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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml: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 xml:id='ex-nix-conf'><title>Nix configuration file</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 xml: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 xml: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 xml:id="conf-gc-reserved-space"><term><literal>gc-reserved-space</literal></term>
<listitem><para>This option specifies how much space should be
reserved in normal use so that the garbage collector can run
succesfully. Since the garbage collector must perform Berkeley DB
transactions, it needs some disk space for itself. However, when
the disk is full, this space is not available, so the collector
would not be able to run precisely when it is most needed.</para>
<para>For this reason, when Nix is run, it allocates a file
<filename>/nix/var/nix/db/reserved</filename> of the size
specified by this option. When the garbage collector is run, this
file is deleted before the Berkeley DB environment is opened.
This should give it enough room to proceed.</para>
<para>The default is <literal>1048576</literal> (1
MiB).</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>
<varlistentry xml:id="conf-build-max-jobs"><term><literal>build-max-jobs</literal></term>
<listitem><para>This option defines the maximum number of jobs
that Nix will try to build in parallel. The default is
<literal>1</literal>. You should generally set it to the number
of CPUs in your system (e.g., <literal>2</literal> on a Athlon 64
X2). It can be overriden using the <option
linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
command line switch.</para></listitem>
</varlistentry>
<varlistentry><term><literal>system</literal></term>
<listitem><para>This option specifies the canonical Nix system
name of the current installation, such as
<literal>i686-linux</literal> or
<literal>powerpc-darwin</literal>. Nix can only build derivations
whose <literal>system</literal> attribute equals the value
specified here. In general, it never makes sense to modify this
value from its default, since you can use it to ‘lie’ about the
platform you are building on (e.g., perform a Mac OS build on a
Linux machine; the result would obviously be wrong). It only
makes sense if the Nix binaries can run on multiple platforms,
e.g., ‘universal binaries’ that run on <literal>powerpc-darwin</literal> and
<literal>i686-darwin</literal>.</para>
<para>It defaults to the canonical Nix system name detected by
<filename>configure</filename> at build time.</para></listitem>
</varlistentry>
</variablelist>
</para>
</section>
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