I broke my NixOS installation while resizing my swap partition from the NixOS Live CD. After the resize, the system no longer booted because the swap UUID had changed. I could still enter the installed system with nixos-enter, but rebuilding from there did not solve the whole problem because the bootloader could not be installed.

What finally worked was using nixos-install --flake from the Live CD against my existing system. This post documents those recovery steps in case you run into the same issue.

nixos-enter

What to do? I checked the wiki and found nixos-enter, which can be used after booting into the NixOS Live CD.

mount /dev/disk/by-label/<ROOT_LABEL> /mnt/
nixos-enter

This worked, and I was able to start nvim and modify my Nix config.

Next, I tried to run sudo nixos-rebuild switch --flake .#<hostname>, but it failed because it could not install the bootloader.

nixos-install

After searching for other solutions, I finally found nixos-install, which is also used from the NixOS Live CD (I started it from a USB stick). Most examples show how to use it without flakes:

mount /dev/disk/by-label/nixos /mnt
mkdir -p /mnt/boot
mount /dev/disk/by-label/boot /mnt/boot
nixos-generate-config --root /mnt
# Edit config
cd /mnt
nixos-install

At first, I did not realize that this was the solution to my problem. But then I read how to use it with a flake-based config.

mount /dev/disk/by-label/nixos /mnt
mount /dev/disk/by-label/boot /mnt/boot
nixos-install --flake /mnt/path/to/flake#<hostname>

This almost worked. I needed to add options for pipe operators:

sudo nixos-install --flake /mnt/path/to/flake#<hostname> \
  --option extra-experimental-features 'nix-command flakes pipe-operators'

During the installation, I was asked to set a root password. Apart from that, it worked, and I was finally able to boot into my Nix system.

I hope this helps. At least for me, the solution was not obvious. I almost gave up and started a full reinstallation.