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<sect1 id="sec-conf-file"><title>Nix configuration file</title>
<para>A number of persistent settings of Nix are stored in the file
<filename><replaceable>prefix</replaceable>/etc/nix/nix.conf</filename>.
This file is a list of <literal><replaceable>name</replaceable> =
<replaceable>value</replaceable></literal> pairs, one per line.
Comments start with a <literal>#</literal> character. An example
configuration file is shown in <xref linkend="ex-nix-conf" />.</para>
<example id='ex-nix-conf'><title>Nix expression for GNU Hello</title>
<programlisting>
gc-keep-outputs = true # Nice for developers
gc-keep-derivations = true # Idem
env-keep-derivations = false
</programlisting>
</example>
<para>The following variables are currently available:
<variablelist>
<varlistentry id="conf-gc-keep-outputs"><term><literal>gc-keep-outputs</literal></term>
<listitem><para>If <literal>true</literal>, the garbage collector
will keep the outputs of non-garbage derivations. If
<literal>false</literal> (default), outputs will be deleted unless
they are GC roots themselves (or reachable from other roots).</para>
<para>In general, outputs must be registered as roots separately.
However, even if the output of a derivation is registered as a
root, the collector will still delete store paths that are used
only at build time (e.g., the C compiler, or source tarballs
downloaded from the network). To prevent it from doing so, set
this option to <literal>true</literal>.</para></listitem>
</varlistentry>
<varlistentry id="conf-gc-keep-derivations"><term><literal>gc-keep-derivations</literal></term>
<listitem><para>If <literal>true</literal> (default), the garbage
collector will keep the derivations from which non-garbage store
paths were built. If <literal>false</literal>, they will be
deleted unless explicitly registered as a root (or reachable from
other roots).</para>
<para>Keeping derivation around is useful for querying and
traceability (e.g., it allows you to ask with what dependencies or
options a store path was built), so by default this option is on.
Turn it off to safe a bit of disk space (or a lot if
<literal>gc-keep-outputs</literal> is also turned on).</para></listitem>
</varlistentry>
<varlistentry><term><literal>env-keep-derivations</literal></term>
<listitem><para>If <literal>false</literal> (default), derivations
are not stored in Nix user environments. That is, the derivation
any build-time-only dependencies may be garbage-collected.</para>
<para>If <literal>true</literal>, when you add a Nix derivation to
a user environment, the path of the derivation is stored in the
user environment. Thus, the derivation will not be
garbage-collected until the user environment generation is deleted
(<command>nix-env --delete-generations</command>). To prevent
build-time-only dependencies from being collected, you should also
turn on <literal>gc-keep-outputs</literal>.</para>
<para>The difference between this option and
<literal>gc-keep-derivations</literal> is that this one is
“sticky”: it applies to any user environment created while this
option was enabled, while <literal>gc-keep-derivations</literal>
only applies at the moment the garbage collector is
run.</para></listitem>
</varlistentry>
</variablelist>
</para>
</sect1>
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