path: root/doc/manual/opt-common.xml

<section xmlns="http://docbook.org/ns/docbook" xml:id="sec-common-options">

<title>Common options</title>


<para>Most Nix commands accept the following command-line options:</para>

<variablelist>

<varlistentry><term><option>--help</option></term>
  
  <listitem><para>Prints out a summary of the command syntax and
  exits.</para></listitem>
  
</varlistentry>


<varlistentry><term><option>--version</option></term>
  
  <listitem><para>Prints out the Nix version number on standard output
  and exits.</para></listitem>
</varlistentry>


<varlistentry><term><option>--verbose</option></term>
  <term><option>-v</option></term>

  <listitem>
    
  <para>Increases the level of verbosity of diagnostic messages
  printed on standard error.  For each Nix operation, the information
  printed on standard output is well-defined; any diagnostic
  information is printed on standard error, never on standard
  output.</para>

  <para>This option may be specified repeatedly.  Currently, the
  following verbosity levels exist:</para>

  <variablelist>
    
    <varlistentry><term>0</term>
    <listitem><para>“Errors only”: only print messages
    explaining why the Nix invocation failed.</para></listitem>
    </varlistentry>
      
    <varlistentry><term>1</term>
    <listitem><para>“Informational”: print
    <emphasis>useful</emphasis> messages about what Nix is doing.
    This is the default.</para></listitem>
    </varlistentry>
      
    <varlistentry><term>2</term>
    <listitem><para>“Talkative”: print more informational
    messages.</para></listitem>
    </varlistentry>

    <varlistentry><term>3</term>
    <listitem><para>“Chatty”: print even more
    informational messages.</para></listitem>
    </varlistentry>

    <varlistentry><term>4</term>
    <listitem><para>“Debug”: print debug
    information.</para></listitem>
    </varlistentry>

    <varlistentry><term>5</term>
    <listitem><para>“Vomit”: print vast amounts of debug
    information.</para></listitem>
    </varlistentry>
    
  </variablelist>

  </listitem>
  
</varlistentry>


<varlistentry><term><option>--no-build-output</option></term>
  <term><option>-Q</option></term>

  <listitem><para>By default, output written by builders to standard
  output and standard error is echoed to the Nix command's standard
  error.  This option suppresses this behaviour.  Note that the
  builder's standard output and error are always written to a log file
  in
  <filename><replaceable>prefix</replaceable>/nix/var/log/nix</filename>.</para></listitem>
  
</varlistentry>


<varlistentry xml:id="opt-max-jobs"><term><option>--max-jobs</option></term>
  <term><option>-j</option></term>

  <listitem><para>Sets the maximum number of build jobs that Nix will
  perform in parallel to the specified number.  The default is
  specified by the <link
  linkend='conf-build-max-jobs'><literal>build-max-jobs</literal></link>
  configuration setting, which itself defaults to
  <literal>1</literal>.  A higher value is useful on SMP systems or to
  exploit I/O latency.  </para></listitem>
  
</varlistentry>


<varlistentry><term><option>--keep-going</option></term>
  <term><option>-k</option></term>

  <listitem><para>Keep going in case of failed builds, to the
  greatest extent possible.  That is, if building an input of some
  derivation fails, Nix will still build the other inputs, but not the
  derivation itself.  Without this option, Nix stops if any build
  fails (except for builds of substitutes), possibly killing builds in
  progress (in case of parallel or distributed builds).</para></listitem>
  
</varlistentry>


<varlistentry><term><option>--keep-failed</option></term>
  <term><option>-K</option></term>

  <listitem><para>Specifies that in case of a build failure, the
  temporary directory (usually in <filename>/tmp</filename>) in which
  the build takes place should not be deleted.  The path of the build
  directory is printed as an informational message.
    </para>
  </listitem>
</varlistentry>


<varlistentry><term><option>--fallback</option></term>

  <listitem>

  <para>Whenever Nix attempts to build a derivation for which
  substitutes are known for each output path, but realising the output
  paths through the substitutes fails, fall back on building the
  derivation.</para>

  <para>The most common scenario in which this is useful is when we
  have registered substitutes in order to perform binary distribution
  from, say, a network repository.  If the repository is down, the
  realisation of the derivation will fail.  When this option is
  specified, Nix will build the derivation instead.  Thus,
  installation from binaries falls back on nstallation from source.
  This option is not the default since it is generally not desirable
  for a transient failure in obtaining the substitutes to lead to a
  full build from source (with the related consumption of
  resources).</para>

  </listitem>
  
</varlistentry>


<varlistentry><term><option>--readonly-mode</option></term>

  <listitem><para>When this option is used, no attempt is made to open
  the Nix database.  Most Nix operations do need database access, so
  those operations will fail.</para></listitem>
  
</varlistentry>


<varlistentry xml:id="opt-log-type"><term><option>--log-type</option>
<replaceable>type</replaceable></term>

  <listitem>

  <para>This option determines how the output written to standard
  error is formatted.  Nix’s diagnostic messages are typically
  <emphasis>nested</emphasis>.  For instance, when tracing Nix
  expression evaluation (<command>nix-env -vvvvv</command>, messages
  from subexpressions are nested inside their parent expressions.  Nix
  builder output is also often nested.  For instance, the Nix Packages
  generic builder nests the various build tasks (unpack, configure,
  compile, etc.), and the GNU Make in <literal>stdenv-linux</literal>
  has been patched to provide nesting for recursive Make
  invocations.</para>

  <para><replaceable>type</replaceable> can be one of the
  following:

  <variablelist>

    <varlistentry><term><literal>pretty</literal></term>

      <listitem><para>Pretty-print the output, indicating different
      nesting levels using spaces.  This is the
      default.</para></listitem>

    </varlistentry>

    <varlistentry><term><literal>escapes</literal></term>

      <listitem><para>Indicate nesting using escape codes that can be
      interpreted by the <command>nix-log2xml</command> tool in the
      Nix source distribution.  The resulting XML file can be fed into
      the <command>log2html.xsl</command> stylesheet to create an HTML
      file that can be browsed interactively, using Javascript to
      expand and collapse parts of the output.</para></listitem>

    </varlistentry>

    <varlistentry><term><literal>flat</literal></term>

      <listitem><para>Remove all nesting.</para></listitem>

    </varlistentry>

  </variablelist>    
  
  </para>

  </listitem>
  
</varlistentry>


<varlistentry><term><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>

  <listitem><para>This option is accepted by
  <command>nix-env</command>, <command>nix-instantiate</command> and
  <command>nix-build</command>.  When evaluating Nix expressions, the
  expression evaluator will automatically try to call functions that
  it encounters.  It can automatically call functions for which every
  argument has a <link linkend='ss-functions'>default value</link>
  (e.g., <literal>{<replaceable>argName</replaceable> ?
  <replaceable>defaultValue</replaceable>}:
  <replaceable>...</replaceable></literal>).  With
  <option>--arg</option>, you can also call functions that have
  arguments without a default value (or override a default value).
  That is, if the evaluator encounters a function with an argument
  named <replaceable>name</replaceable>, it will call it with value
  <replaceable>value</replaceable>.</para>

  <para>For instance, the file
  <literal>pkgs/top-level/all-packages.nix</literal> in Nixpkgs is
  actually a function:

<programlisting>
{ # The system (e.g., `i686-linux') for which to build the packages.
  system ? __currentSystem
  <replaceable>...</replaceable>
}: <replaceable>...</replaceable></programlisting>

  So if you call this Nix expression (e.g., when you do
  <literal>nix-env -i <replaceable>pkgname</replaceable></literal>),
  the function will be called automatically using the value <link
  linkend='builtin-currentSystem'><literal>__currentSystem</literal></link>
  for the <literal>system</literal> argument.  You can override this
  using <option>--arg</option>, e.g., <literal>nix-env -i
  <replaceable>pkgname</replaceable> --arg system
  \"i686-freebsd\"</literal>.  (Note that since the argument is a Nix
  string literal, you have to escape the quotes.)</para></listitem>

</varlistentry>


<varlistentry xml:id="opt-attr"><term><option>--attr</option> / <option>-A</option>
<replaceable>attrPath</replaceable></term>

  <listitem><para>In <command>nix-env</command>,
  <command>nix-instantiate</command> and <command>nix-build</command>,
  <option>--attr</option> allows you to select an attribute from the
  top-level Nix expression being evaluated.  The <emphasis>attribute
  path</emphasis> <replaceable>attrPath</replaceable> is a sequence of
  attribute names separated by dots.  For instance, given a top-level
  Nix expression <replaceable>e</replaceable>, the attribute path
  <literal>xorg.xorgserver</literal> would cause the expression
  <literal><replaceable>e</replaceable>.xorg.xorgserver</literal> to
  be used.  See <link
  linkend='refsec-nix-env-install-examples'><command>nix-env
  --install</command></link> for some concrete examples.</para>

  <para>In addition to attribute names, you can also specify array
  indices.  For instance, the attribute path
  <literal>foo.3.bar</literal> selects the <literal>bar</literal>
  attribute of the fourth element of the array in the
  <literal>foo</literal> attribute of the top-level
  expression.</para></listitem>

</varlistentry>


</variablelist>


</section>