Document nix-shell #!-scripts
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Eelco Dolstra
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@ -149,6 +149,15 @@ also <xref linkend="sec-common-options" />.</phrase></para>
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</varlistentry>
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<varlistentry><term><option>-i</option> <replaceable>interpreter</replaceable></term>
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<listitem><para>The chained script interpreter to be invoked by
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<command>nix-shell</command>. Only applicable in
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<literal>#!</literal>-scripts (described <link
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linkend="ssec-nix-shell-shebang">below</link>).</para>
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</listitem></varlistentry>
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</variablelist>
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<para>The following common options are supported:</para>
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@ -203,6 +212,121 @@ $ nix-shell -p sqlite xorg.libX11
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</refsection>
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<refsection xml:id="ssec-nix-shell-shebang"><title>Use as a <literal>#!</literal>-interpreter</title>
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<para>You can use <command>nix-shell</command> as a script interpreter
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to allow scripts written in arbitrary languages to obtain their own
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dependencies via Nix. This is done by starting the script with the
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following lines:
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<programlisting>
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#! /usr/bin/env nix-shell
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#! nix-shell -i <replaceable>real-interpreter</replaceable> -p <replaceable>packages</replaceable>
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</programlisting>
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where <replaceable>real-interpreter</replaceable> is the “real” script
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interpreter that will be invoked by <command>nix-shell</command> after
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it has obtained the dependencies and initialised the environment, and
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<replaceable>packages</replaceable> are the attribute names of the
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dependencies in Nixpkgs.</para>
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<para>The lines starting with <literal>#! nix-shell</literal> specify
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<command>nix-shell</command> options (see above). Note that you cannot
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write <literal>#1 /usr/bin/env nix-shell -i ...</literal> because
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<command>/usr/bin/env</command> does not support passing options to
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the interpreter.</para>
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<para>For example, here is a Python script that depends on Python and
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the <literal>prettytable</literal> package:
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<programlisting>
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#! /usr/bin/env nix-shell
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#! nix-shell -i python -p python pythonPackages.prettytable
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import prettytable
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# Print a simple table.
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t = prettytable.PrettyTable(["N", "N^2"])
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for n in range(1, 10): t.add_row([n, n * n])
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print t
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</programlisting>
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</para>
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<para>Similarly, the following is a Perl script that specifies that it
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requires Perl and the <literal>HTML::TokeParser::Simple</literal> and
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<literal>LWP</literal> packages:
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<programlisting>
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#! /usr/bin/env nix-shell
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#! nix-shell -i perl -p perl perlPackages.HTMLTokeParserSimple perlPackages.LWP
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use HTML::TokeParser::Simple;
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# Fetch nixos.org and print all hrefs.
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my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');
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while (my $token = $p->get_tag("a")) {
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my $href = $token->get_attr("href");
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print "$href\n" if $href;
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}
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</programlisting>
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</para>
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<para>Finally, the following Haskell script uses a specific branch of
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Nixpkgs/NixOS (the 14.12 stable branch):
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<programlisting><![CDATA[
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#! /usr/bin/env nix-shell
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#! nix-shell -i runghc -p haskellPackages.ghc haskellPackages.HTTP haskellPackages.tagsoup
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#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz
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import Network.HTTP
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import Text.HTML.TagSoup
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-- Fetch nixos.org and print all hrefs.
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main = do
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resp <- Network.HTTP.simpleHTTP (getRequest "http://nixos.org/")
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body <- getResponseBody resp
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let tags = filter (isTagOpenName "a") $ parseTags body
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let tags' = map (fromAttrib "href") tags
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mapM_ putStrLn $ filter (/= "") tags'
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]]></programlisting>
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If you want to be even more precise, you can specify a specific
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revision of Nixpkgs:
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<programlisting>
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#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/0672315759b3e15e2121365f067c1c8c56bb4722.tar.gz
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</programlisting>
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</para>
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<para>The examples above all used <option>-p</option> to get
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dependencies from Nixpkgs. You can also use a Nix expression to build
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your own dependencies. For example, the Python example could have been
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written as:
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<programlisting>
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#! /usr/bin/env nix-shell
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#! nix-shell deps.nix -i python
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</programlisting>
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where the file <filename>deps.nix</filename> in the same directory
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as the <literal>#!</literal>-script contains:
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<programlisting>
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with import <nixpkgs> {};
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runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""
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</programlisting>
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</para>
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</refsection>
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<refsection condition="manpage"><title>Environment variables</title>
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<variablelist>
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