nixos-rebuild-boot.md

Inspired from: https://github.com/SayantanRC/URLs/blob/master/repair-grub-live-usb.md

Rebuild NixOS bootloader

Requirements

1. Already installed NixOS partition for which bootloader needs to installed.
2. Patience and some terminal knowledge is helpful.

Procedure

Assuming linux is installed in /dev/sda3. This will be different for your system. Use Gparted program to check.

1. Boot from the USB drive. Open a terminal with "Alt+Ctrl+T".
2. Mount the partition.

   sudo mkdir -p /cdrom
   sudo mount /dev/sda3 /cdrom
   

3. Bind mount some necessary partitions from live USB environment.

   sudo mount -o bind /dev /cdrom/dev/
   sudo mount -o bind /proc /cdrom/proc/
   sudo mount -o bind /sys /cdrom/sys/
   

   We also need to bind mount resolv.conf to be able to get internet access in chroot environment.
   See how to connect to WiFi using terminal before running this command.

   sudo mount -o bind /etc/resolv.conf /cdrom/etc/resolv.conf
   

4. Chroot into the mount point.

   sudo chroot /cdrom /nix/var/nix/profiles/system/activate
   sudo chroot /cdrom /run/current-system/sw/bin/bash
   

5. Now we are in the environment of the actual NixOS installation. Usually the bootloader is installed in /dev/sda1.
   Hence we need to mount it. In NixOS we need to mount in under /boot.

   sudo mount /dev/sda1 /boot
   

6. Rebuild NixOS boot.

   sudo nixos-rebuild boot --option sandbox false
   

7. Reinstall bootloader

   sudo NIXOS_INSTALL_BOOTLOADER=1 /nix/var/nix/profiles/system/bin/switch-to-configuration boot
   

8. Finally, unmount partitions. In chroot environment (not all may be successful):

   sudo umount /boot
   exit
   

   In live USB:

   sudo umount /cdrom/proc
   sudo umount /cdrom/sys
   sudo umount /cdrom/dev
   sudo umount /cdrom/etc/resolv.conf
   sudo umount /cdrom
   

Reboot and enjoy.

Sources:
NixOS/nix#5076 (comment)
https://nixos.wiki/wiki/Change_root
https://www.reddit.com/r/NixOS/comments/ouxjzh/how_to_reinstall_bootloader