<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-installing-from-other-distro">
  <title>Installing from another Linux distribution</title>
  <para>
    Because Nix (the package manager) &amp; 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:
  </para>
  <orderedlist numeration="arabic">
    <listitem>
      <para>
        Install NixOS on another partition, from your existing Linux
        distribution (without the use of a USB or optical device!)
      </para>
    </listitem>
    <listitem>
      <para>
        Install NixOS on the same partition (in place!), from your
        existing non-NixOS Linux distribution using
        <literal>NIXOS_LUSTRATE</literal>.
      </para>
    </listitem>
    <listitem>
      <para>
        Install NixOS on your hard drive from the Live CD of any Linux
        distribution.
      </para>
    </listitem>
  </orderedlist>
  <para>
    The first steps to all these are the same:
  </para>
  <orderedlist numeration="arabic">
    <listitem>
      <para>
        Install the Nix package manager:
      </para>
      <para>
        Short version:
      </para>
      <programlisting>
$ curl -L https://nixos.org/nix/install | sh
$ . $HOME/.nix-profile/etc/profile.d/nix.sh # …or open a fresh shell
</programlisting>
      <para>
        More details in the
        <link xlink:href="https://nixos.org/nix/manual/#chap-quick-start">
        Nix manual</link>
      </para>
    </listitem>
    <listitem>
      <para>
        Switch to the NixOS channel:
      </para>
      <para>
        If you've just installed Nix on a non-NixOS distribution, you
        will be on the <literal>nixpkgs</literal> channel by default.
      </para>
      <programlisting>
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixpkgs-unstable
</programlisting>
      <para>
        As that channel gets released without running the NixOS tests,
        it will be safer to use the <literal>nixos-*</literal> channels
        instead:
      </para>
      <programlisting>
$ nix-channel --add https://nixos.org/channels/nixos-version nixpkgs
</programlisting>
      <para>
        You may want to throw in a
        <literal>nix-channel --update</literal> for good measure.
      </para>
    </listitem>
    <listitem>
      <para>
        Install the NixOS installation tools:
      </para>
      <para>
        You'll need <literal>nixos-generate-config</literal> and
        <literal>nixos-install</literal>, but this also makes some man
        pages and <literal>nixos-enter</literal> available, just in case
        you want to chroot into your NixOS partition. NixOS installs
        these by default, but you don't have NixOS yet..
      </para>
      <programlisting>
$ nix-env -f '&lt;nixpkgs&gt;' -iA nixos-install-tools
</programlisting>
    </listitem>
    <listitem>
      <note>
        <para>
          The following 5 steps are only for installing NixOS to another
          partition. For installing NixOS in place using
          <literal>NIXOS_LUSTRATE</literal>, skip ahead.
        </para>
      </note>
      <para>
        Prepare your target partition:
      </para>
      <para>
        At this point it is time to prepare your target partition.
        Please refer to the partitioning, file-system creation, and
        mounting steps of <xref linkend="sec-installation" />
      </para>
      <para>
        If you're about to install NixOS in place using
        <literal>NIXOS_LUSTRATE</literal> there is nothing to do for
        this step.
      </para>
    </listitem>
    <listitem>
      <para>
        Generate your NixOS configuration:
      </para>
      <programlisting>
$ sudo `which nixos-generate-config` --root /mnt
</programlisting>
      <para>
        You'll probably want to edit the configuration files. Refer to
        the <literal>nixos-generate-config</literal> step in
        <xref linkend="sec-installation" /> for more information.
      </para>
      <para>
        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
        <literal>configuration.nix</literal>:
      </para>
      <programlisting language="bash">
boot.loader.grub.extraEntries = ''
  menuentry &quot;Ubuntu&quot; {
    search --set=ubuntu --fs-uuid 3cc3e652-0c1f-4800-8451-033754f68e6e
    configfile &quot;($ubuntu)/boot/grub/grub.cfg&quot;
  }
'';
</programlisting>
      <para>
        (You can find the appropriate UUID for your partition in
        <literal>/dev/disk/by-uuid</literal>)
      </para>
    </listitem>
    <listitem>
      <para>
        Create the <literal>nixbld</literal> group and user on your
        original distribution:
      </para>
      <programlisting>
$ sudo groupadd -g 30000 nixbld
$ sudo useradd -u 30000 -g nixbld -G nixbld nixbld
</programlisting>
    </listitem>
    <listitem>
      <para>
        Download/build/install NixOS:
      </para>
      <warning>
        <para>
          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.
        </para>
      </warning>
      <note>
        <para>
          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
          <literal>PATH=&quot;$PATH:/usr/sbin:/sbin&quot;</literal> in
          the following command.
        </para>
      </note>
      <programlisting>
$ sudo PATH=&quot;$PATH&quot; NIX_PATH=&quot;$NIX_PATH&quot; `which nixos-install` --root /mnt
</programlisting>
      <para>
        Again, please refer to the <literal>nixos-install</literal> step
        in <xref linkend="sec-installation" /> for more information.
      </para>
      <para>
        That should be it for installation to another partition!
      </para>
    </listitem>
    <listitem>
      <para>
        Optionally, you may want to clean up your non-NixOS
        distribution:
      </para>
      <programlisting>
$ sudo userdel nixbld
$ sudo groupdel nixbld
</programlisting>
      <para>
        If you do not wish to keep the Nix package manager installed
        either, run something like
        <literal>sudo rm -rv ~/.nix-* /nix</literal> and remove the line
        that the Nix installer added to your
        <literal>~/.profile</literal>.
      </para>
    </listitem>
    <listitem>
      <note>
        <para>
          The following steps are only for installing NixOS in place
          using <literal>NIXOS_LUSTRATE</literal>:
        </para>
      </note>
      <para>
        Generate your NixOS configuration:
      </para>
      <programlisting>
$ sudo `which nixos-generate-config` --root /
</programlisting>
      <para>
        Note that this will place the generated configuration files in
        <literal>/etc/nixos</literal>. You'll probably want to edit the
        configuration files. Refer to the
        <literal>nixos-generate-config</literal> step in
        <xref linkend="sec-installation" /> for more information.
      </para>
      <para>
        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 initalize 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 <literal>sudo passwd -l root</literal> if you use
        <literal>sudo</literal>)
      </para>
      <programlisting language="bash">
users.users.root.initialHashedPassword = &quot;&quot;;
</programlisting>
    </listitem>
    <listitem>
      <para>
        Build the NixOS closure and install it in the
        <literal>system</literal> profile:
      </para>
      <programlisting>
$ nix-env -p /nix/var/nix/profiles/system -f '&lt;nixpkgs/nixos&gt;' -I nixos-config=/etc/nixos/configuration.nix -iA system
</programlisting>
    </listitem>
    <listitem>
      <para>
        Change ownership of the <literal>/nix</literal> tree to root
        (since your Nix install was probably single user):
      </para>
      <programlisting>
$ sudo chown -R 0:0 /nix
</programlisting>
    </listitem>
    <listitem>
      <para>
        Set up the <literal>/etc/NIXOS</literal> and
        <literal>/etc/NIXOS_LUSTRATE</literal> files:
      </para>
      <para>
        <literal>/etc/NIXOS</literal> officializes that this is now a
        NixOS partition (the bootup scripts require its presence).
      </para>
      <para>
        <literal>/etc/NIXOS_LUSTRATE</literal> tells the NixOS bootup
        scripts to move <emphasis>everything</emphasis> that's in the
        root partition to <literal>/old-root</literal>. 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:
      </para>
      <itemizedlist>
        <listitem>
          <para>
            The <literal>/nix</literal> directory
          </para>
        </listitem>
        <listitem>
          <para>
            The <literal>/boot</literal> directory
          </para>
        </listitem>
        <listitem>
          <para>
            Any file or directory listed in
            <literal>/etc/NIXOS_LUSTRATE</literal> (one per line)
          </para>
        </listitem>
      </itemizedlist>
      <note>
        <para>
          Support for <literal>NIXOS_LUSTRATE</literal> was added in
          NixOS 16.09. The act of &quot;lustrating&quot; refers to the
          wiping of the existing distribution. Creating
          <literal>/etc/NIXOS_LUSTRATE</literal> can also be used on
          NixOS to remove all mutable files from your root partition
          (anything that's not in <literal>/nix</literal> or
          <literal>/boot</literal> gets &quot;lustrated&quot; on the
          next boot.
        </para>
        <para>
          lustrate /ˈlʌstreɪt/ verb.
        </para>
        <para>
          purify by expiatory sacrifice, ceremonial washing, or some
          other ritual action.
        </para>
      </note>
      <para>
        Let's create the files:
      </para>
      <programlisting>
$ sudo touch /etc/NIXOS
$ sudo touch /etc/NIXOS_LUSTRATE
</programlisting>
      <para>
        Let's also make sure the NixOS configuration files are kept once
        we reboot on NixOS:
      </para>
      <programlisting>
$ echo etc/nixos | sudo tee -a /etc/NIXOS_LUSTRATE
</programlisting>
    </listitem>
    <listitem>
      <para>
        Finally, move the <literal>/boot</literal> directory of your
        current distribution out of the way (the lustrate process will
        take care of the rest once you reboot, but this one must be
        moved out now because NixOS needs to install its own boot files:
      </para>
      <warning>
        <para>
          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.
        </para>
      </warning>
      <programlisting>
$ sudo mv -v /boot /boot.bak &amp;&amp;
sudo /nix/var/nix/profiles/system/bin/switch-to-configuration boot
</programlisting>
      <para>
        Cross your fingers, reboot, hopefully you should get a NixOS
        prompt!
      </para>
    </listitem>
    <listitem>
      <para>
        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:
      </para>
      <programlisting>
# 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
</programlisting>
      <para>
        This may work as is or you might also need to reinstall the boot
        loader.
      </para>
      <para>
        And of course, if you're happy with NixOS and no longer need the
        old distribution:
      </para>
      <programlisting>
sudo rm -rf /old-root
</programlisting>
    </listitem>
    <listitem>
      <para>
        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,
        <link xlink:href="https://github.com/elitak/nixos-infect">nixos-infect</link>
        uses the lustrate process to convert Digital Ocean droplets to
        NixOS from other distributions automatically.
      </para>
    </listitem>
  </orderedlist>
</section>
