Installing from another Linux distribution

Because Nix (the package manager) & Nixpkgs (the Nix packages collection) can both be installed on any (most?) Linux distributions, they can be used to install NixOS in various creative ways. You can, for instance:

Install NixOS on another partition, from your existing Linux distribution (without the use of a USB or optical device!)
Install NixOS on the same partition (in place!), from your existing non-NixOS Linux distribution using NIXOS_LUSTRATE.
Install NixOS on your hard drive from the Live CD of any Linux distribution.

The first steps to all these are the same:

Install the Nix package manager:

Short version:

$ curl -L https://nixos.org/nix/install | sh
$ . $HOME/.nix-profile/etc/profile.d/nix.sh # …or open a fresh shell

More details in the Nix manual

Switch to the NixOS channel:

If you've just installed Nix on a non-NixOS distribution, you will be on the nixpkgs channel by default.
```
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixpkgs-unstable
```
As that channel gets released without running the NixOS tests, it will be safer to use the nixos-* channels instead:
```
$ nix-channel --add https://nixos.org/channels/nixos-<version> nixpkgs
```
Where <version> corresponds to the latest version available on channels.nixos.org.

You may want to throw in a nix-channel --update for good measure.
Install the NixOS installation tools:

You'll need nixos-generate-config and nixos-install, but this also makes some man pages and nixos-enter available, just in case you want to chroot into your NixOS partition. NixOS installs these by default, but you don't have NixOS yet..
```
$ nix-env -f '<nixpkgs>' -iA nixos-install-tools
```
::: {.note} The following 5 steps are only for installing NixOS to another partition. For installing NixOS in place using NIXOS_LUSTRATE, skip ahead. :::

Prepare your target partition:

At this point it is time to prepare your target partition. Please refer to the partitioning, file-system creation, and mounting steps of

If you're about to install NixOS in place using NIXOS_LUSTRATE there is nothing to do for this step.
Generate your NixOS configuration:
```
$ sudo `which nixos-generate-config` --root /mnt
```
You'll probably want to edit the configuration files. Refer to the nixos-generate-config step in for more information.

Consider setting up the NixOS bootloader to give you the ability to boot on your existing Linux partition. For instance, if you're using GRUB and your existing distribution is running Ubuntu, you may want to add something like this to your configuration.nix:
```
{
  boot.loader.grub.extraEntries = ''
    menuentry "Ubuntu" {
      search --set=ubuntu --fs-uuid 3cc3e652-0c1f-4800-8451-033754f68e6e
      configfile "($ubuntu)/boot/grub/grub.cfg"
    }
  '';
}
```
(You can find the appropriate UUID for your partition in /dev/disk/by-uuid)

Create the nixbld group and user on your original distribution:

$ sudo groupadd -g 30000 nixbld
$ sudo useradd -u 30000 -g nixbld -G nixbld nixbld

Download/build/install NixOS:

::: {.warning} Once you complete this step, you might no longer be able to boot on existing systems without the help of a rescue USB drive or similar. :::

::: {.note} On some distributions there are separate PATHS for programs intended only for root. In order for the installation to succeed, you might have to use PATH="$PATH:/usr/sbin:/sbin" in the following command. :::
```
$ sudo PATH="$PATH" `which nixos-install` --root /mnt
```
Again, please refer to the nixos-install step in for more information.

That should be it for installation to another partition!
Optionally, you may want to clean up your non-NixOS distribution:
```
$ sudo userdel nixbld
$ sudo groupdel nixbld
```
If you do not wish to keep the Nix package manager installed either, run something like sudo rm -rv ~/.nix-* /nix and remove the line that the Nix installer added to your ~/.profile.
::: {.note} The following steps are only for installing NixOS in place using NIXOS_LUSTRATE: :::

Generate your NixOS configuration:
```
$ sudo `which nixos-generate-config`
```
Note that this will place the generated configuration files in /etc/nixos. You'll probably want to edit the configuration files. Refer to the nixos-generate-config step in for more information.

::: {.note} On UEFI systems, check that your /etc/nixos/hardware-configuration.nix did the right thing with the EFI System Partition. In NixOS, by default, both systemd-boot and grub expect it to be mounted on /boot. However, the configuration generator bases its configuration on the current mount points at the time it is run. If the current system and NixOS's bootloader configuration don't agree on where the EFI System Partition is to be mounted, you'll need to manually alter the mount point in hardware-configuration.nix before building the system closure. :::

::: {.note} The lustrate process will not work if the option is set to true. If you want to use this option, wait until after the first boot into the NixOS system to enable it and rebuild. :::

You'll likely want to set a root password for your first boot using the configuration files because you won't have a chance to enter a password until after you reboot. You can initialize the root password to an empty one with this line: (and of course don't forget to set one once you've rebooted or to lock the account with sudo passwd -l root if you use sudo)
```
{
  users.users.root.initialHashedPassword = "";
}
```

Build the NixOS closure and install it in the system profile:

$ nix-env -p /nix/var/nix/profiles/system -f '<nixpkgs/nixos>' -I nixos-config=/etc/nixos/configuration.nix -iA system

Change ownership of the /nix tree to root (since your Nix install was probably single user):
```
$ sudo chown -R 0:0 /nix
```
Set up the /etc/NIXOS and /etc/NIXOS_LUSTRATE files:

/etc/NIXOS officializes that this is now a NixOS partition (the bootup scripts require its presence).

/etc/NIXOS_LUSTRATE tells the NixOS bootup scripts to move everything that's in the root partition to /old-root. This will move your existing distribution out of the way in the very early stages of the NixOS bootup. There are exceptions (we do need to keep NixOS there after all), so the NixOS lustrate process will not touch:
- The /nix directory
- The /boot directory
- Any file or directory listed in /etc/NIXOS_LUSTRATE (one per line)
::: {.note} The act of "lustrating" refers to the wiping of the existing distribution. Creating /etc/NIXOS_LUSTRATE can also be used on NixOS to remove all mutable files from your root partition (anything that's not in /nix or /boot gets "lustrated" on the next boot.

lustrate /ˈlʌstreɪt/ verb.

purify by expiatory sacrifice, ceremonial washing, or some other ritual action. :::

Let's create the files:
```
$ sudo touch /etc/NIXOS
$ sudo touch /etc/NIXOS_LUSTRATE
```
Let's also make sure the NixOS configuration files are kept once we reboot on NixOS:
```
$ echo etc/nixos | sudo tee -a /etc/NIXOS_LUSTRATE
```
Finally, install NixOS's boot system, backing up the current boot system's files in the process.

The details of this step can vary depending on the bootloader configuration in NixOS and the bootloader in use by the current system.

The commands below should work for:
- BIOS systems.
- UEFI systems where both the current system and NixOS mount the EFI System Partition on /boot. Both systemd-boot and grub expect this by default in NixOS, but other distributions vary.
::: {.warning} Once you complete this step, your current distribution will no longer be bootable! If you didn't get all the NixOS configuration right, especially those settings pertaining to boot loading and root partition, NixOS may not be bootable either. Have a USB rescue device ready in case this happens. :::

::: {.warning} On UEFI systems, anything on the EFI System Partition will be removed by these commands, such as other coexisting OS's bootloaders. :::
```
$ sudo mkdir /boot.bak && sudo mv /boot/* /boot.bak &&
sudo NIXOS_INSTALL_BOOTLOADER=1 /nix/var/nix/profiles/system/bin/switch-to-configuration boot
```
Cross your fingers, reboot, hopefully you should get a NixOS prompt!

In other cases, most commonly where the EFI System Partition of the current system is instead mounted on /boot/efi, the goal is to:
- Make sure /boot (and the EFI System Partition, if mounted elsewhere) are mounted how the NixOS configuration would mount them.
- Clear them of files related to the current system, backing them up outside of /boot. NixOS will move the backups into /old-root along with everything else when it first boots.
- Instruct the NixOS closure built earlier to install its bootloader with:
```
sudo NIXOS_INSTALL_BOOTLOADER=1 /nix/var/nix/profiles/system/bin/switch-to-configuration boot
```
If for some reason you want to revert to the old distribution, you'll need to boot on a USB rescue disk and do something along these lines:
```
# mkdir root
# mount /dev/sdaX root
# mkdir root/nixos-root
# mv -v root/* root/nixos-root/
# mv -v root/nixos-root/old-root/* root/
# mv -v root/boot.bak root/boot  # We had renamed this by hand earlier
# umount root
# reboot
```
This may work as is or you might also need to reinstall the boot loader.

And of course, if you're happy with NixOS and no longer need the old distribution:
```
sudo rm -rf /old-root
```
It's also worth noting that this whole process can be automated. This is especially useful for Cloud VMs, where provider do not provide NixOS. For instance, nixos-infect uses the lustrate process to convert Digital Ocean droplets to NixOS from other distributions automatically.

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Installing from another Linux distribution

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