346 lines
12 KiB
XML
346 lines
12 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>Development</title>
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<para>This chapter has some random notes on hacking on
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NixOS.</para>
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<section>
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<title>Extending NixOS</title>
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<para>A unique syntax is used to express all system, hardware, computer and
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service configurations. This syntax helps for reading and writing new
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configuration files. It is coming with some extra strategies defined in
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NixPkgs which are used to merge and evaluate all configuration files.</para>
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<para>A configuration file is the same as your own computer
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configuration.</para>
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<example xml:id='conf-syntax'><title>Usual configuration file</title>
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<programlisting>
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{config, modulesPath, pkgs, ...}: <co xml:id='conf-syntax-1' />
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let
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inherit (pkgs.lib) mkOption mkIf mkThenElse types; <co xml:id='conf-syntax-2' />
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cfg = config.services.locate; <co xml:id='conf-syntax-4' />
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locatedb = "/var/cache/locatedb";
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logfile = "/var/log/updatedb";
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cmd = "root updatedb --localuser=nobody --output=${locatedb} > ${logfile}";
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in
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{
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imports = [ <co xml:id='conf-syntax-6' />
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(modulesPath + /services/scheduling/cron.nix)
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];
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options = { <co xml:id='conf-syntax-3' />
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services.locate = {
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enable = mkOption {
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default = false;
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example = true;
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type = types.bool;
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description = ''
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If enabled, NixOS will periodically update the database of
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files used by the <command>locate</command> command.
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'';
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};
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period = mkOption {
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default = "15 02 * * *";
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type = types.uniq type.string;
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description = ''
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This option defines (in the format used by cron) when the
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locate database is updated.
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The default is to update at 02:15 (at night) every day.
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'';
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};
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};
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};
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config = mkIf cfg.enable { <co xml:id='conf-syntax-5' />
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services.cron = {
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systemCronJobs = mkThenElse { <co xml:id='conf-syntax-7' />
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thenPart = "${cfg.period} root ${cmd}";
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elsePart = "";
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};
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};
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};
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}</programlisting>
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</example>
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<para><xref linkend='conf-syntax' /> shows the <emphasis>configuration
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file</emphasis> for the locate service which uses cron to update the
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database at some dates which can be defined by the user. This nix
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expression is coming
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from <filename>upstart-jobs/cron/locate.nix</filename>. It shows a simple
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example of a service that can be either distributed on many computer that
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are using the same configuration or to shared with the community. This
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file is divided in two with the interface and the implementation. Both
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the interface and the implementation declare a <emphasis>configuration
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set</emphasis>.
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<calloutlist>
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<callout arearefs='conf-syntax-1'>
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<para>This line declares the arguments of the configuration file. You
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can omit this line if there is no reference to <varname>pkgs</varname>
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and <varname>config</varname> inside the configuration file.</para>
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<para>The argument <varname>pkgs</varname> refers to NixPkgs and allow
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you to access all attributes contained inside it. In this
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example <varname>pkgs</varname> is used to retrieve common functions to
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ease the writing of configuration files
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like <varname>mkOption</varname>, <varname>mkIf</varname>
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and <varname>mkThenElse</varname>.</para>
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<para>The argument <varname>config</varname> corresponds to the whole
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NixOS configuration. This is a set which is build by merging all
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configuration files imported to set up the system. Thus all options
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declared are contained inside this variable. In this
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example <varname>config</varname> is used to retrieve the status of
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the <varname>enable</varname> flag. The important point of this
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argument is that it contains either the result of the merge of different
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settings or the default value, therefore you cannot assume
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that <option>config.services.locate.enable</option> is always false
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because it may have been defined in another configuration file.</para>
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</callout>
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<callout arearefs='conf-syntax-2'>
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<para>This line is used to import a functions that are useful for
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writing this configuration file.</para>
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</callout>
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<callout arearefs='conf-syntax-3'>
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<para>The variable <varname>options</varname> is
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a <emphasis>configuration set</emphasis> which is only used to declare
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options with the function <varname>mkOption</varname> imported
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from <filename>pkgs/lib/default.nix</filename>. Options may contained
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any attribute but only the following have a special
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meaning: <varname>default</varname>, <varname>example</varname>,
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<varname>description</varname>, <varname>merge</varname>
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and <varname>apply</varname>.</para>
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<para>The <varname>merge</varname> attribute is used to merge all values
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defined in all configuration files and this function return a value
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which has the same type as the default value. If the merge function is
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not defined, then a default function
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(<varname>pkgs.lib.mergeDefaultOption</varname>) is used to merge
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values. The <varname>merge</varname> attribute is a function which
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expect two arguments: the location of the option and the list of values
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which have to be merged.</para>
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<para>The <varname>apply</varname> attribute is a function used to
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process the option. Thus the value return
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by <option>config.<replaceable>option</replaceable></option> would be
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the result of the <varname>apply</varname> function called with either
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the <varname>default</varname> value or the result of
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the <varname>merge</varname> function.</para>
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</callout>
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<callout arearefs='conf-syntax-4'>
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<para>This line is a common trick used to reduce the amount of
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writing. In this case <varname>cfg</varname> is just a sugar over
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<option>config.services.locate</option></para>
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</callout>
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<callout arearefs='conf-syntax-5'>
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<para>This line is used to declare a special <emphasis>IF</emphasis>
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statement. If you had put a usual <emphasis>IF</emphasis> statement
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here, with the same condition, then you will get an infinite loop. The
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reason is that your condition ask for the value of the
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option <option>config.services.locate.enable</option> but in order to
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get this value you have to evaluate all configuration sets including the
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configuration set contained inside your file.</para>
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<para>To remove this extra complexity, <varname>mkIf</varname> has been
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introduced to get rid of possible infinite loop and to factor your
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writing.</para>
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</callout>
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<callout arearefs='conf-syntax-6'>
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<para>The attribute <varname>imports</varname> contains a list of other
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module location. These modules should provide option declarations for
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the current module in order to be evaluated safely.</para>
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<para>When a dependence on a NixOS module has to be made, then you
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should use the argument <varname>modulesPath</varname> to prefix the
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location of your NixOS repository.</para>
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</callout>
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<callout arearefs='conf-syntax-7'>
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<para>The attribute <varname>config</varname>, which should not be
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confused with the argument of the same name, contains a set of option
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definitions defined by this module. When there is
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neither <varname>imports</varname> nor <varname>options</varname>, then
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this attribute set can be return without any enclosing. This feature
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allow you to write your <filename>configuration.nix</filename> as a
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module which does not contains any option declarations.</para>
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<para>As <varname>mkIf</varname> does not need
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any <emphasis>then</emphasis> part or <emphasis>else</emphasis> part,
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then you can specify one on each option definition with the
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function <varname>mkThenElse</varname>.</para>
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<para>To avoid a lot of <varname>mkThenElse</varname> with empty
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<emphasis>else</emphasis> part, a sugar has been added to infer the
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corresponding <emphasis>empty</emphasis>-value of your option when the
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function <varname>mkThenElse</varname> is not used.</para>
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<para>If your <emphasis>then</emphasis> part
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and <emphasis>else</emphasis> part are identical, then you should use
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the function <varname>mkAlways</varname> to ignore the condition.</para>
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<para>If you need to add another condition, then you can add <varname>mkIf</varname> to on
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the appropriate location. Thus the <emphasis>then</emphasis> part will
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only be used if all conditions declared with <varname>mkIf</varname>
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are satisfied.</para>
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</callout>
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</calloutlist>
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</para>
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</section>
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<section>
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<title>Building specific parts of NixOS</title>
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<para>
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<screen>
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$ nix-build /etc/nixos/nixos -A <replaceable>attr</replaceable></screen>
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where <replaceable>attr</replaceable> is an attribute in
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<filename>/etc/nixos/nixos/default.nix</filename>. Attributes of interest include:
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<variablelist>
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<varlistentry>
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<term><varname>config</varname></term>
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<listitem><para>The computer configuration generated from
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the <envar>NIXOS_CONFIG</envar> environment variable (default
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is <filename>/etc/nixos/configuration.nix</filename>) with the NixOS
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default set of modules.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>system</varname></term>
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<listitem><para>The derivation which build your computer system. It is
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built by the command <command>nixos-rebuild
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build</command></para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>vm</varname></term>
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<listitem><para>The derivation which build your computer system inside a
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virtual machine. It is built by the command <command>nixos-rebuild
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build-vm</command></para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>
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Most parts of NixOS can be build through the <varname>config</varname>
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attribute set. This attribute set allows you to have a view of the merged
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option definitions and all its derivations. Important derivations are store
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inside the option <option>system.build</option> and can be listed with the
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command <command>nix-instantiate --xml --eval-only /etc/nixos/nixos -A
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config.system.build</command>
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</para>.
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</section>
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<section>
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<title>Building your own NixOS CD</title>
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<para>Building a NixOS CD is as easy as configuring your own computer. The
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idea is to use another module which will replace
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your <filename>configuration.nix</filename> to configure the system that
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would be install on the CD.</para>
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<para>Default CD/DVD configurations are available
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inside <filename>nixos/modules/installer/cd-dvd</filename>. To build them
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you have to set <envar>NIXOS_CONFIG</envar> before
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running <command>nix-build</command> to build the ISO.
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<screen>
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$ export NIXOS_CONFIG=/etc/nixos/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix
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$ nix-build /etc/nixos/nixos -A config.system.build.isoImage</screen>
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</para>
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<para>Before burning your CD/DVD, you can check the content of the image by mounting anywhere like
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suggested by the following command:
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<screen>
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$ mount -o loop -t iso9660 ./result/iso/cd.iso /mnt/iso</screen>
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</para>
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</section>
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<section>
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<title>Testing the installer</title>
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<para>Building, burning, and booting from an installation CD is rather
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tedious, so here is a quick way to see if the installer works
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properly:
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<screen>
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$ export NIXOS_CONFIG=/etc/nixos/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix
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$ nix-build /etc/nixos/nixos -A config.system.build.nixosInstall
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$ dd if=/dev/zero of=diskimage seek=2G count=0 bs=1
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$ yes | mke2fs -j diskimage
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$ mount -o loop diskimage /mnt
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$ ./result/bin/nixos-install</screen>
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</para>
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</section>
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<section>
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<title>Testing the <literal>initrd</literal></title>
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<para>A quick way to test whether the kernel and the initial ramdisk
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boot correctly is to use QEMU’s <option>-kernel</option> and
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<option>-initrd</option> options:
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<screen>
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$ nix-build /etc/nixos/nixos -A config.system.build.initialRamdisk -o initrd
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$ nix-build /etc/nixos/nixos -A config.system.build.kernel -o kernel
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$ qemu-system-x86_64 -kernel ./kernel/vmlinuz -initrd ./initrd/initrd -hda /dev/null
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</screen>
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</para>
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</section>
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</chapter>
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