295 lines
9.5 KiB
XML
295 lines
9.5 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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xml:id="chap-functions">
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<title>Functions reference</title>
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<para>
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The nixpkgs repository has several utility functions to manipulate Nix expressions.
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</para>
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<section xml:id="sec-pkgs-overridePackages">
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<title>pkgs.overridePackages</title>
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<para>
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This function inside the nixpkgs expression (<varname>pkgs</varname>)
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can be used to override the set of packages itself.
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</para>
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<para>
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Warning: this function is expensive and must not be used from within
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the nixpkgs repository.
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</para>
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<para>
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Example usage:
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<programlisting>let
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pkgs = import <nixpkgs> {};
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newpkgs = pkgs.overridePackages (self: super: {
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foo = super.foo.override { ... };
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};
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in ...</programlisting>
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</para>
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<para>
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The resulting <varname>newpkgs</varname> will have the new <varname>foo</varname>
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expression, and all other expressions depending on <varname>foo</varname> will also
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use the new <varname>foo</varname> expression.
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</para>
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<para>
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The behavior of this function is similar to <link
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linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>.
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</para>
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<para>
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The <varname>self</varname> parameter refers to the final package set with the
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applied overrides. Using this parameter may lead to infinite recursion if not
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used consciously.
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</para>
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<para>
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The <varname>super</varname> parameter refers to the old package set.
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It's equivalent to <varname>pkgs</varname> in the above example.
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</para>
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</section>
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<section xml:id="sec-pkg-override">
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<title><pkg>.override</title>
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<para>
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The function <varname>override</varname> is usually available for all the
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derivations in the nixpkgs expression (<varname>pkgs</varname>).
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</para>
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<para>
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It is used to override the arguments passed to a function.
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</para>
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<para>
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Example usages:
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<programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
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<programlisting>pkgs.overridePackages (self: super: {
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foo = super.foo.override { barSupport = true ; };
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})</programlisting>
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<programlisting>mypkg = pkgs.callPackage ./mypkg.nix {
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mydep = pkgs.mydep.override { ... };
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})</programlisting>
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</para>
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<para>
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In the first example, <varname>pkgs.foo</varname> is the result of a function call
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with some default arguments, usually a derivation.
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Using <varname>pkgs.foo.override</varname> will call the same function with
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the given new arguments.
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</para>
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</section>
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<section xml:id="sec-pkg-overrideDerivation">
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<title><pkg>.overrideDerivation</title>
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<warning>
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<para>Do not use this function in Nixpkgs. Because it breaks
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package abstraction and doesn’t provide error checking for
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function arguments, it is only intended for ad-hoc customisation
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(such as in <filename>~/.nixpkgs/config.nix</filename>).</para>
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</warning>
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<para>
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The function <varname>overrideDerivation</varname> is usually available for all the
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derivations in the nixpkgs expression (<varname>pkgs</varname>).
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</para>
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<para>
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It is used to create a new derivation by overriding the attributes of
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the original derivation according to the given function.
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</para>
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<para>
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Example usage:
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<programlisting>mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
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name = "sed-4.2.2-pre";
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src = fetchurl {
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url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
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sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
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};
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patches = [];
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});</programlisting>
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</para>
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<para>
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In the above example, the name, src and patches of the derivation
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will be overridden, while all other attributes will be retained from the
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original derivation.
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</para>
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<para>
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The argument <varname>oldAttrs</varname> is used to refer to the attribute set of
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the original derivation.
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</para>
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</section>
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<section xml:id="sec-lib-makeOverridable">
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<title>lib.makeOverridable</title>
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<para>
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The function <varname>lib.makeOverridable</varname> is used to make the result
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of a function easily customizable. This utility only makes sense for functions
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that accept an argument set and return an attribute set.
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</para>
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<para>
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Example usage:
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<programlisting>f = { a, b }: { result = a+b; }
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c = lib.makeOverridable f { a = 1; b = 2; }</programlisting>
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</para>
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<para>
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The variable <varname>c</varname> is the value of the <varname>f</varname> function
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applied with some default arguments. Hence the value of <varname>c.result</varname>
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is <literal>3</literal>, in this example.
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</para>
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<para>
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The variable <varname>c</varname> however also has some additional functions, like
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<link linkend="sec-pkg-override">c.override</link> which can be used to
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override the default arguments. In this example the value of
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<varname>(c.override { a = 4; }).result</varname> is 6.
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</para>
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</section>
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<section xml:id="sec-fhs-environments">
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<title>buildFHSChrootEnv/buildFHSUserEnv</title>
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<para>
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<function>buildFHSChrootEnv</function> and
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<function>buildFHSUserEnv</function> provide a way to build and run
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FHS-compatible lightweight sandboxes. They get their own isolated root with
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binded <filename>/nix/store</filename>, so their footprint in terms of disk
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space needed is quite small. This allows one to run software which is hard or
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unfeasible to patch for NixOS -- 3rd-party source trees with FHS assumptions,
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games distributed as tarballs, software with integrity checking and/or external
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self-updated binaries.
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</para>
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<para>
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<function>buildFHSChrootEnv</function> allows to create persistent
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environments, which can be constructed, deconstructed and entered by
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multiple users at once. A downside is that it requires
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<literal>root</literal> access for both those who create and destroy and
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those who enter it. It can be useful to create environments for daemons that
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one can enter and observe.
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</para>
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<para>
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<function>buildFHSUserEnv</function> uses Linux namespaces feature to create
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temporary lightweight environments which are destroyed after all child
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processes exit. It does not require root access, and can be useful to create
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sandboxes and wrap applications.
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</para>
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<para>
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Those functions both rely on <function>buildFHSEnv</function>, which creates
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an actual directory structure given a list of necessary packages and extra
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build commands.
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<function>buildFHSChrootEnv</function> and <function>buildFHSUserEnv</function>
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both accept those arguments which are passed to
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<function>buildFHSEnv</function>:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>name</literal></term>
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<listitem><para>Environment name.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>targetPkgs</literal></term>
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<listitem><para>Packages to be installed for the main host's architecture
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(i.e. x86_64 on x86_64 installations).</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>multiPkgs</literal></term>
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<listitem><para>Packages to be installed for all architectures supported by
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a host (i.e. i686 and x86_64 on x86_64 installations).</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>extraBuildCommands</literal></term>
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<listitem><para>Additional commands to be executed for finalizing the
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directory structure.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>extraBuildCommandsMulti</literal></term>
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<listitem><para>Like <literal>extraBuildCommandsMulti</literal>, but
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executed only on multilib architectures.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>
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Additionally, <function>buildFHSUserEnv</function> accepts
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<literal>runScript</literal> parameter, which is a command that would be
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executed inside the sandbox and passed all the command line arguments. It
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default to <literal>bash</literal>.
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</para>
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<para>
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It also uses <literal>CHROOTENV_EXTRA_BINDS</literal> environment variable
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for binding extra directories in the sandbox to outside places. The format of
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the variable is <literal>/mnt=test-mnt:/data</literal>, where
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<literal>/mnt</literal> would be mounted as <literal>/test-mnt</literal>
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and <literal>/data</literal> would be mounted as <literal>/data</literal>.
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<literal>extraBindMounts</literal> array argument to
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<function>buildFHSUserEnv</function> function is prepended to this variable.
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Latter entries take priority if defined several times -- i.e. in case of
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<literal>/data=data1:/data=data2</literal> the actual bind path would be
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<literal>/data2</literal>.
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</para>
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<para>
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One can create a simple environment using a <literal>shell.nix</literal>
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like that:
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</para>
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<programlisting><![CDATA[
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{ pkgs ? import <nixpkgs> {} }:
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(pkgs.buildFHSUserEnv {
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name = "simple-x11-env";
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targetPkgs = pkgs: (with pkgs;
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[ udev
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alsaLib
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]) ++ (with pkgs.xorg;
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[ libX11
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libXcursor
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libXrandr
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]);
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multiPkgs = pkgs: (with pkgs;
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[ udev
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alsaLib
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]) ++ (with [];
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runScript = "bash";
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}).env
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]]></programlisting>
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<para>
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Running <literal>nix-shell</literal> would then drop you into a shell with
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these libraries and binaries available. You can use this to run
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closed-source applications which expect FHS structure without hassles:
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simply change <literal>runScript</literal> to the application path,
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e.g. <filename>./bin/start.sh</filename> -- relative paths are supported.
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</para>
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</section>
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</chapter>
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