<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>
