<appendix xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink">

<title>Glossary</title>


<glosslist>


<glossentry xml:id="gloss-derivation"><glossterm>derivation</glossterm>

  <glossdef><para>A description of a build action.  The result of a
  derivation is a store object.  Derivations are typically specified
  in Nix expressions using the <link
  linkend="ssec-derivation"><function>derivation</function>
  primitive</link>.  These are translated into low-level
  <emphasis>store derivations</emphasis> (implicitly by
  <command>nix-env</command> and <command>nix-build</command>, or
  explicitly by <command>nix-instantiate</command>).</para></glossdef>

</glossentry>


<glossentry><glossterm>store</glossterm>

  <glossdef><para>The location in the file system where store objects
  live.  Typically <filename>/nix/store</filename>.</para></glossdef>

</glossentry>


<glossentry><glossterm>store path</glossterm>

  <glossdef><para>The location in the file system of a store object,
  i.e., an immediate child of the Nix store
  directory.</para></glossdef>

</glossentry>


<glossentry><glossterm>store object</glossterm>

  <glossdef><para>A file that is an immediate child of the Nix store
  directory.  These can be regular files, but also entire directory
  trees.  Store objects can be sources (objects copied from outside of
  the store), derivation outputs (objects produced by running a build
  action), or derivations (files describing a build
  action).</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-substitute"><glossterm>substitute</glossterm>

  <glossdef><para>A substitute is a command invocation stored in the
  Nix database that describes how to build a store object, bypassing
  the normal build mechanism (i.e., derivations).  Typically, the
  substitute builds the store object by downloading a pre-built
  version of the store object from some server.</para></glossdef>

</glossentry>


<glossentry><glossterm>purity</glossterm>

  <glossdef><para>The assumption that equal Nix derivations when run
  always produce the same output.  This cannot be guaranteed in
  general (e.g., a builder can rely on external inputs such as the
  network or the system time) but the Nix model assumes
  it.</para></glossdef>

</glossentry>


<glossentry><glossterm>Nix expression</glossterm>

  <glossdef><para>A high-level description of software packages and
  compositions thereof.  Deploying software using Nix entails writing
  Nix expressions for your packages.  Nix expressions are translated
  to derivations that are stored in the Nix store.  These derivations
  can then be built.</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-reference"><glossterm>reference</glossterm>

  <glossdef>
    <para>A store path <varname>P</varname> is said to have a
    reference to a store path <varname>Q</varname> if the store object
    at <varname>P</varname> contains the path <varname>Q</varname>
    somewhere. The <emphasis>references</emphasis> of a store path are
    the set of store paths to which it has a reference.
    </para>
    <para>A derivation can reference other derivations and sources
    (but not output paths), whereas an output path only references other
    output paths.
    </para>
  </glossdef>

</glossentry>

<glossentry xml:id="gloss-reachable"><glossterm>reachable</glossterm>

  <glossdef><para>A store path <varname>Q</varname> is reachable from
  another store path <varname>P</varname> if <varname>Q</varname> is in the
  <link linkend="gloss-closure">closure</link> of the
  <link linkend="gloss-reference">references</link> relation.
  </para></glossdef>
</glossentry>

<glossentry xml:id="gloss-closure"><glossterm>closure</glossterm>

  <glossdef><para>The closure of a store path is the set of store
  paths that are directly or indirectly “reachable” from that store
  path; that is, it’s the closure of the path under the <link
  linkend="gloss-reference">references</link> relation. For a package, the
  closure of its derivation is equivalent to the build-time
  dependencies, while the closure of its output path is equivalent to its
  runtime dependencies. For correct deployment it is necessary to deploy whole
  closures, since otherwise at runtime files could be missing. The command
  <command>nix-store -qR</command> prints out closures of store paths.
  </para>
  <para>As an example, if the store object at path <varname>P</varname> contains
  a reference to path <varname>Q</varname>, then <varname>Q</varname> is
  in the closure of <varname>P</varname>. Further, if <varname>Q</varname>
  references <varname>R</varname> then <varname>R</varname> is also in
  the closure of <varname>P</varname>.
  </para></glossdef>

</glossentry>


<glossentry xml:id="gloss-output-path"><glossterm>output path</glossterm>

  <glossdef><para>A store path produced by a derivation.</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-deriver"><glossterm>deriver</glossterm>

  <glossdef><para>The deriver of an <link
  linkend="gloss-output-path">output path</link> is the store
  derivation that built it.</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-validity"><glossterm>validity</glossterm>

  <glossdef><para>A store path is considered
  <emphasis>valid</emphasis> if it exists in the file system, is
  listed in the Nix database as being valid, and if all paths in its
  closure are also valid.</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-user-env"><glossterm>user environment</glossterm>

  <glossdef><para>An automatically generated store object that
  consists of a set of symlinks to “active” applications, i.e., other
  store paths.  These are generated automatically by <link
  linkend="sec-nix-env"><command>nix-env</command></link>.  See <xref
  linkend="sec-profiles" />.</para>

  </glossdef>

</glossentry>


<glossentry xml:id="gloss-profile"><glossterm>profile</glossterm>

  <glossdef><para>A symlink to the current <link
  linkend="gloss-user-env">user environment</link> of a user, e.g.,
  <filename>/nix/var/nix/profiles/default</filename>.</para></glossdef>

</glossentry>


<glossentry xml:id="gloss-nar"><glossterm>NAR</glossterm>

  <glossdef><para>A <emphasis>N</emphasis>ix
  <emphasis>AR</emphasis>chive.  This is a serialisation of a path in
  the Nix store.  It can contain regular files, directories and
  symbolic links.  NARs are generated and unpacked using
  <command>nix-store --dump</command> and <command>nix-store
  --restore</command>.</para></glossdef>

</glossentry>



</glosslist>


</appendix>
