<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-shell">

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

<refnamediv>
  <refname>nix-shell</refname>
  <refpurpose>start an interactive shell based on a Nix expression</refpurpose>
</refnamediv>

<refsynopsisdiv>
  <cmdsynopsis>
    <command>nix-shell</command>
    <arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
    <arg><option>--argstr</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>--command</option> <replaceable>cmd</replaceable></arg>
    <arg><option>--run</option> <replaceable>cmd</replaceable></arg>
    <arg><option>--exclude</option> <replaceable>regexp</replaceable></arg>
    <arg><option>--pure</option></arg>
    <arg><option>--keep</option> <replaceable>name</replaceable></arg>
    <group choice='req'>
      <arg choice='plain'>
        <group choice='req'>
          <arg choice='plain'><option>--packages</option></arg>
          <arg choice='plain'><option>-p</option></arg>
        </group>
        <arg choice='plain' rep='repeat'><replaceable>packages</replaceable></arg>
      </arg>
      <arg><replaceable>path</replaceable></arg>
    </group>
  </cmdsynopsis>
</refsynopsisdiv>

<refsection><title>Description</title>

<para>The command <command>nix-shell</command> will build the
dependencies of the specified derivation, but not the derivation
itself.  It will then start an interactive shell in which all
environment variables defined by the derivation
<replaceable>path</replaceable> have been set to their corresponding
values, and the script <literal>$stdenv/setup</literal> has been
sourced.  This is useful for reproducing the environment of a
derivation for development.</para>

<para>If <replaceable>path</replaceable> is not given,
<command>nix-shell</command> defaults to
<filename>shell.nix</filename> if it exists, and
<filename>default.nix</filename> otherwise.</para>

<para>If <replaceable>path</replaceable> starts with
<literal>http://</literal> or <literal>https://</literal>, it is
interpreted as the URL of a tarball that will be downloaded and
unpacked to a temporary location. The tarball must include a single
top-level directory containing at least a file named
<filename>default.nix</filename>.</para>

<para>If the derivation defines the variable
<varname>shellHook</varname>, it will be evaluated after
<literal>$stdenv/setup</literal> has been sourced.  Since this hook is
not executed by regular Nix builds, it allows you to perform
initialisation specific to <command>nix-shell</command>.  For example,
the derivation attribute

<programlisting>
shellHook =
  ''
    echo "Hello shell"
  '';
</programlisting>

will cause <command>nix-shell</command> to print <literal>Hello shell</literal>.</para>

</refsection>


<refsection><title>Options</title>

<para>All options not listed here are passed to <command>nix-store
--realise</command>, except for <option>--arg</option> and
<option>--attr</option> / <option>-A</option> which are passed to
<command>nix-instantiate</command>.  <phrase condition="manual">See
also <xref linkend="sec-common-options" />.</phrase></para>

<variablelist>

  <varlistentry><term><option>--command</option> <replaceable>cmd</replaceable></term>

    <listitem><para>In the environment of the derivation, run the
    shell command <replaceable>cmd</replaceable>. This command is
    executed in an interactive shell. (Use <option>--run</option> to
    use a non-interactive shell instead.) However, a call to
    <literal>exit</literal> is implicitly added to the command, so the
    shell will exit after running the command. To prevent this, add
    <literal>return</literal> at the end; e.g. <literal>--command
    "echo Hello; return"</literal> will print <literal>Hello</literal>
    and then drop you into the interactive shell. This can be useful
    for doing any additional initialisation.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--run</option> <replaceable>cmd</replaceable></term>

    <listitem><para>Like <option>--command</option>, but executes the
    command in a non-interactive shell. This means (among other
    things) that if you hit Ctrl-C while the command is running, the
    shell exits.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--exclude</option> <replaceable>regexp</replaceable></term>

    <listitem><para>Do not build any dependencies whose store path
    matches the regular expression <replaceable>regexp</replaceable>.
    This option may be specified multiple times.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--pure</option></term>

    <listitem><para>If this flag is specified, the environment is
    almost entirely cleared before the interactive shell is started,
    so you get an environment that more closely corresponds to the
    “real” Nix build.  A few variables, in particular
    <envar>HOME</envar>, <envar>USER</envar> and
    <envar>DISPLAY</envar>, are retained.  Note that
    <filename>~/.bashrc</filename> and (depending on your Bash
    installation) <filename>/etc/bashrc</filename> are still sourced,
    so any variables set there will affect the interactive
    shell.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--packages</option> / <option>-p</option> <replaceable>packages</replaceable>…</term>

    <listitem><para>Set up an environment in which the specified
    packages are present.  The command line arguments are interpreted
    as attribute names inside the Nix Packages collection.  Thus,
    <literal>nix-shell -p libjpeg openjdk</literal> will start a shell
    in which the packages denoted by the attribute names
    <varname>libjpeg</varname> and <varname>openjdk</varname> are
    present.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>-i</option> <replaceable>interpreter</replaceable></term>

    <listitem><para>The chained script interpreter to be invoked by
    <command>nix-shell</command>. Only applicable in
    <literal>#!</literal>-scripts (described <link
    linkend="ssec-nix-shell-shebang">below</link>).</para>

    </listitem></varlistentry>

  <varlistentry><term><option>--keep</option> <replaceable>name</replaceable></term>

    <listitem><para>When a <option>--pure</option> shell is started,
    keep the listed environment variables.</para></listitem>

  </varlistentry>

</variablelist>

<para>The following common options are supported:</para>

<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>Environment variables</title>

<variablelist>

  <varlistentry><term><envar>NIX_BUILD_SHELL</envar></term>
    
    <listitem><para>Shell used to start the interactive environment. 
    Defaults to the <command>bash</command> found in <envar>PATH</envar>.</para></listitem>

  </varlistentry>

</variablelist>

</refsection>


<refsection><title>Examples</title>

<para>To build the dependencies of the package Pan, and start an
interactive shell in which to build it:

<screen>
$ nix-shell '&lt;nixpkgs>' -A pan
[nix-shell]$ unpackPhase
[nix-shell]$ cd pan-*
[nix-shell]$ configurePhase
[nix-shell]$ buildPhase
[nix-shell]$ ./pan/gui/pan
</screen>

To clear the environment first, and do some additional automatic
initialisation of the interactive shell:

<screen>
$ nix-shell '&lt;nixpkgs>' -A pan --pure \
    --command 'export NIX_DEBUG=1; export NIX_CORES=8; return'
</screen>

Nix expressions can also be given on the command line.  For instance,
the following starts a shell containing the packages
<literal>sqlite</literal> and <literal>libX11</literal>:

<screen>
$ nix-shell -E 'with import &lt;nixpkgs> { }; runCommand "dummy" { buildInputs = [ sqlite xorg.libX11 ]; } ""'
</screen>

A shorter way to do the same is:

<screen>
$ nix-shell -p sqlite xorg.libX11
[nix-shell]$ echo $NIX_LDFLAGS
… -L/nix/store/j1zg5v…-sqlite-3.8.0.2/lib -L/nix/store/0gmcz9…-libX11-1.6.1/lib …
</screen>

The <command>-p</command> flag looks up Nixpkgs in the Nix search
path. You can override it by passing <option>-I</option> or setting
<envar>NIX_PATH</envar>. For example, the following gives you a shell
containing the Pan package from a specific revision of Nixpkgs:

<screen>
$ nix-shell -p pan -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/8a3eea054838b55aca962c3fbde9c83c102b8bf2.tar.gz

[nix-shell:~]$ pan --version
Pan 0.139
</screen>

</para>

</refsection>


<refsection xml:id="ssec-nix-shell-shebang"><title>Use as a <literal>#!</literal>-interpreter</title>

<para>You can use <command>nix-shell</command> as a script interpreter
to allow scripts written in arbitrary languages to obtain their own
dependencies via Nix. This is done by starting the script with the
following lines:

<programlisting>
#! /usr/bin/env nix-shell
#! nix-shell -i <replaceable>real-interpreter</replaceable> -p <replaceable>packages</replaceable>
</programlisting>

where <replaceable>real-interpreter</replaceable> is the “real” script
interpreter that will be invoked by <command>nix-shell</command> after
it has obtained the dependencies and initialised the environment, and
<replaceable>packages</replaceable> are the attribute names of the
dependencies in Nixpkgs.</para>

<para>The lines starting with <literal>#! nix-shell</literal> specify
<command>nix-shell</command> options (see above). Note that you cannot
write <literal>#! /usr/bin/env nix-shell -i ...</literal> because
many operating systems only allow one argument in
<literal>#!</literal> lines.</para>

<para>For example, here is a Python script that depends on Python and
the <literal>prettytable</literal> package:

<programlisting>
#! /usr/bin/env nix-shell
#! nix-shell -i python -p python pythonPackages.prettytable

import prettytable

# Print a simple table.
t = prettytable.PrettyTable(["N", "N^2"])
for n in range(1, 10): t.add_row([n, n * n])
print t
</programlisting>

</para>

<para>Similarly, the following is a Perl script that specifies that it
requires Perl and the <literal>HTML::TokeParser::Simple</literal> and
<literal>LWP</literal> packages:

<programlisting>
#! /usr/bin/env nix-shell
#! nix-shell -i perl -p perl perlPackages.HTMLTokeParserSimple perlPackages.LWP

use HTML::TokeParser::Simple;

# Fetch nixos.org and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');

while (my $token = $p->get_tag("a")) {
    my $href = $token->get_attr("href");
    print "$href\n" if $href;
}
</programlisting>

</para>

<para>Sometimes you need to pass a simple Nix expression to customize
a package like Terraform:

<programlisting><![CDATA[
#! /usr/bin/env nix-shell
#! nix-shell -i bash -p "terraform.withPlugins (plugins: [ plugins.openstack ])"

terraform apply
]]></programlisting>

<note><para>You must use double quotes (<literal>"</literal>) when
passing a simple Nix expression in a nix-shell shebang.</para></note>
</para>

<para>Finally, using the merging of multiple nix-shell shebangs the
following Haskell script uses a specific branch of Nixpkgs/NixOS (the
18.03 stable branch):

<programlisting><![CDATA[
#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p "haskellPackages.ghcWithPackages (ps: [ps.HTTP ps.tagsoup])"
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-18.03.tar.gz

import Network.HTTP
import Text.HTML.TagSoup

-- Fetch nixos.org and print all hrefs.
main = do
  resp <- Network.HTTP.simpleHTTP (getRequest "http://nixos.org/")
  body <- getResponseBody resp
  let tags = filter (isTagOpenName "a") $ parseTags body
  let tags' = map (fromAttrib "href") tags
  mapM_ putStrLn $ filter (/= "") tags'
]]></programlisting>

If you want to be even more precise, you can specify a specific
revision of Nixpkgs:

<programlisting>
#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/0672315759b3e15e2121365f067c1c8c56bb4722.tar.gz
</programlisting>

</para>

<para>The examples above all used <option>-p</option> to get
dependencies from Nixpkgs. You can also use a Nix expression to build
your own dependencies. For example, the Python example could have been
written as:

<programlisting>
#! /usr/bin/env nix-shell
#! nix-shell deps.nix -i python
</programlisting>

where the file <filename>deps.nix</filename> in the same directory
as the <literal>#!</literal>-script contains:

<programlisting>
with import &lt;nixpkgs> {};

runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""
</programlisting>

</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>
