nixpkgs/nixos/doc/manual/installation/installing-from-other-distro.xml

<!-- vim: set expandtab ts=2 softtabstop=2 shiftwidth=2 smarttab textwidth=80 wrapmargin=2 -->
<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>
$ bash &lt;(curl https://nixos.org/nix/install)
$ . $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>
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 mananager
                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>users.extraUsers.root.initialHashedPassword = "";</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>