<chapter xmlns="http://docbook.org/ns/docbook"
	 xmlns:xlink="http://www.w3.org/1999/xlink"
	 xml:id="chap-functions">

<title>Functions reference</title>

<para>
  The nixpkgs repository has several utility functions to manipulate Nix expressions.
</para>

<section xml:id="sec-pkgs-overridePackages">
  <title>pkgs.overridePackages</title>

  <para>
    This function inside the nixpkgs expression (<varname>pkgs</varname>)
    can be used to override the set of packages itself.
  </para>
  <para>
    Warning: this function is expensive and must not be used from within
    the nixpkgs repository.
  </para>
  <para>
    Example usage:

    <programlisting>let
  pkgs = import &lt;nixpkgs&gt; {};
  newpkgs = pkgs.overridePackages (self: super: {
    foo = super.foo.override { ... };
  };
in ...</programlisting>
  </para>

  <para>
    The resulting <varname>newpkgs</varname> will have the new <varname>foo</varname>
    expression, and all other expressions depending on <varname>foo</varname> will also
    use the new <varname>foo</varname> expression.
  </para>

  <para>
    The behavior of this function is similar to <link 
    linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>.
  </para>

  <para>
    The <varname>self</varname> parameter refers to the final package set with the
    applied overrides. Using this parameter may lead to infinite recursion if not
    used consciously.
  </para>

  <para>
    The <varname>super</varname> parameter refers to the old package set.
    It's equivalent to <varname>pkgs</varname> in the above example.
  </para>

  <para>
    Note that in previous versions of nixpkgs, this method replaced any changes from <link 
    linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>,
    along with that from previous calls if this function was called repeatedly.
    Now those previous changes will be preserved so this function can be "chained" meaningfully.
    To recover the old behavior, make sure <varname>config.packageOverrides</varname> is unset,
    and call this only once off a "freshly" imported nixpkgs:

    <programlisting>let
  pkgs = import &lt;nixpkgs&gt; { config: {}; };
  newpkgs = pkgs.overridePackages ...;
in ...</programlisting>
  </para>

</section>

<section xml:id="sec-pkg-override">
  <title>&lt;pkg&gt;.override</title>

  <para>
    The function <varname>override</varname> is usually available for all the
    derivations in the nixpkgs expression (<varname>pkgs</varname>).
  </para>
  <para>
    It is used to override the arguments passed to a function.
  </para>
  <para>
    Example usages:

    <programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
    <programlisting>pkgs.overridePackages (self: super: {
  foo = super.foo.override { barSupport = true ; };
})</programlisting>
    <programlisting>mypkg = pkgs.callPackage ./mypkg.nix {
  mydep = pkgs.mydep.override { ... };
})</programlisting>
  </para>

  <para>
    In the first example, <varname>pkgs.foo</varname> is the result of a function call
    with some default arguments, usually a derivation.
    Using <varname>pkgs.foo.override</varname> will call the same function with
    the given new arguments.
  </para>

</section>

<section xml:id="sec-pkg-overrideAttrs">
  <title>&lt;pkg&gt;.overrideAttrs</title>

  <para>
    The function <varname>overrideAttrs</varname> allows overriding the
    attribute set passed to a <varname>stdenv.mkDerivation</varname> call,
    producing a new derivation based on the original one.
    This function is available on all derivations produced by the
    <varname>stdenv.mkDerivation</varname> function, which is most packages
    in the nixpkgs expression <varname>pkgs</varname>.
  </para>

  <para>
    Example usage:

    <programlisting>helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
  separateDebugInfo = true;
});</programlisting>
  </para>

  <para>
    In the above example, the <varname>separateDebugInfo</varname> attribute is
    overriden to be true, thus building debug info for
    <varname>helloWithDebug</varname>, while all other attributes will be
    retained from the original <varname>hello</varname> package.
  </para>

  <para>
    The argument <varname>oldAttrs</varname> is conventionally used to refer to
    the attr set originally passed to <varname>stdenv.mkDerivation</varname>.
  </para>

  <note>
    <para>
      Note that <varname>separateDebugInfo</varname> is processed only by the
      <varname>stdenv.mkDerivation</varname> function, not the generated, raw
      Nix derivation. Thus, using <varname>overrideDerivation</varname> will
      not work in this case, as it overrides only the attributes of the final
      derivation. It is for this reason that <varname>overrideAttrs</varname>
      should be preferred in (almost) all cases to
      <varname>overrideDerivation</varname>, i.e. to allow using
      <varname>sdenv.mkDerivation</varname> to process input arguments, as well
      as the fact that it is easier to use (you can use the same attribute
      names you see in your Nix code, instead of the ones generated (e.g.
      <varname>buildInputs</varname> vs <varname>nativeBuildInputs</varname>,
      and involves less typing.
    </para>
  </note>

</section>


<section xml:id="sec-pkg-overrideDerivation">
  <title>&lt;pkg&gt;.overrideDerivation</title>

  <warning>
    <para>You should prefer <varname>overrideAttrs</varname> in almost all
    cases, see its documentation for the reasons why.
    <varname>overrideDerivation</varname> is not deprecated and will continue
    to work, but is less nice to use and does not have as many abilities as
    <varname>overrideAttrs</varname>.
    </para>
  </warning>

  <warning>
    <para>Do not use this function in Nixpkgs as it evaluates a Derivation
    before modifying it, which breaks package abstraction and removes
    error-checking of function arguments. In addition, this
    evaluation-per-function application incurs a performance penalty,
    which can become a problem if many overrides are used.
    It is only intended for ad-hoc customisation, such as in
    <filename>~/.nixpkgs/config.nix</filename>.
   </para>
  </warning>

  <para>
    The function <varname>overrideDerivation</varname> creates a new derivation
    based on an existing one by overriding the original's attributes with
    the attribute set produced by the specified function.
    This function is available on all
    derivations defined using the <varname>makeOverridable</varname> function.
    Most standard derivation-producing functions, such as
    <varname>stdenv.mkDerivation</varname>, are defined using this
    function, which means most packages in the nixpkgs expression,
    <varname>pkgs</varname>, have this function.
  </para> 

  <para>
    Example usage:

    <programlisting>mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
  name = "sed-4.2.2-pre";
  src = fetchurl {
    url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
    sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
  };
  patches = [];
});</programlisting>
  </para>

  <para>
    In the above example, the <varname>name</varname>, <varname>src</varname>,
    and <varname>patches</varname> of the derivation will be overridden, while
    all other attributes will be retained from the original derivation.
  </para>

  <para>
    The argument <varname>oldAttrs</varname> is used to refer to the attribute set of
    the original derivation.
  </para>

  <note>
    <para>
      A package's attributes are evaluated *before* being modified by
      the <varname>overrideDerivation</varname> function.
      For example, the <varname>name</varname> attribute reference
      in <varname>url = "mirror://gnu/hello/${name}.tar.gz";</varname>
      is filled-in *before* the <varname>overrideDerivation</varname> function
      modifies the attribute set. This means that overriding the
      <varname>name</varname> attribute, in this example, *will not* change the
      value of the <varname>url</varname> attribute. Instead, we need to override
      both the <varname>name</varname> *and* <varname>url</varname> attributes.
    </para>
  </note>

</section>

<section xml:id="sec-lib-makeOverridable">
  <title>lib.makeOverridable</title>

  <para>
    The function <varname>lib.makeOverridable</varname> is used to make the result
    of a function easily customizable. This utility only makes sense for functions
    that accept an argument set and return an attribute set.
  </para>

  <para>
    Example usage:

    <programlisting>f = { a, b }: { result = a+b; }
c = lib.makeOverridable f { a = 1; b = 2; }</programlisting>

  </para>

  <para>
    The variable <varname>c</varname> is the value of the <varname>f</varname> function
    applied with some default arguments. Hence the value of <varname>c.result</varname>
    is <literal>3</literal>, in this example.
  </para>

  <para>
    The variable <varname>c</varname> however also has some additional functions, like
    <link linkend="sec-pkg-override">c.override</link> which can be used to
    override the default arguments. In this example the value of
    <varname>(c.override { a = 4; }).result</varname> is 6.
  </para>

</section>


<section xml:id="sec-fhs-environments">
  <title>buildFHSUserEnv</title>

  <para>
    <function>buildFHSUserEnv</function> provides a way to build and run
    FHS-compatible lightweight sandboxes. It creates an isolated root with
    bound <filename>/nix/store</filename>, so its footprint in terms of disk
    space needed is quite small. This allows one to run software which is hard or
    unfeasible to patch for NixOS -- 3rd-party source trees with FHS assumptions,
    games distributed as tarballs, software with integrity checking and/or external
    self-updated binaries. It uses Linux namespaces feature to create
    temporary lightweight environments which are destroyed after all child
    processes exit, without root user rights requirement. Accepted arguments are:
  </para>

  <variablelist>
    <varlistentry>
    <term><literal>name</literal></term>

    <listitem><para>Environment name.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>targetPkgs</literal></term>

    <listitem><para>Packages to be installed for the main host's architecture
    (i.e. x86_64 on x86_64 installations). Along with libraries binaries are also
    installed.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>multiPkgs</literal></term>

    <listitem><para>Packages to be installed for all architectures supported by
    a host (i.e. i686 and x86_64 on x86_64 installations). Only libraries are
    installed by default.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraBuildCommands</literal></term>

    <listitem><para>Additional commands to be executed for finalizing the
    directory structure.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraBuildCommandsMulti</literal></term>

    <listitem><para>Like <literal>extraBuildCommands</literal>, but
    executed only on multilib architectures.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraOutputsToInstall</literal></term>

    <listitem><para>Additional derivation outputs to be linked for both
    target and multi-architecture packages.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>extraInstallCommands</literal></term>

    <listitem><para>Additional commands to be executed for finalizing the
    derivation with runner script.</para></listitem>
    </varlistentry>

    <varlistentry>
    <term><literal>runScript</literal></term>

    <listitem><para>A command that would be executed inside the sandbox and
    passed all the command line arguments. It defaults to
    <literal>bash</literal>.</para></listitem>
    </varlistentry>
  </variablelist>

  <para>
    One can create a simple environment using a <literal>shell.nix</literal>
    like that:
  </para>

<programlisting><![CDATA[
{ pkgs ? import <nixpkgs> {} }:

(pkgs.buildFHSUserEnv {
  name = "simple-x11-env";
  targetPkgs = pkgs: (with pkgs;
    [ udev
      alsaLib
    ]) ++ (with pkgs.xorg;
    [ libX11
      libXcursor
      libXrandr
    ]);
  multiPkgs = pkgs: (with pkgs;
    [ udev
      alsaLib
    ]);
  runScript = "bash";
}).env
]]></programlisting>

  <para>
    Running <literal>nix-shell</literal> would then drop you into a shell with
    these libraries and binaries available. You can use this to run
    closed-source applications which expect FHS structure without hassles:
    simply change <literal>runScript</literal> to the application path,
    e.g. <filename>./bin/start.sh</filename> -- relative paths are supported.
  </para>
</section>

<section xml:id="sec-pkgs-dockerTools">
 <title>pkgs.dockerTools</title>

 <para>
  <varname>pkgs.dockerTools</varname> is a set of functions for creating and
  manipulating Docker images according to the
  <link xlink:href="https://github.com/docker/docker/blob/master/image/spec/v1.md#docker-image-specification-v100">
   Docker Image Specification v1.0.0
  </link>. Docker itself is not used to perform any of the operations done by these
  functions.
 </para>

 <warning>
  <para>
   The <varname>dockerTools</varname> API is unstable and may be subject to
   backwards-incompatible changes in the future.
  </para>
 </warning>

 <section xml:id="ssec-pkgs-dockerTools-buildImage">
  <title>buildImage</title>

  <para>
   This function is analogous to the <command>docker build</command> command,
   in that can used to build a Docker-compatible repository tarball containing
   a single image with one or multiple layers. As such, the result
   is suitable for being loaded in Docker with <command>docker load</command>.
  </para>

  <para>
   The parameters of <varname>buildImage</varname> with relative example values are
   described below:
  </para>

  <example xml:id='ex-dockerTools-buildImage'><title>Docker build</title>
  <programlisting>
  buildImage {
    name = "redis"; <co xml:id='ex-dockerTools-buildImage-1' />
    tag = "latest"; <co xml:id='ex-dockerTools-buildImage-2' />
    
    fromImage = someBaseImage; <co xml:id='ex-dockerTools-buildImage-3' />
    fromImageName = null; <co xml:id='ex-dockerTools-buildImage-4' />
    fromImageTag = "latest"; <co xml:id='ex-dockerTools-buildImage-5' />
    
    contents = pkgs.redis; <co xml:id='ex-dockerTools-buildImage-6' />
    runAsRoot = '' <co xml:id='ex-dockerTools-buildImage-runAsRoot' />
      #!${stdenv.shell}
      mkdir -p /data
    '';

    config = { <co xml:id='ex-dockerTools-buildImage-8' />
      Cmd = [ "/bin/redis-server" ];
      WorkingDir = "/data";
      Volumes = {
        "/data" = {};
      };
    };
  }
  </programlisting>
  </example>

  <para>The above example will build a Docker image <literal>redis/latest</literal>
   from the given base image. Loading and running this image in Docker results in
   <literal>redis-server</literal> being started automatically.
  </para>

  <calloutlist>
   <callout arearefs='ex-dockerTools-buildImage-1'>
    <para>
     <varname>name</varname> specifies the name of the resulting image.
     This is the only required argument for <varname>buildImage</varname>.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-2'>
    <para>
     <varname>tag</varname> specifies the tag of the resulting image.
     By default it's <literal>latest</literal>.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-3'>
    <para>
     <varname>fromImage</varname> is the repository tarball containing the base image.
     It must be a valid Docker image, such as exported by <command>docker save</command>.
     By default it's <literal>null</literal>, which can be seen as equivalent
     to <literal>FROM scratch</literal> of a <filename>Dockerfile</filename>.
    </para>
   </callout>
   
   <callout arearefs='ex-dockerTools-buildImage-4'>
    <para>
     <varname>fromImageName</varname> can be used to further specify
     the base image within the repository, in case it contains multiple images.
     By default it's <literal>null</literal>, in which case
     <varname>buildImage</varname> will peek the first image available
     in the repository.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-5'>
    <para>
     <varname>fromImageTag</varname> can be used to further specify the tag
     of the base image within the repository, in case an image contains multiple tags.
     By default it's <literal>null</literal>, in which case
     <varname>buildImage</varname> will peek the first tag available for the base image.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-6'>
    <para>
     <varname>contents</varname> is a derivation that will be copied in the new
     layer of the resulting image. This can be similarly seen as
     <command>ADD contents/ /</command> in a <filename>Dockerfile</filename>.
     By default it's <literal>null</literal>.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-runAsRoot'>
    <para>
     <varname>runAsRoot</varname> is a bash script that will run as root
     in an environment that overlays the existing layers of the base image with
     the new resulting layer, including the previously copied
     <varname>contents</varname> derivation.
     This can be similarly seen as
     <command>RUN ...</command> in a <filename>Dockerfile</filename>.
     
     <note>
      <para>
       Using this parameter requires the <literal>kvm</literal>
       device to be available.
      </para>
     </note>
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-buildImage-8'>
    <para>
     <varname>config</varname> is used to specify the configuration of the
     containers that will be started off the built image in Docker.
     The available options are listed in the
     <link xlink:href="https://github.com/docker/docker/blob/master/image/spec/v1.md#container-runconfig-field-descriptions">
      Docker Image Specification v1.0.0
     </link>.
    </para>
   </callout>

  </calloutlist>

  <para>
   After the new layer has been created, its closure
   (to which <varname>contents</varname>, <varname>config</varname> and
   <varname>runAsRoot</varname> contribute) will be copied in the layer itself.
   Only new dependencies that are not already in the existing layers will be copied.
  </para>

  <para>
   At the end of the process, only one new single layer will be produced and
   added to the resulting image.
  </para>

  <para>
   The resulting repository will only list the single image
   <varname>image/tag</varname>. In the case of <xref linkend='ex-dockerTools-buildImage'/>
   it would be <varname>redis/latest</varname>.
  </para>

  <para>
   It is possible to inspect the arguments with which an image was built
   using its <varname>buildArgs</varname> attribute.
  </para>

 </section>

 <section xml:id="ssec-pkgs-dockerTools-fetchFromRegistry">
  <title>pullImage</title>

  <para>
   This function is analogous to the <command>docker pull</command> command,
   in that can be used to fetch a Docker image from a Docker registry.
   Currently only registry <literal>v1</literal> is supported.
   By default <link xlink:href="https://hub.docker.com/">Docker Hub</link>
   is used to pull images.
  </para>

  <para>
   Its parameters are described in the example below:
  </para>

  <example xml:id='ex-dockerTools-pullImage'><title>Docker pull</title>
  <programlisting>
  pullImage {
    imageName = "debian"; <co xml:id='ex-dockerTools-pullImage-1' />
    imageTag = "jessie"; <co xml:id='ex-dockerTools-pullImage-2' />
    imageId = null; <co xml:id='ex-dockerTools-pullImage-3' />
    sha256 = "1bhw5hkz6chrnrih0ymjbmn69hyfriza2lr550xyvpdrnbzr4gk2"; <co xml:id='ex-dockerTools-pullImage-4' />

    indexUrl = "https://index.docker.io"; <co xml:id='ex-dockerTools-pullImage-5' />
    registryVersion = "v1";
  }
  </programlisting>
  </example>

  <calloutlist>
   <callout arearefs='ex-dockerTools-pullImage-1'>
    <para>
     <varname>imageName</varname> specifies the name of the image to be downloaded,
     which can also include the registry namespace (e.g. <literal>library/debian</literal>).
     This argument is required.
    </para>
   </callout>
   
   <callout arearefs='ex-dockerTools-pullImage-2'>
    <para>
     <varname>imageTag</varname> specifies the tag of the image to be downloaded.
     By default it's <literal>latest</literal>.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-pullImage-3'>
    <para>
     <varname>imageId</varname>, if specified this exact image will be fetched, instead
     of <varname>imageName/imageTag</varname>. However, the resulting repository
     will still be named <varname>imageName/imageTag</varname>.
     By default it's <literal>null</literal>.
    </para>
   </callout>

   <callout arearefs='ex-dockerTools-pullImage-4'>
    <para>
     <varname>sha256</varname> is the checksum of the whole fetched image.
     This argument is required.
    </para>

    <note>
     <para>The checksum is computed on the unpacked directory, not on the final tarball.</para>
    </note>

   </callout>

   <callout arearefs='ex-dockerTools-pullImage-5'>
    <para>
     In the above example the default values are shown for the variables
     <varname>indexUrl</varname> and <varname>registryVersion</varname>.
     Hence by default the Docker.io registry is used to pull the images.
    </para>
   </callout>
  </calloutlist>
   
 </section>
  
 <section xml:id="ssec-pkgs-dockerTools-exportImage">
  <title>exportImage</title>

  <para>
   This function is analogous to the <command>docker export</command> command,
   in that can used to flatten a Docker image that contains multiple layers.
   It is in fact the result of the merge of all the layers of the image.
   As such, the result is suitable for being imported in Docker
   with <command>docker import</command>.
  </para>

  <note>
   <para>
    Using this function requires the <literal>kvm</literal>
    device to be available.
   </para>
  </note>

  <para>
   The parameters of <varname>exportImage</varname> are the following:
  </para>

  <example xml:id='ex-dockerTools-exportImage'><title>Docker export</title>
  <programlisting>
  exportImage {
    fromImage = someLayeredImage;
    fromImageName = null;
    fromImageTag = null;
    
    name = someLayeredImage.name;
  }
  </programlisting>
  </example>

  <para>
   The parameters relative to the base image have the same synopsis as
   described in <xref linkend='ssec-pkgs-dockerTools-buildImage'/>, except that
   <varname>fromImage</varname> is the only required argument in this case.
  </para>

  <para>
   The <varname>name</varname> argument is the name of the derivation output,
   which defaults to <varname>fromImage.name</varname>.
  </para>
 </section>

 <section xml:id="ssec-pkgs-dockerTools-shadowSetup">
  <title>shadowSetup</title>

  <para>
   This constant string is a helper for setting up the base files for managing
   users and groups, only if such files don't exist already.
   It is suitable for being used in a
   <varname>runAsRoot</varname> <xref linkend='ex-dockerTools-buildImage-runAsRoot'/> script for cases like
   in the example below:
  </para>
  
  <example xml:id='ex-dockerTools-shadowSetup'><title>Shadow base files</title>
  <programlisting>
  buildImage {
    name = "shadow-basic";

    runAsRoot = ''
      #!${stdenv.shell}
      ${shadowSetup}
      groupadd -r redis
      useradd -r -g redis redis
      mkdir /data
      chown redis:redis /data
    '';
  }
  </programlisting>
  </example>

  <para>
   Creating base files like <literal>/etc/passwd</literal> or
   <literal>/etc/login.defs</literal> are necessary for shadow-utils to
   manipulate users and groups.
  </para>
  
 </section>
 
</section>

</chapter>
