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<section
    xmlns="http://docbook.org/ns/docbook"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xi="http://www.w3.org/2001/XInclude"
    version="5.0"
    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>
  <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>
  <listitem>
   <para>
    Install the Nix package manager:
   </para>
   <para>
    Short version:
   </para>
<screen>
$ curl https://nixos.org/nix/install | sh
$ . $HOME/.nix-profile/etc/profile.d/nix.sh # …or open a fresh shell</screen>
   <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>
<screen>
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixpkgs-unstable</screen>
   <para>
    As that channel gets released without running the NixOS tests, it will be
    safer to use the <literal>nixos-*</literal> channels instead:
   </para>
<screen>
$ nix-channel --add https://nixos.org/channels/nixos-<replaceable>version</replaceable> nixpkgs</screen>
   <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> and we'll throw in some man pages and
    <literal>nixos-enter</literal> just in case you want to chroot into your
    NixOS partition. They are installed by default on NixOS, but you don't have
    NixOS yet..
   </para>
<screen>$ nix-env -iE "_: with import &lt;nixpkgs/nixos&gt; { configuration = {}; }; with config.system.build; [ nixos-generate-config nixos-install nixos-enter manual.manpages ]"</screen>
  </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>
<screen>$ sudo `which nixos-generate-config` --root /mnt</screen>
   <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>
<xref linkend="opt-boot.loader.grub.extraEntries"/> = ''
  menuentry "Ubuntu" {
    search --set=ubuntu --fs-uuid 3cc3e652-0c1f-4800-8451-033754f68e6e
    configfile "($ubuntu)/boot/grub/grub.cfg"
  }
'';</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>
<screen>
$ sudo groupadd -g 30000 nixbld
$ sudo useradd -u 30000 -g nixbld -G nixbld nixbld</screen>
  </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>
<screen>$ sudo PATH="$PATH" NIX_PATH="$NIX_PATH" `which nixos-install` --root /mnt</screen>
   <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>
<screen>
$ sudo userdel nixbld
$ sudo groupdel nixbld</screen>
   <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>
<screen>$ sudo `which nixos-generate-config` --root /</screen>
   <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>
<link linkend="opt-users.users._name__.initialHashedPassword">users.users.root.initialHashedPassword</link> = "";
              </programlisting>
  </listitem>
  <listitem>
   <para>
    Build the NixOS closure and install it in the <literal>system</literal>
    profile:
   </para>
<screen>$ nix-env -p /nix/var/nix/profiles/system -f '&lt;nixpkgs/nixos&gt;' -I nixos-config=/etc/nixos/configuration.nix -iA system</screen>
  </listitem>
  <listitem>
   <para>
    Change ownership of the <literal>/nix</literal> tree to root (since your
    Nix install was probably single user):
   </para>
<screen>$ sudo chown -R 0.0 /nix</screen>
  </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 "lustrating" 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 "lustrated" 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>
<screen>
$ sudo touch /etc/NIXOS
$ sudo touch /etc/NIXOS_LUSTRATE
            </screen>
   <para>
    Let's also make sure the NixOS configuration files are kept once we reboot
    on NixOS:
   </para>
<screen>
$ echo etc/nixos | sudo tee -a /etc/NIXOS_LUSTRATE
            </screen>
  </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>
<screen>
$ sudo mv -v /boot /boot.bak &amp;&amp;
    sudo /nix/var/nix/profiles/system/bin/switch-to-configuration boot</screen>
   <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>
<screen>
# 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</screen>
   <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>
<screen>sudo rm -rf /old-root</screen>
  </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>
