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<chapter xmlns=""
<title>Installing NixOS</title>
<title>Obtaining NixOS</title>
<para>NixOS ISO images can be downloaded from the <link
homepage</link>. These can be burned onto a CD. It is also possible
to copy them onto a USB stick and install NixOS from there. For
details, see the <link
<listitem><para>Boot from the CD.</para></listitem>
<listitem><para>The CD contains a basic NixOS installation. (It
also contains Memtest86+, useful if you want to test new hardware.)
When it’s finished booting, it should have detected most of your
hardware and brought up networking (check
<command>ifconfig</command>). Networking is necessary for the
installer, since it will download lots of stuff (such as source
tarballs or Nixpkgs channel binaries). It’s best if you have a DHCP
server on your network. Otherwise configure networking manually
using <command>ifconfig</command>.</para></listitem>
<listitem><para>The NixOS manual is available on virtual console 8
(press Alt+F8 to access).</para></listitem>
<listitem><para>Login as <literal>root</literal>, empty
<listitem><para>If you downloaded the graphical ISO image, you can
run <command>start display-manager</command> to start KDE.</para></listitem>
<listitem><para>The NixOS installer doesn’t do any partitioning or
formatting yet, so you need to that yourself. Use the following
<listitem><para>For partitioning:
<listitem><para>For initialising Ext4 partitions:
<command>mkfs.ext4</command>. It is recommended that you assign a
unique symbolic label to the file system using the option
<option>-L <replaceable>label</replaceable></option>. This will
make the file system configuration independent from device
<listitem><para>For creating swap partitions:
<command>mkswap</command>. Again it’s recommended to assign a
label to the swap partition: <option>-L
<listitem><para>For creating LVM volumes, the LVM commands, e.g.,
$ pvcreate /dev/sda1 /dev/sdb1
$ vgcreate MyVolGroup /dev/sda1 /dev/sdb1
$ lvcreate --size 2G --name bigdisk MyVolGroup
$ lvcreate --size 1G --name smalldisk MyVolGroup</screen>
<listitem><para>For creating software RAID devices, use
<listitem><para>Mount the target file system on which NixOS should
be installed on <filename>/mnt</filename>.</para></listitem>
<para>You now need to create a file
<filename>/mnt/etc/nixos/configuration.nix</filename> that
specifies the intended configuration of the system. This is
because NixOS has a <emphasis>declarative</emphasis> configuration
model: you create or edit a description of the configuration that
you want to be built and activated, and then NixOS takes care of
realising that configuration. The command
<command>nixos-option</command> can generate an initial
configuration file for you:
$ nixos-option --install</screen>
It tries to figure out the kernel modules necessary for mounting
the root device, as well as various other hardware
characteristics. However, it doesn’t try to figure out the
<option>fileSystems</option> option yet.</para>
<para>You should edit
<filename>/mnt/etc/nixos/configuration.nix</filename> to suit your
needs. The text editors <command>nano</command> and
<command>vim</command> are available.</para>
<para>You need to specify a root file system in
<option>fileSystems</option> and the target device for the Grub boot
loader in <option>boot.loader.grub.device</option>. See
<xref linkend="ch-options"/> for a list of the available configuration
<note><para>It is very important that you specify in the option
<option>boot.initrd.kernelModules</option> all kernel modules that
are necessary for mounting the root file system, otherwise the
installed system will not be able to boot. (If this happens, boot
from the CD again, mount the target file system on
<filename>/mnt</filename>, fix
<filename>/mnt/etc/nixos/configuration.nix</filename> and rerun
<filename>nixos-install</filename>.) In most cases,
<command>nixos-option --install</command> will figure out the
required modules.</para></note>
<para>Examples of real-world NixOS configuration files can be
found at <link
<listitem><para>If your machine has a limited amount of memory, you
may want to activate swap devices now (<command>swapon
<replaceable>device</replaceable></command>). The installer (or
rather, the build actions that it may spawn) may need quite a bit of
RAM, depending on your configuration.</para></listitem>
<listitem><para>Optionally, you can run
$ nixos-checkout</screen>
to make the installer use the latest NixOS/Nixpkgs sources from the
Git repository, rather than the sources on CD.</para></listitem>
<listitem><para>Do the installation:
$ nixos-install</screen>
Cross fingers.</para></listitem>
<listitem><para>If everything went well:
$ reboot</screen>
<para>You should now be able to boot into the installed NixOS.
The Grub boot menu shows a list of <emphasis>available
configurations</emphasis> (initially just one). Every time you
change the NixOS configuration (see <xref
linkend="sec-changing-config" />), a new item appears in the menu.
This allows you to easily roll back to another configuration if
something goes wrong.</para>
<para>You should log in and change the <literal>root</literal>
password with <command>passwd</command>.</para>
<para>You’ll probably want to create some user accounts as well,
which can be done with <command>useradd</command>:
$ useradd -c 'Eelco Dolstra' -m eelco
$ passwd eelco</screen>
<para>You may also want to install some software. For instance,
$ nix-env -qa \*</screen>
shows what packages are available, and
$ nix-env -i w3m</screen>
install the <literal>w3m</literal> browser.</para>
<para><xref linkend="ex-install-sequence" /> shows a typical sequence
of commands for installing NixOS on an empty hard drive (here
<filename>/dev/sda</filename>). <xref linkend="ex-config" /> shows a
corresponding configuration Nix expression.</para>
<example xml:id='ex-install-sequence'><title>Commands for installing NixOS on <filename>/dev/sda</filename></title>
$ fdisk /dev/sda <lineannotation>(or whatever device you want to install on)</lineannotation>
$ mkfs.ext4 -L nixos /dev/sda1 <lineannotation>(idem)</lineannotation>
$ mkswap -L swap /dev/sda2 <lineannotation>(idem)</lineannotation>
$ mount LABEL=nixos /mnt
$ nixos-option --install
$ nano /mnt/etc/nixos/configuration.nix
<lineannotation>(in particular, set the fileSystems and swapDevices options)</lineannotation>
$ nixos-install
$ reboot</screen>
<example xml:id='ex-config'><title>NixOS configuration</title>
boot.loader.grub.device = "/dev/sda";
fileSystems."/".device = "/dev/disk/by-label/nixos";
swapDevices =
[ { device = "/dev/disk/by-label/swap"; } ];
services.sshd.enable = true;
<section xml:id="sec-changing-config">
<title>Changing the configuration</title>
<para>The file <filename>/etc/nixos/configuration.nix</filename>
contains the current configuration of your machine. Whenever you’ve
changed something to that file, you should do
$ nixos-rebuild switch</screen>
to build the new configuration, make it the default configuration for
booting, and try to realise the configuration in the running system
(e.g., by restarting system services).</para>
<para>You can also do
$ nixos-rebuild test</screen>
to build the configuration and switch the running system to it, but
without making it the boot default. So if (say) the configuration
locks up your machine, you can just reboot to get back to a working
<para>There is also
$ nixos-rebuild boot</screen>
to build the configuration and make it the boot default, but not
switch to it now (so it will only take effect after the next
<para>Finally, you can do
$ nixos-rebuild build</screen>
to build the configuration but nothing more. This is useful to see
whether everything compiles cleanly.</para>
<para>If you have a machine that supports hardware virtualisation, you
can also test the new configuration in a sandbox by building and
running a <emphasis>virtual machine</emphasis> that contains the
desired configuration. Just do
$ nixos-rebuild build-vm
$ ./result/bin/run-*-vm
The VM does not have use any data from your host system, so your
existing user accounts and home directories will not be
<section xml:id="sec-upgrading">
<title>Upgrading NixOS</title>
<para>The best way to keep your NixOS installation up to date is to
use the <literal>nixos-unstable</literal> channel. (A channel is a
Nix mechanism for distributing Nix expressions and associated
binaries.) The NixOS channel is updated automatically from NixOS’s
Git repository after running certain tests and building most
<para>NixOS automatically subscribes you to the NixOS channel. If for
some reason this is not the case, just do
$ nix-channel --add
You can then upgrade NixOS to the latest version in the channel by
$ nix-channel --update nixos
and running the <command>nixos-rebuild</command> command as described
in <xref linkend="sec-changing-config"/>.</para>