<refentry 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-nix-instantiate">

<refmeta>
  <refentrytitle>nix-instantiate</refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo class="source">Nix</refmiscinfo>
  <refmiscinfo class="version"><xi:include href="../version.txt" parse="text"/></refmiscinfo>
</refmeta>

<refnamediv>
  <refname>nix-instantiate</refname>
  <refpurpose>instantiate store derivations from Nix expressions</refpurpose>
</refnamediv>

<refsynopsisdiv>
  <cmdsynopsis>
    <command>nix-instantiate</command>
    <group>
      <arg choice='plain'><option>--parse</option></arg>
      <arg choice='plain'>
        <option>--eval</option>
        <arg><option>--strict</option></arg>
        <arg><option>--json</option></arg>
        <arg><option>--xml</option></arg>
      </arg>
    </group>
    <arg><option>--read-write-mode</option></arg>
    <arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
    <arg>
      <group choice='req'>
        <arg choice='plain'><option>--attr</option></arg>
        <arg choice='plain'><option>-A</option></arg>
      </group>
      <replaceable>attrPath</replaceable>
    </arg>
    <arg><option>--add-root</option> <replaceable>path</replaceable></arg>
    <arg><option>--indirect</option></arg>
    <arg><option>--<arg>no</arg>trace-file-access</option></arg>
    <group>
      <arg choice='plain'><option>--expr</option></arg>
      <arg choice='plain'><option>-E</option></arg>
    </group>
    <arg choice='plain' rep='repeat'><replaceable>files</replaceable></arg>
  </cmdsynopsis>
  <cmdsynopsis>
    <command>nix-instantiate</command>
    <arg choice='plain'><option>--find-file</option></arg>
    <arg choice='plain' rep='repeat'><replaceable>files</replaceable></arg>
  </cmdsynopsis>
</refsynopsisdiv>


<refsection><title>Description</title>

<para>The command <command>nix-instantiate</command> generates <link
linkend="gloss-derivation">store derivations</link> from (high-level)
Nix expressions.  It evaluates the Nix expressions in each of
<replaceable>files</replaceable> (which defaults to
<replaceable>./default.nix</replaceable>).  Each top-level expression
should evaluate to a derivation, a list of derivations, or a set of
derivations.  The paths of the resulting store derivations are printed
on standard output.</para>

<para>If <replaceable>files</replaceable> is the character
<literal>-</literal>, then a Nix expression will be read from standard
input.</para>

<para condition="manual">See also <xref linkend="sec-common-options"
/> for a list of common options.</para>

</refsection>


<refsection><title>Options</title>

<variablelist>

  <varlistentry>
    <term><option>--add-root</option> <replaceable>path</replaceable></term>
    <term><option>--indirect</option></term>

    <listitem><para>See the <link linkend="opt-add-root">corresponding
    options</link> in <command>nix-store</command>.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--parse</option></term>

    <listitem><para>Just parse the input files, and print their
    abstract syntax trees on standard output in ATerm
    format.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--eval</option></term>

    <listitem><para>Just parse and evaluate the input files, and print
    the resulting values on standard output.  No instantiation of
    store derivations takes place.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--find-file</option></term>

    <listitem><para>Look up the given files in Nix’s search path (as
    specified by the <envar linkend="env-NIX_PATH">NIX_PATH</envar>
    environment variable).  If found, print the corresponding absolute
    paths on standard output.  For instance, if
    <envar>NIX_PATH</envar> is
    <literal>nixpkgs=/home/alice/nixpkgs</literal>, then
    <literal>nix-instantiate --find-file nixpkgs/default.nix</literal>
    will print
    <literal>/home/alice/nixpkgs/default.nix</literal>.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--strict</option></term>

    <listitem><para>When used with <option>--eval</option>,
    recursively evaluate list elements and attributes.  Normally, such
    sub-expressions are left unevaluated (since the Nix expression
    language is lazy).</para>

    <warning><para>This option can cause non-termination, because lazy
    data structures can be infinitely large.</para></warning>

    </listitem>

  </varlistentry>

  <varlistentry><term><option>--<arg>no</arg>trace-file-access</option></term>
    <listitem><para>While instantiating the expression, the evaluator will
    print the full path to any files it reads with the prefix
    <envar>trace-file-access: </envar> to the standard error.</para>
    </listitem>
  </varlistentry>

  <varlistentry><term><option>--json</option></term>

    <listitem><para>When used with <option>--eval</option>, print the resulting
    value as an JSON representation of the abstract syntax tree rather
    than as an ATerm.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--xml</option></term>

    <listitem><para>When used with <option>--eval</option>, print the resulting
    value as an XML representation of the abstract syntax tree rather than as
    an ATerm. The schema is the same as that used by the <link
    linkend="builtin-toXML"><function>toXML</function> built-in</link>.
    </para></listitem>

  </varlistentry>

  <varlistentry><term><option>--read-write-mode</option></term>

    <listitem><para>When used with <option>--eval</option>, perform
    evaluation in read/write mode so nix language features that
    require it will still work (at the cost of needing to do
    instantiation of every evaluated derivation). If this option is
    not enabled, there may be uninstantiated store paths in the final
    output.</para>

    </listitem>

  </varlistentry>

</variablelist>

<variablelist condition="manpage">
  <xi:include href="opt-common.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(//db:variablelist[@xml:id='opt-common']/*)" />
</variablelist>

</refsection>


<refsection><title>Examples</title>

<para>Instantiating store derivations from a Nix expression, and
building them using <command>nix-store</command>:

<screen>
$ nix-instantiate test.nix <lineannotation>(instantiate)</lineannotation>
/nix/store/cigxbmvy6dzix98dxxh9b6shg7ar5bvs-perl-BerkeleyDB-0.26.drv

$ nix-store -r $(nix-instantiate test.nix) <lineannotation>(build)</lineannotation>
<replaceable>...</replaceable>
/nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26 <lineannotation>(output path)</lineannotation>

$ ls -l /nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26
dr-xr-xr-x    2 eelco    users        4096 1970-01-01 01:00 lib
...</screen>

</para>

<para>You can also give a Nix expression on the command line:

<screen>
$ nix-instantiate -E 'with import &lt;nixpkgs> { }; hello'
/nix/store/j8s4zyv75a724q38cb0r87rlczaiag4y-hello-2.8.drv
</screen>

This is equivalent to:

<screen>
$ nix-instantiate '&lt;nixpkgs>' -A hello
</screen>

</para>

<para>Parsing and evaluating Nix expressions:

<screen>
$ nix-instantiate --parse -E '1 + 2'
1 + 2

$ nix-instantiate --eval -E '1 + 2'
3

$ nix-instantiate --eval --xml -E '1 + 2'
<![CDATA[<?xml version='1.0' encoding='utf-8'?>
<expr>
  <int value="3" />
</expr>]]></screen>

</para>

<para>The difference between non-strict and strict evaluation:

<screen>
$ nix-instantiate --eval --xml -E 'rec { x = "foo"; y = x; }'
<replaceable>...</replaceable><![CDATA[
  <attr name="x">
    <string value="foo" />
  </attr>
  <attr name="y">
    <unevaluated />
  </attr>]]>
<replaceable>...</replaceable></screen>

Note that <varname>y</varname> is left unevaluated (the XML
representation doesn’t attempt to show non-normal forms).

<screen>
$ nix-instantiate --eval --xml --strict -E 'rec { x = "foo"; y = x; }'
<replaceable>...</replaceable><![CDATA[
  <attr name="x">
    <string value="foo" />
  </attr>
  <attr name="y">
    <string value="foo" />
  </attr>]]>
<replaceable>...</replaceable></screen>

</para>

</refsection>

<refsection><title>Conformance</title>
  <para>The <option>--trace-file-access</option> option is a nonstandard
  extension added by Tvix in 2020.</para>
</refsection>

<refsection condition="manpage"><title>Environment variables</title>

<variablelist>
  <xi:include href="env-common.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(//db:variablelist[@xml:id='env-common']/*)" />
</variablelist>

</refsection>


</refentry>
