doc/manual/opt-verbose.xml


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<varlistentry>
  <term><option>--verbose</option> / <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>
            <quote>Errors only</quote>: only print messages explaining
            why the Nix invocation failed.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>1</term>
        <listitem>
          <para>
            <quote>Informational</quote>: print
            <emphasis>useful</emphasis> messages about what Nix is
            doing.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>2</term>
        <listitem>
          <para>
            <quote>Talkative</quote>: print more informational messages.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>3</term>
        <listitem>
          <para>
            <quote>Chatty</quote>: print even more informational messages.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>4</term>
        <listitem>
          <para>
            <quote>Debug</quote>: print debug information:
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>5</term>
        <listitem>
          <para>
            <quote>Vomit</quote>: print vast amounts of debug
            information.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

  </listitem>
</varlistentry>

<varlistentry>
  <term><option>--build-output</option> / <option>-B</option></term>
  <listitem>
    <para>
      Causes the output written by build actions to standard output
      and standard error to be echoed to standard error, regardless of
      verbosity level.  By default, it is only echoed at a verbosity
      level of at least 4 (<quote>Debug</quote>), and is suppressed at
      lower levels.  Note that it is always written to a log file in
      <filename><replaceable>prefix</replaceable>/nix/var/log/nix</filename>.
    </para>
  </listitem>
</varlistentry>