<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>