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<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>
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