<chapter 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 xml:id="opt-common">
<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> / <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> / <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> / <option>-j</option>
<replaceable>number</replaceable></term>
<listitem><para>Sets the maximum number of build jobs that Nix will
perform in parallel to the specified number. Specify
<literal>auto</literal> to use the number of CPUs in the system.
The default is specified by the <link
linkend='conf-max-jobs'><literal>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 xml:id="opt-cores"><term><option>--cores</option></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 defaults to the value of the <link
linkend='conf-cores'><literal>cores</literal></link>
configuration setting, if set, or <literal>1</literal> otherwise.
The value <literal>0</literal> means that the builder should use all
available CPU cores in the system.</para></listitem>
</varlistentry>
<varlistentry xml:id="opt-max-silent-time"><term><option>--max-silent-time</option></term>
<listitem><para>Sets the maximum number of seconds that a builder
can go without producing any data on standard output or standard
error. The default is specified by the <link
linkend='conf-max-silent-time'><literal>max-silent-time</literal></link>
configuration setting. <literal>0</literal> means no
time-out.</para></listitem>
</varlistentry>
<varlistentry xml:id="opt-timeout"><term><option>--timeout</option></term>
<listitem><para>Sets the maximum number of seconds that a builder
can run. The default is specified by the <link
linkend='conf-timeout'><literal>timeout</literal></link>
configuration setting. <literal>0</literal> means no
timeout.</para></listitem>
</varlistentry>
<varlistentry><term><option>--keep-going</option> / <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> / <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 installation 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>--no-build-hook</option></term>
<listitem>
<para>Disables the build hook mechanism. This allows to ignore remote
builders if they are setup on the machine.</para>
<para>It's useful in cases where the bandwidth between the client and the
remote builder is too low. In that case it can take more time to upload the
sources to the remote builder and fetch back the result than to do the
computation locally.</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><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 top-level <literal>default.nix</literal> in
Nixpkgs is actually a function:
<programlisting>
{ # The system (e.g., `i686-linux') for which to build the packages.
system ? builtins.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>builtins.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><term><option>--argstr</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
<listitem><para>This option is like <option>--arg</option>, only the
value is not a Nix expression but a string. So instead of
<literal>--arg system \"i686-linux\"</literal> (the outer quotes are
to keep the shell happy) you can say <literal>--argstr system
i686-linux</literal>.</para></listitem>
</varlistentry>
<varlistentry xml:id="opt-attr"><term><option>--attr</option> / <option>-A</option>
<replaceable>attrPath</replaceable></term>
<listitem><para>Select an attribute from the top-level Nix
expression being evaluated. (<command>nix-env</command>,
<command>nix-instantiate</command>, <command>nix-build</command> and
<command>nix-shell</command> only.) 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>
<varlistentry><term><option>--expr</option> / <option>-E</option></term>
<listitem><para>Interpret the command line arguments as a list of
Nix expressions to be parsed and evaluated, rather than as a list
of file names of Nix expressions.
(<command>nix-instantiate</command>, <command>nix-build</command>
and <command>nix-shell</command> only.)</para></listitem>
</varlistentry>
<varlistentry xml:id="opt-I"><term><option>-I</option> <replaceable>path</replaceable></term>
<listitem><para>Add a path to the Nix expression search path. This
option may be given multiple times. See the <envar
linkend="env-NIX_PATH">NIX_PATH</envar> environment variable for
information on the semantics of the Nix search path. Paths added
through <option>-I</option> take precedence over
<envar>NIX_PATH</envar>.</para></listitem>
</varlistentry>
<varlistentry><term><option>--option</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
<listitem><para>Set the Nix configuration option
<replaceable>name</replaceable> to <replaceable>value</replaceable>.
This overrides settings in the Nix configuration file (see
<citerefentry><refentrytitle>nix.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para></listitem>
</varlistentry>
<varlistentry><term><option>--repair</option></term>
<listitem><para>Fix corrupted or missing store paths by
redownloading or rebuilding them. Note that this is slow because it
requires computing a cryptographic hash of the contents of every
path in the closure of the build. Also note the warning under
<command>nix-store --repair-path</command>.</para></listitem>
</varlistentry>
</variablelist>
</chapter>
