[Nix-dev] How can I boot into nixos from a grub installed on other drive ? (chroot problem)

[Matt]

hi,

My macbook SATA wire broke, I plugged its harddrive with nixos (16.XX)
installed into my workstation and now I would like to boot on it. My
main workstation harddrive is ubuntu with grub.
I would like to add a grub entry to boot on it (the grub is on the
ubuntu harddrive).

>From Ubuntu, I mounted the nixos hardrive in /mnt/nixos I played a bit
with os-prober to have grub-update generate the nixos entry but it
fails to do so.

Finally I stumbled upon:
http://lists.science.uu.nl/pipermail/nix-dev/2015-August/017932.html
which seems like a good idea ?! i.e., have nixos generate its own grub
entries (without installing grub) and then have the grub ony my ubuntu
drive include those config file.

The problem I now have is to be able to generate the nixos grub
entries, which require to run "nixos-rebuild switch" from my ubuntu
partition.

I've tried to chroot but it fails miserably ? How can I circumvent that ?
===
LANG=C sudo chroot . nix/store/*-nixos-system-*/init
<<< NixOS Stage 2 >>>

running activation script...
setting up /etc...
starting systemd...
Trying to run as user instance, but the system has not been booted with systemd.
===

Here is the content of /mnt/nixos/boot , I don't know much about EFI
but I wonder if it can be a problem, ie., if I can boot those kernels
on a different laptop.
==
ubuntu$ ls -R /mnt/nixos/boot

./boot:
efi/  loader/

./boot/efi:
nixos/

./boot/efi/nixos:
8ii7mf9a1c7d9g09p176z7682smyrg96-initrd-initrd.efi
mjy1zi0iygs5d676yplrdckjhw011prm-linux-4.4.39-bzImage.efi
hjbgyi20c2w17vsdcxpprgyzvzp528pp-initrd-initrd.efi
v440b1as5dpbwv5id2ijkwip5qv6c3mz-initrd-initrd.efi
jhhfsg8lpvnf16xryk6xdb18mdv52ggd-initrd-initrd.efi
y10bb973l89wl5n3gwx8zl0ml4h24hg6-linux-4.4.45-bzImage.efi
jxiz0q5q98yi5bw8hajzdnwrjkx5m6iz-linux-4.4.47-bzImage.efi
ywrjsjk0l5rak4qmhww3n7fs2gb9kxrn-initrd-initrd.efi

./boot/loader:
entries/  loader.conf

./boot/loader/entries:
nixos-generation-10.conf  nixos-generation-16.conf
nixos-generation-21.conf  nixos-generation-27.conf
nixos-generation-6.conf
nixos-generation-11.conf  nixos-generation-17.conf
nixos-generation-22.conf  nixos-generation-28.conf
nixos-generation-7.conf
nixos-generation-12.conf  nixos-generation-18.conf
nixos-generation-23.conf  nixos-generation-2.conf
nixos-generation-8.conf
nixos-generation-13.conf  nixos-generation-19.conf
nixos-generation-24.conf  nixos-generation-3.conf
nixos-generation-9.conf
nixos-generation-14.conf  nixos-generation-1.conf
nixos-generation-25.conf  nixos-generation-4.conf
nixos-generation-15.conf  nixos-generation-20.conf
nixos-generation-26.conf  nixos-generation-5.conf
==

Apologies for the long mail and thanks for making nix(OS).

Best regards
Matt