqb7aws91rw55v-ssh_host_rsa_key.pub ./squashfs-root/wqfpawgsigwnz2bk1ygkfya7802jxl9c-iputils-20180629 ./squashfs-root/p54mjqlrngzzyb2892489b4hffgz03g2-aws-sdk-cpp-1.5.17 ./squashfs-root/jaiq6xgyhhl84826lrsxbgdy5sm9n8wx-nixos.conf ...
The derivation is defined as a call to
nixpkgs/nixos/lib/make-squashfs.nix, passing toplevel as argument. The closure is constructed by
A call to
nix-store --load-db with the registration file found in the rootfs is done in a runit script. I wonder if it could be done directly when the rootfs is mounted.