<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 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-build-cores'><literal>build-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-build-max-silent-time'><literal>build-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-build-timeout'><literal>build-timeout</literal></link> configuration setting. <literal>0</literal> means no timeout.</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 ? 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>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> <varlistentry><term><option>--show-trace</option></term> <listitem><para>Causes Nix to print out a stack trace in case of Nix expression evaluation errors.</para></listitem> </varlistentry> </variablelist> </section>