lix/doc/manual/conf-file.xml

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          xmlns:xlink="http://www.w3.org/1999/xlink"
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          xml:id="sec-conf-file">

<refmeta>
  <refentrytitle>nix.conf</refentrytitle>
  <manvolnum>5</manvolnum>
  <refmiscinfo class="source">Nix</refmiscinfo>
  <refmiscinfo class="version"><xi:include href="version.txt" parse="text"/></refmiscinfo>
</refmeta>

<refnamediv>
  <refname>nix.conf</refname>
  <refpurpose>Nix configuration file</refpurpose>
</refnamediv>

<refsection><title>Description</title>

<para>A number of persistent settings of Nix are stored in the file
<filename><replaceable>sysconfdir</replaceable>/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.  Here is an example
configuration file:</para>

<programlisting>
gc-keep-outputs = true       # Nice for developers
gc-keep-derivations = true   # Idem
env-keep-derivations = false
</programlisting>

<para>You can override settings using the <option>--option</option>
flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>

<para>The following settings 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><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 an Athlon 64
    X2).  It can be overridden using the <option
    linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
    command line switch.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-cores"><term><literal>build-cores</literal></term>

    <listitem><para>Sets the value of the
    <envar>NIX_BUILD_CORES</envar> environment variable in the
    invocation of builders.  Builders can use this variable at their
    discretion to control the maximum amount of parallelism.  For
    instance, in Nixpkgs, if the derivation attribute
    <varname>enableParallelBuilding</varname> is set to
    <literal>true</literal>, the builder passes the
    <option>-j<replaceable>N</replaceable></option> flag to GNU Make.
    It can be overridden using the <option
    linkend='opt-cores'>--cores</option> command line switch and
    defaults to <literal>1</literal>.  The value <literal>0</literal>
    means that the builder should use all available CPU cores in the
    system.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-max-silent-time"><term><literal>build-max-silent-time</literal></term>

    <listitem>

      <para>This option defines the maximum number of seconds that a
      builder can go without producing any data on standard output or
      standard error.  This is useful (for instance in an automated
      build system) to catch builds that are stuck in an infinite
      loop, or to catch remote builds that are hanging due to network
      problems.  It can be overridden using the <option
      linkend="opt-max-silent-time">--max-silent-time</option> command
      line switch.</para>

      <para>The value <literal>0</literal> means that there is no
      timeout.  This is also the default.</para>

    </listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-timeout"><term><literal>build-timeout</literal></term>

    <listitem>

      <para>This option defines the maximum number of seconds that a
      builder can run.  This is useful (for instance in an automated
      build system) to catch builds that are stuck in an infinite loop
      but keep writing to their standard output or standard error.  It
      can be overridden using the <option
      linkend="opt-timeout">--timeout</option> command line
      switch.</para>

      <para>The value <literal>0</literal> means that there is no
      timeout.  This is also the default.</para>

    </listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-max-log-size"><term><literal>build-max-log-size</literal></term>

    <listitem>

      <para>This option defines the maximum number of bytes that a
      builder can write to its stdout/stderr.  If the builder exceeds
      this limit, it’s killed.  A value of <literal>0</literal> (the
      default) means that there is no limit.</para>

    </listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-users-group"><term><literal>build-users-group</literal></term>

    <listitem><para>This options specifies the Unix group containing
    the Nix build user accounts.  In multi-user Nix installations,
    builds should not be performed by the Nix account since that would
    allow users to arbitrarily modify the Nix store and database by
    supplying specially crafted builders; and they cannot be performed
    by the calling user since that would allow him/her to influence
    the build result.</para>

    <para>Therefore, if this option is non-empty and specifies a valid
    group, builds will be performed under the user accounts that are a
    member of the group specified here (as listed in
    <filename>/etc/group</filename>).  Those user accounts should not
    be used for any other purpose!</para>

    <para>Nix will never run two builds under the same user account at
    the same time.  This is to prevent an obvious security hole: a
    malicious user writing a Nix expression that modifies the build
    result of a legitimate Nix expression being built by another user.
    Therefore it is good to have as many Nix build user accounts as
    you can spare.  (Remember: uids are cheap.)</para>

    <para>The build users should have permission to create files in
    the Nix store, but not delete them.  Therefore,
    <filename>/nix/store</filename> should be owned by the Nix
    account, its group should be the group specified here, and its
    mode should be <literal>1775</literal>.</para>

    <para>If the build users group is empty, builds will be performed
    under the uid of the Nix process (that is, the uid of the caller
    if <envar>NIX_REMOTE</envar> is empty, the uid under which the Nix
    daemon runs if <envar>NIX_REMOTE</envar> is
    <literal>daemon</literal>).  Obviously, this should not be used in
    multi-user settings with untrusted users.</para>

    </listitem>

  </varlistentry>


  <varlistentry><term><literal>build-use-chroot</literal></term>

    <listitem><para>If set to <literal>true</literal>, builds will be
    performed in a <emphasis>chroot environment</emphasis>, i.e., the
    build will be isolated from the normal file system hierarchy and
    will only see the Nix store, the temporary build directory, and
    the directories configured with the <link
    linkend='conf-build-chroot-dirs'><literal>build-chroot-dirs</literal>
    option</link> (such as <filename>/proc</filename> and
    <filename>/dev</filename>).  This is useful to prevent undeclared
    dependencies on files in directories such as
    <filename>/usr/bin</filename>.</para>

    <para>The use of a chroot requires that Nix is run as root (but
    you can still use the <link
    linkend='conf-build-users-group'>“build users” feature</link> to
    perform builds under different users than root).  Currently,
    chroot builds only work on Linux because Nix uses “bind mounts” to
    make the Nix store and other directories available inside the
    chroot.</para>

    </listitem>

  </varlistentry>


  <varlistentry xml:id="conf-build-chroot-dirs"><term><literal>build-chroot-dirs</literal></term>

    <listitem><para>When builds are performed in a chroot environment,
    Nix will mount some directories from the normal file system
    hierarchy inside the chroot.  These are the Nix store, the
    temporary build directory (usually
    <filename>/tmp/nix-build-<replaceable>drvname</replaceable>-<replaceable>number</replaceable></filename>),
    the <literal>/proc</literal> filesystem, and the directories
    listed here.  The default is <literal>/dev /dev/pts</literal>,
    since these contain files needed by many builds (such as
    <filename>/dev/null</filename>).  You can use the syntax
    <literal><replaceable>target</replaceable>=<replaceable>source</replaceable></literal>
    to mount a path in a different location in the chroot; for
    instance, <literal>/bin=/nix-bin</literal> will mount the
    directory <literal>/nix-bin</literal> as <literal>/bin</literal>
    inside the chroot.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>build-use-substitutes</literal></term>

    <listitem><para>If set to <literal>true</literal> (default), Nix
    will use binary substitutes if available.  This option can be
    disabled to force building from source.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>build-fallback</literal></term>

    <listitem><para>If set to <literal>true</literal>, Nix will fall
    back to building from source if a binary substitute fails.  This
    is equivalent to the <option>--fallback</option> flag.  The
    default is <literal>false</literal>.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>build-cache-failures</literal></term>

    <listitem><para>If set to <literal>true</literal>, Nix will
    “cache” build failures, meaning that it will remember (in its
    database) that a derivation previously failed.  If you then try to
    build the derivation again, Nix will immediately fail rather than
    perform the build again.  Failures in fixed-output derivations
    (such as <function>fetchurl</function> calls) are never cached.
    The “failed” status of a derivation can be cleared using
    <command>nix-store --clear-failed-paths</command>.  By default,
    failure caching is disabled.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>build-keep-log</literal></term>

    <listitem><para>If set to <literal>true</literal> (the default),
    Nix will write the build log of a derivation (i.e. the standard
    output and error of its builder) to the directory
    <filename>/nix/var/log/nix/drvs</filename>.  The build log can be
    retrieved using the command <command>nix-store -l
    <replaceable>path</replaceable></command>.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>build-compress-log</literal></term>

    <listitem><para>If set to <literal>true</literal> (the default),
    build logs written to <filename>/nix/var/log/nix/drvs</filename>
    will be compressed on the fly using bzip2.  Otherwise, they will
    not be compressed.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>use-binary-caches</literal></term>

    <listitem><para>If set to <literal>true</literal> (the default),
    Nix will check the binary caches specified by
    <option>binary-caches</option> and related options to obtain
    binary substitutes.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>binary-caches</literal></term>

    <listitem><para>A list of URLs of binary caches, separated by
    whitespace.  The default is
    <literal>http://cache.nixos.org</literal>.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>binary-caches-files</literal></term>

    <listitem><para>A list of names of files that will be read to
    obtain additional binary cache URLs.  The default is
    <literal>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/channels/binary-caches/*</literal>.
    Note that when you’re using the Nix daemon,
    <replaceable>username</replaceable> is always equal to
    <literal>root</literal>, so Nix will only use the binary caches
    provided by the channels installed by root.  Do not set this
    option to read files created by untrusted users!</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>trusted-binary-caches</literal></term>

    <listitem><para>A list of URLs of binary caches, separated by
    whitespace.  These are not used by default, but can be enabled by
    users of the Nix daemon by specifying <literal>--option
    binary-caches <replaceable>urls</replaceable></literal> on the
    command line.  Unprivileged users are only allowed to pass a
    subset of the URLs listed in <literal>binary-caches</literal> and
    <literal>trusted-binary-caches</literal>.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>extra-binary-caches</literal></term>

    <listitem><para>Additional binary caches appended to those
    specified in <option>binary-caches</option> and
    <option>binary-caches-files</option>.  When used by unprivileged
    users, untrusted binary caches (i.e. those not listed in
    <option>trusted-binary-caches</option>) are silently
    ignored.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>binary-caches-parallel-connections</literal></term>

    <listitem><para>The maximum number of parallel HTTP connections
    used by the binary cache substituter to get NAR info files.  This
    number should be high to minimise latency.  It defaults to
    150.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>force-manifest</literal></term>

    <listitem><para>If this option is set to <literal>false</literal>
    (default) and a Nix channel provides both a manifest and a binary
    cache, only the binary cache will be used.  If set to
    <literal>true</literal>, the manifest will be fetched as well.
    This is useful if you want to use binary patches (which are
    currently not supported by binary caches).</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>


  <varlistentry><term><literal>fsync-metadata</literal></term>

    <listitem><para>If set to <literal>true</literal>, changes to the
    Nix store metadata (in <filename>/nix/var/nix/db</filename>) are
    synchronously flushed to disk.  This improves robustness in case
    of system crashes, but reduces performance.  The default is
    <literal>true</literal>.</para></listitem>

  </varlistentry>


  <varlistentry><term><literal>auto-optimise-store</literal></term>

    <listitem><para>If set to <literal>true</literal>, Nix
    automatically detects files in the store that have identical
    contents, and replaces them with hard links to a single copy.
    This saves disk space.  If set to <literal>false</literal> (the
    default), you can still run <command>nix-store
    --optimise</command> to get rid of duplicate
    files.</para></listitem>

  </varlistentry>


</variablelist>

</para>

</refsection>

</refentry>