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</style></head><body><div lang="en" class="book"><div class="titlepage"><div><div><h1 class="title"><a name="idm1"></a>The Ninja build system</h1></div><div><div class="author"><h3 class="author"><span class="firstname">v1.8.2,</span> <span class="othername">Sep</span> <span class="surname">2017</span></h3></div></div></div><hr></div><div class="toc"><ul class="toc"><li><span class="chapter"><a href="#_introduction">Introduction</a></span><ul><li><span class="section"><a href="#_philosophical_overview">Philosophical overview</a></span></li><li><span class="section"><a href="#_design_goals">Design goals</a></span></li><li><span class="section"><a href="#_comparison_to_make">Comparison to Make</a></span></li></ul></li><li><span class="chapter"><a href="#_using_ninja_for_your_project">Using Ninja for your project</a></span><ul><li><span class="section"><a href="#_running_ninja">Running Ninja</a></span></li><li><span class="section"><a href="#_environment_variables">Environment variables</a></span></li><li><span class="section"><a href="#_extra_tools">Extra tools</a></span></li></ul></li><li><span class="chapter"><a href="#_writing_your_own_ninja_files">Writing your own Ninja files</a></span><ul><li><span class="section"><a href="#_conceptual_overview">Conceptual overview</a></span></li><li><span class="section"><a href="#_syntax_example">Syntax example</a></span></li><li><span class="section"><a href="#_variables">Variables</a></span></li><li><span class="section"><a href="#_rules">Rules</a></span></li><li><span class="section"><a href="#_build_statements">Build statements</a></span></li><li><span class="section"><a href="#_generating_ninja_files_from_code">Generating Ninja files from code</a></span></li></ul></li><li><span class="chapter"><a href="#_more_details">More details</a></span><ul><li><span class="section"><a href="#_the_literal_phony_literal_rule">The <code class="literal">phony</code> rule</a></span></li><li><span class="section"><a href="#_default_target_statements">Default target statements</a></span></li><li><span class="section"><a href="#ref_log">The Ninja log</a></span></li><li><span class="section"><a href="#ref_versioning">Version compatibility</a></span></li><li><span class="section"><a href="#ref_headers">C/C++ header dependencies</a></span><ul><li><span class="section"><a href="#_depfile">depfile</a></span></li><li><span class="section"><a href="#_deps">deps</a></span></li></ul></li><li><span class="section"><a href="#ref_pool">Pools</a></span><ul><li><span class="section"><a href="#_the_literal_console_literal_pool">The <code class="literal">console</code> pool</a></span></li></ul></li></ul></li><li><span class="chapter"><a href="#_ninja_file_reference">Ninja file reference</a></span><ul><li><span class="section"><a href="#_lexical_syntax">Lexical syntax</a></span></li><li><span class="section"><a href="#ref_toplevel">Top-level variables</a></span></li><li><span class="section"><a href="#ref_rule">Rule variables</a></span><ul><li><span class="section"><a href="#ref_rule_command">Interpretation of the <code class="literal">command</code> variable</a></span></li></ul></li><li><span class="section"><a href="#ref_outputs">Build outputs</a></span></li><li><span class="section"><a href="#ref_dependencies">Build dependencies</a></span></li><li><span class="section"><a href="#_variable_expansion">Variable expansion</a></span></li><li><span class="section"><a href="#ref_scope">Evaluation and scoping</a></span></li></ul></li></ul></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_introduction"></a>Introduction</h1></div></div></div><p>Ninja is yet another build system.  It takes as input the
interdependencies of files (typically source code and output
executables) and orchestrates building them, <span class="emphasis"><em>quickly</em></span>.</p><p>Ninja joins a sea of other build systems.  Its distinguishing goal is
to be fast.  It is born from
<a class="ulink" href="http://neugierig.org/software/chromium/notes/2011/02/ninja.html" target="_top">my
work on the Chromium browser project</a>, which has over 30,000 source
files and whose other build systems (including one built from custom
non-recursive Makefiles) would take ten seconds to start building
after changing one file.  Ninja is under a second.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_philosophical_overview"></a>Philosophical overview</h2></div></div></div><p>Where other build systems are high-level languages, Ninja aims to be
an assembler.</p><p>Build systems get slow when they need to make decisions.  When you are
in a edit-compile cycle you want it to be as fast as possible — you
want the build system to do the minimum work necessary to figure out
what needs to be built immediately.</p><p>Ninja contains the barest functionality necessary to describe
arbitrary dependency graphs.  Its lack of syntax makes it impossible
to express complex decisions.</p><p>Instead, Ninja is intended to be used with a separate program
generating its input files.  The generator program (like the
<code class="literal">./configure</code> found in autotools projects) can analyze system
dependencies and make as many decisions as possible up front so that
incremental builds stay fast.  Going beyond autotools, even build-time
decisions like "which compiler flags should I use?"  or "should I
build a debug or release-mode binary?"  belong in the <code class="literal">.ninja</code> file
generator.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_design_goals"></a>Design goals</h2></div></div></div><p>Here are the design goals of Ninja:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
very fast (i.e., instant) incremental builds, even for very large
  projects.
</li><li class="listitem">
very little policy about how code is built.  Different projects and
  higher-level build systems have different opinions about how code
  should be built; for example, should built objects live alongside
  the sources or should all build output go into a separate directory?
  Is there a "package" rule that builds a distributable package of
  the project?  Sidestep these decisions by trying to allow either to
  be implemented, rather than choosing, even if that results in
  more verbosity.
</li><li class="listitem">
get dependencies correct, and in particular situations that are
  difficult to get right with Makefiles (e.g. outputs need an implicit
  dependency on the command line used to generate them; to build C
  source code you need to use gcc’s <code class="literal">-M</code> flags for header
  dependencies).
</li><li class="listitem">
when convenience and speed are in conflict, prefer speed.
</li></ul></div><p>Some explicit <span class="emphasis"><em>non-goals</em></span>:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
convenient syntax for writing build files by hand.  <span class="emphasis"><em>You should
  generate your ninja files using another program</em></span>.  This is how we
  can sidestep many policy decisions.
</li><li class="listitem">
built-in rules. <span class="emphasis"><em>Out of the box, Ninja has no rules for
  e.g. compiling C code.</em></span>
</li><li class="listitem">
build-time customization of the build. <span class="emphasis"><em>Options belong in
  the program that generates the ninja files</em></span>.
</li><li class="listitem">
build-time decision-making ability such as conditionals or search
  paths. <span class="emphasis"><em>Making decisions is slow.</em></span>
</li></ul></div><p>To restate, Ninja is faster than other build systems because it is
painfully simple.  You must tell Ninja exactly what to do when you
create your project’s <code class="literal">.ninja</code> files.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_comparison_to_make"></a>Comparison to Make</h2></div></div></div><p>Ninja is closest in spirit and functionality to Make, relying on
simple dependencies between file timestamps.</p><p>But fundamentally, make has a lot of <span class="emphasis"><em>features</em></span>: suffix rules,
functions, built-in rules that e.g. search for RCS files when building
source.  Make’s language was designed to be written by humans.  Many
projects find make alone adequate for their build problems.</p><p>In contrast, Ninja has almost no features; just those necessary to get
builds correct while punting most complexity to generation of the
ninja input files.  Ninja by itself is unlikely to be useful for most
projects.</p><p>Here are some of the features Ninja adds to Make.  (These sorts of
features can often be implemented using more complicated Makefiles,
but they are not part of make itself.)</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
Ninja has special support for discovering extra dependencies at build
  time, making it easy to get <a class="link" href="#ref_headers" title="C/C++ header dependencies">header dependencies</a>
  correct for C/C++ code.
</li><li class="listitem">
A build edge may have multiple outputs.
</li><li class="listitem">
Outputs implicitly depend on the command line that was used to generate
  them, which means that changing e.g. compilation flags will cause
  the outputs to rebuild.
</li><li class="listitem">
Output directories are always implicitly created before running the
  command that relies on them.
</li><li class="listitem">
Rules can provide shorter descriptions of the command being run, so
  you can print e.g. <code class="literal">CC foo.o</code> instead of a long command line while
  building.
</li><li class="listitem">
Builds are always run in parallel, based by default on the number of
  CPUs your system has.  Underspecified build dependencies will result
  in incorrect builds.
</li><li class="listitem">
Command output is always buffered.  This means commands running in
  parallel don’t interleave their output, and when a command fails we
  can print its failure output next to the full command line that
  produced the failure.
</li></ul></div></div></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_using_ninja_for_your_project"></a>Using Ninja for your project</h1></div></div></div><p>Ninja currently works on Unix-like systems and Windows. It’s seen the
most testing on Linux (and has the best performance there) but it runs
fine on Mac OS X and FreeBSD.</p><p>If your project is small, Ninja’s speed impact is likely unnoticeable.
(However, even for small projects it sometimes turns out that Ninja’s
limited syntax forces simpler build rules that result in faster
builds.)  Another way to say this is that if you’re happy with the
edit-compile cycle time of your project already then Ninja won’t help.</p><p>There are many other build systems that are more user-friendly or
featureful than Ninja itself.  For some recommendations: the Ninja
author found <a class="ulink" href="http://gittup.org/tup/" target="_top">the tup build system</a> influential
in Ninja’s design, and thinks <a class="ulink" href="https://github.com/apenwarr/redo" target="_top">redo</a>'s
design is quite clever.</p><p>Ninja’s benefit comes from using it in conjunction with a smarter
meta-build system.</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<a class="ulink" href="http://code.google.com/p/gyp/" target="_top">gyp</a>
</span></dt><dd>
The meta-build system used to
generate build files for Google Chrome and related projects (v8,
node.js).  gyp can generate Ninja files for all platforms supported by
Chrome. See the
<a class="ulink" href="https://chromium.googlesource.com/chromium/src/+/master/docs/ninja_build.md" target="_top">Chromium Ninja documentation for more details</a>.
</dd><dt><span class="term">
<a class="ulink" href="https://cmake.org/" target="_top">CMake</a>
</span></dt><dd>
A widely used meta-build system that
can generate Ninja files on Linux as of CMake version 2.8.8.  Newer versions
of CMake support generating Ninja files on Windows and Mac OS X too.
</dd><dt><span class="term">
<a class="ulink" href="https://github.com/ninja-build/ninja/wiki/List-of-generators-producing-ninja-build-files" target="_top">others</a>
</span></dt><dd>
Ninja ought to fit perfectly into other meta-build software
like <a class="ulink" href="http://industriousone.com/premake" target="_top">premake</a>.  If you do this work,
please let us know!
</dd></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_running_ninja"></a>Running Ninja</h2></div></div></div><p>Run <code class="literal">ninja</code>.  By default, it looks for a file named <code class="literal">build.ninja</code> in
the current directory and builds all out-of-date targets.  You can
specify which targets (files) to build as command line arguments.</p><p>There is also a special syntax <code class="literal">target^</code> for specifying a target
as the first output of some rule containing the source you put in
the command line, if one exists. For example, if you specify target as
<code class="literal">foo.c^</code> then <code class="literal">foo.o</code> will get built (assuming you have those targets
in your build files).</p><p><code class="literal">ninja -h</code> prints help output.  Many of Ninja’s flags intentionally
match those of Make; e.g <code class="literal">ninja -C build -j 20</code> changes into the
<code class="literal">build</code> directory and runs 20 build commands in parallel.  (Note that
Ninja defaults to running commands in parallel anyway, so typically
you don’t need to pass <code class="literal">-j</code>.)</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_environment_variables"></a>Environment variables</h2></div></div></div><p>Ninja supports one environment variable to control its behavior:
<code class="literal">NINJA_STATUS</code>, the progress status printed before the rule being run.</p><p>Several placeholders are available:</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="literal">%s</code>
</span></dt><dd>
The number of started edges.
</dd><dt><span class="term">
<code class="literal">%t</code>
</span></dt><dd>
The total number of edges that must be run to complete the build.
</dd><dt><span class="term">
<code class="literal">%p</code>
</span></dt><dd>
The percentage of started edges.
</dd><dt><span class="term">
<code class="literal">%r</code>
</span></dt><dd>
The number of currently running edges.
</dd><dt><span class="term">
<code class="literal">%u</code>
</span></dt><dd>
The number of remaining edges to start.
</dd><dt><span class="term">
<code class="literal">%f</code>
</span></dt><dd>
The number of finished edges.
</dd><dt><span class="term">
<code class="literal">%o</code>
</span></dt><dd>
Overall rate of finished edges per second
</dd><dt><span class="term">
<code class="literal">%c</code>
</span></dt><dd>
Current rate of finished edges per second (average over builds
specified by <code class="literal">-j</code> or its default)
</dd><dt><span class="term">
<code class="literal">%e</code>
</span></dt><dd>
Elapsed time in seconds.  <span class="emphasis"><em>(Available since Ninja 1.2.)</em></span>
</dd><dt><span class="term">
<code class="literal">%%</code>
</span></dt><dd>
A plain <code class="literal">%</code> character.
</dd></dl></div><p>The default progress status is <code class="literal">"[%f/%t] "</code> (note the trailing space
to separate from the build rule). Another example of possible progress status
could be <code class="literal">"[%u/%r/%f] "</code>.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_extra_tools"></a>Extra tools</h2></div></div></div><p>The <code class="literal">-t</code> flag on the Ninja command line runs some tools that we have
found useful during Ninja’s development.  The current tools are:</p><div class="horizontal"><table class="horizontal" border="0"><colgroup><col><col></colgroup><tbody valign="top"><tr><td valign="top">
<p>
<code class="literal">query</code>
</p>
</td><td valign="top">
<p>
dump the inputs and outputs of a given target.
</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">browse</code>
</p>
</td><td valign="top">
<p>
browse the dependency graph in a web browser.  Clicking a
file focuses the view on that file, showing inputs and outputs.  This
feature requires a Python installation. By default port 8000 is used
and a web browser will be opened. This can be changed as follows:
</p>
<pre class="screen">ninja -t browse --port=8000 --no-browser mytarget</pre>
</td></tr><tr><td valign="top">
<p>
<code class="literal">graph</code>
</p>
</td><td valign="top">
<p>
output a file in the syntax used by <code class="literal">graphviz</code>, a automatic
graph layout tool.  Use it like:
</p>
<pre class="screen">ninja -t graph mytarget | dot -Tpng -ograph.png</pre>
<p>In the Ninja source tree, <code class="literal">ninja graph.png</code>
generates an image for Ninja itself.  If no target is given generate a
graph for all root targets.</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">targets</code>
</p>
</td><td valign="top">
<p>
output a list of targets either by rule or by depth.  If used
like <code class="literal">ninja -t targets rule <span class="emphasis"><em>name</em></span></code> it prints the list of targets
using the given rule to be built.  If no rule is given, it prints the source
files (the leaves of the graph).  If used like
<code class="literal">ninja -t targets depth <span class="emphasis"><em>digit</em></span></code> it
prints the list of targets in a depth-first manner starting by the root
targets (the ones with no outputs). Indentation is used to mark dependencies.
If the depth is zero it prints all targets. If no arguments are provided
<code class="literal">ninja -t targets depth 1</code> is assumed. In this mode targets may be listed
several times. If used like this <code class="literal">ninja -t targets all</code> it
prints all the targets available without indentation and it is faster
than the <span class="emphasis"><em>depth</em></span> mode.
</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">commands</code>
</p>
</td><td valign="top">
<p>
given a list of targets, print a list of commands which, if
executed in order, may be used to rebuild those targets, assuming that all
output files are out of date.
</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">clean</code>
</p>
</td><td valign="top">
<p>
remove built files. By default it removes all built files
except for those created by the generator.  Adding the <code class="literal">-g</code> flag also
removes built files created by the generator (see <a class="link" href="#ref_rule" title="Rule variables">the rule reference for the <code class="literal">generator</code> attribute</a>).  Additional arguments are
targets, which removes the given targets and recursively all files
built for them.
</p>
<p>If used like <code class="literal">ninja -t clean -r <span class="emphasis"><em>rules</em></span></code> it removes all files built using
the given rules.</p>
<p>Files created but not referenced in the graph are not removed. This
tool takes in account the <code class="literal">-v</code> and the <code class="literal">-n</code> options (note that <code class="literal">-n</code>
implies <code class="literal">-v</code>).</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">compdb</code>
</p>
</td><td valign="top">
<p>
given a list of rules, each of which is expected to be a
C family language compiler rule whose first input is the name of the
source file, prints on standard output a compilation database in the
<a class="ulink" href="http://clang.llvm.org/docs/JSONCompilationDatabase.html" target="_top">JSON format</a> expected
by the Clang tooling interface.
<span class="emphasis"><em>Available since Ninja 1.2.</em></span>
</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">deps</code>
</p>
</td><td valign="top">
<p>
show all dependencies stored in the <code class="literal">.ninja_deps</code> file. When given a
target, show just the target’s dependencies. <span class="emphasis"><em>Available since Ninja 1.4.</em></span>
</p>
</td></tr><tr><td valign="top">
<p>
<code class="literal">recompact</code>
</p>
</td><td valign="top">
<p>
recompact the <code class="literal">.ninja_deps</code> file. <span class="emphasis"><em>Available since Ninja 1.4.</em></span>
</p>
</td></tr></tbody></table></div></div></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_writing_your_own_ninja_files"></a>Writing your own Ninja files</h1></div></div></div><p>The remainder of this manual is only useful if you are constructing
Ninja files yourself: for example, if you’re writing a meta-build
system or supporting a new language.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_conceptual_overview"></a>Conceptual overview</h2></div></div></div><p>Ninja evaluates a graph of dependencies between files, and runs
whichever commands are necessary to make your build target up to date
as determined by file modification times.  If you are familiar with
Make, Ninja is very similar.</p><p>A build file (default name: <code class="literal">build.ninja</code>) provides a list of <span class="emphasis"><em>rules</em></span> — short names for longer commands, like how to run the compiler — along with a list of <span class="emphasis"><em>build</em></span> statements saying how to build files
using the rules — which rule to apply to which inputs to produce
which outputs.</p><p>Conceptually, <code class="literal">build</code> statements describe the dependency graph of your
project, while <code class="literal">rule</code> statements describe how to generate the files
along a given edge of the graph.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_syntax_example"></a>Syntax example</h2></div></div></div><p>Here’s a basic <code class="literal">.ninja</code> file that demonstrates most of the syntax.
It will be used as an example for the following sections.</p><pre class="screen">cflags = -Wall

rule cc
  command = gcc $cflags -c $in -o $out

build foo.o: cc foo.c</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_variables"></a>Variables</h2></div></div></div><p>Despite the non-goal of being convenient to write by hand, to keep
build files readable (debuggable), Ninja supports declaring shorter
reusable names for strings.  A declaration like the following</p><pre class="screen">cflags = -g</pre><p>can be used on the right side of an equals sign, dereferencing it with
a dollar sign, like this:</p><pre class="screen">rule cc
  command = gcc $cflags -c $in -o $out</pre><p>Variables can also be referenced using curly braces like <code class="literal">${in}</code>.</p><p>Variables might better be called "bindings", in that a given variable
cannot be changed, only shadowed.  There is more on how shadowing works
later in this document.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_rules"></a>Rules</h2></div></div></div><p>Rules declare a short name for a command line.  They begin with a line
consisting of the <code class="literal">rule</code> keyword and a name for the rule.  Then
follows an indented set of <code class="literal">variable = value</code> lines.</p><p>The basic example above declares a new rule named <code class="literal">cc</code>, along with the
command to run.  In the context of a rule, the <code class="literal">command</code> variable
defines the command to run, <code class="literal">$in</code> expands to the list of
input files (<code class="literal">foo.c</code>), and <code class="literal">$out</code> to the output files (<code class="literal">foo.o</code>) for the
command.  A full list of special variables is provided in
<a class="link" href="#ref_rule" title="Rule variables">the reference</a>.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_build_statements"></a>Build statements</h2></div></div></div><p>Build statements declare a relationship between input and output
files.  They begin with the <code class="literal">build</code> keyword, and have the format
<code class="literal">build <span class="emphasis"><em>outputs</em></span>: <span class="emphasis"><em>rulename</em></span> <span class="emphasis"><em>inputs</em></span></code>.  Such a declaration says that
all of the output files are derived from the input files.  When the
output files are missing or when the inputs change, Ninja will run the
rule to regenerate the outputs.</p><p>The basic example above describes how to build <code class="literal">foo.o</code>, using the <code class="literal">cc</code>
rule.</p><p>In the scope of a <code class="literal">build</code> block (including in the evaluation of its
associated <code class="literal">rule</code>), the variable <code class="literal">$in</code> is the list of inputs and the
variable <code class="literal">$out</code> is the list of outputs.</p><p>A build statement may be followed by an indented set of <code class="literal">key = value</code>
pairs, much like a rule.  These variables will shadow any variables
when evaluating the variables in the command.  For example:</p><pre class="screen">cflags = -Wall -Werror
rule cc
  command = gcc $cflags -c $in -o $out

# If left unspecified, builds get the outer $cflags.
build foo.o: cc foo.c

# But you can shadow variables like cflags for a particular build.
build special.o: cc special.c
  cflags = -Wall

# The variable was only shadowed for the scope of special.o;
# Subsequent build lines get the outer (original) cflags.
build bar.o: cc bar.c</pre><p>For more discussion of how scoping works, consult <a class="link" href="#ref_scope" title="Evaluation and scoping">the reference</a>.</p><p>If you need more complicated information passed from the build
statement to the rule (for example, if the rule needs "the file
extension of the first input"), pass that through as an extra
variable, like how <code class="literal">cflags</code> is passed above.</p><p>If the top-level Ninja file is specified as an output of any build
statement and it is out of date, Ninja will rebuild and reload it
before building the targets requested by the user.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_generating_ninja_files_from_code"></a>Generating Ninja files from code</h2></div></div></div><p><code class="literal">misc/ninja_syntax.py</code> in the Ninja distribution is a tiny Python
module to facilitate generating Ninja files.  It allows you to make
Python calls like <code class="literal">ninja.rule(name='foo', command='bar',
depfile='$out.d')</code> and it will generate the appropriate syntax.  Feel
free to just inline it into your project’s build system if it’s
useful.</p></div></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_more_details"></a>More details</h1></div></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_the_literal_phony_literal_rule"></a>The <code class="literal">phony</code> rule</h2></div></div></div><p>The special rule name <code class="literal">phony</code> can be used to create aliases for other
targets.  For example:</p><pre class="screen">build foo: phony some/file/in/a/faraway/subdir/foo</pre><p>This makes <code class="literal">ninja foo</code> build the longer path.  Semantically, the
<code class="literal">phony</code> rule is equivalent to a plain rule where the <code class="literal">command</code> does
nothing, but phony rules are handled specially in that they aren’t
printed when run, logged (see below), nor do they contribute to the
command count printed as part of the build process.</p><p><code class="literal">phony</code> can also be used to create dummy targets for files which
may not exist at build time.  If a phony build statement is written
without any dependencies, the target will be considered out of date if
it does not exist.  Without a phony build statement, Ninja will report
an error if the file does not exist and is required by the build.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_default_target_statements"></a>Default target statements</h2></div></div></div><p>By default, if no targets are specified on the command line, Ninja
will build every output that is not named as an input elsewhere.
You can override this behavior using a default target statement.
A default target statement causes Ninja to build only a given subset
of output files if none are specified on the command line.</p><p>Default target statements begin with the <code class="literal">default</code> keyword, and have
the format <code class="literal">default <span class="emphasis"><em>targets</em></span></code>.  A default target statement must appear
after the build statement that declares the target as an output file.
They are cumulative, so multiple statements may be used to extend
the list of default targets.  For example:</p><pre class="screen">default foo bar
default baz</pre><p>This causes Ninja to build the <code class="literal">foo</code>, <code class="literal">bar</code> and <code class="literal">baz</code> targets by
default.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_log"></a>The Ninja log</h2></div></div></div><p>For each built file, Ninja keeps a log of the command used to build
it.  Using this log Ninja can know when an existing output was built
with a different command line than the build files specify (i.e., the
command line changed) and knows to rebuild the file.</p><p>The log file is kept in the build root in a file called <code class="literal">.ninja_log</code>.
If you provide a variable named <code class="literal">builddir</code> in the outermost scope,
<code class="literal">.ninja_log</code> will be kept in that directory instead.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_versioning"></a>Version compatibility</h2></div></div></div><p><span class="emphasis"><em>Available since Ninja 1.2.</em></span></p><p>Ninja version labels follow the standard major.minor.patch format,
where the major version is increased on backwards-incompatible
syntax/behavioral changes and the minor version is increased on new
behaviors.  Your <code class="literal">build.ninja</code> may declare a variable named
<code class="literal">ninja_required_version</code> that asserts the minimum Ninja version
required to use the generated file.  For example,</p><pre class="screen">ninja_required_version = 1.1</pre><p>declares that the build file relies on some feature that was
introduced in Ninja 1.1 (perhaps the <code class="literal">pool</code> syntax), and that
Ninja 1.1 or greater must be used to build.  Unlike other Ninja
variables, this version requirement is checked immediately when
the variable is encountered in parsing, so it’s best to put it
at the top of the build file.</p><p>Ninja always warns if the major versions of Ninja and the
<code class="literal">ninja_required_version</code> don’t match; a major version change hasn’t
come up yet so it’s difficult to predict what behavior might be
required.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_headers"></a>C/C++ header dependencies</h2></div></div></div><p>To get C/C++ header dependencies (or any other build dependency that
works in a similar way) correct Ninja has some extra functionality.</p><p>The problem with headers is that the full list of files that a given
source file depends on can only be discovered by the compiler:
different preprocessor defines and include paths cause different files
to be used.  Some compilers can emit this information while building,
and Ninja can use that to get its dependencies perfect.</p><p>Consider: if the file has never been compiled, it must be built anyway,
generating the header dependencies as a side effect.  If any file is
later modified (even in a way that changes which headers it depends
on) the modification will cause a rebuild as well, keeping the
dependencies up to date.</p><p>When loading these special dependencies, Ninja implicitly adds extra
build edges such that it is not an error if the listed dependency is
missing.  This allows you to delete a header file and rebuild without
the build aborting due to a missing input.</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_depfile"></a>depfile</h3></div></div></div><p><code class="literal">gcc</code> (and other compilers like <code class="literal">clang</code>) support emitting dependency
information in the syntax of a Makefile.  (Any command that can write
dependencies in this form can be used, not just <code class="literal">gcc</code>.)</p><p>To bring this information into Ninja requires cooperation.  On the
Ninja side, the <code class="literal">depfile</code> attribute on the <code class="literal">build</code> must point to a
path where this data is written.  (Ninja only supports the limited
subset of the Makefile syntax emitted by compilers.)  Then the command
must know to write dependencies into the <code class="literal">depfile</code> path.
Use it like in the following example:</p><pre class="screen">rule cc
  depfile = $out.d
  command = gcc -MMD -MF $out.d [other gcc flags here]</pre><p>The <code class="literal">-MMD</code> flag to <code class="literal">gcc</code> tells it to output header dependencies, and
the <code class="literal">-MF</code> flag tells it where to write them.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_deps"></a>deps</h3></div></div></div><p><span class="emphasis"><em>(Available since Ninja 1.3.)</em></span></p><p>It turns out that for large projects (and particularly on Windows,
where the file system is slow) loading these dependency files on
startup is slow.</p><p>Ninja 1.3 can instead process dependencies just after they’re generated
and save a compacted form of the same information in a Ninja-internal
database.</p><p>Ninja supports this processing in two forms.</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
<code class="literal">deps = gcc</code> specifies that the tool outputs <code class="literal">gcc</code>-style dependencies
   in the form of Makefiles.  Adding this to the above example will
   cause Ninja to process the <code class="literal">depfile</code> immediately after the
   compilation finishes, then delete the <code class="literal">.d</code> file (which is only used
   as a temporary).
</li><li class="listitem"><p class="simpara">
<code class="literal">deps = msvc</code> specifies that the tool outputs header dependencies
   in the form produced by Visual Studio’s compiler’s
   <a class="ulink" href="http://msdn.microsoft.com/en-us/library/hdkef6tk(v=vs.90).aspx" target="_top"><code class="literal">/showIncludes</code>
   flag</a>.  Briefly, this means the tool outputs specially-formatted lines
   to its stdout.  Ninja then filters these lines from the displayed
   output.  No <code class="literal">depfile</code> attribute is necessary, but the localized string
   in front of the the header file path. For instance
   `msvc_deps_prefix = Note: including file: `
   for a English Visual Studio (the default). Should be globally defined.
</p><pre class="screen">msvc_deps_prefix = Note: including file:
rule cc
  deps = msvc
  command = cl /showIncludes -c $in /Fo$out</pre></li></ol></div><p>If the include directory directives are using absolute paths, your depfile
may result in a mixture of relative and absolute paths. Paths used by other
build rules need to match exactly. Therefore, it is recommended to use
relative paths in these cases.</p></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_pool"></a>Pools</h2></div></div></div><p><span class="emphasis"><em>Available since Ninja 1.1.</em></span></p><p>Pools allow you to allocate one or more rules or edges a finite number
of concurrent jobs which is more tightly restricted than the default
parallelism.</p><p>This can be useful, for example, to restrict a particular expensive rule
(like link steps for huge executables), or to restrict particular build
statements which you know perform poorly when run concurrently.</p><p>Each pool has a <code class="literal">depth</code> variable which is specified in the build file.
The pool is then referred to with the <code class="literal">pool</code> variable on either a rule
or a build statement.</p><p>No matter what pools you specify, ninja will never run more concurrent jobs
than the default parallelism, or the number of jobs specified on the command
line (with <code class="literal">-j</code>).</p><pre class="screen"># No more than 4 links at a time.
pool link_pool
  depth = 4

# No more than 1 heavy object at a time.
pool heavy_object_pool
  depth = 1

rule link
  ...
  pool = link_pool

rule cc
  ...

# The link_pool is used here. Only 4 links will run concurrently.
build foo.exe: link input.obj

# A build statement can be exempted from its rule's pool by setting an
# empty pool. This effectively puts the build statement back into the default
# pool, which has infinite depth.
build other.exe: link input.obj
  pool =

# A build statement can specify a pool directly.
# Only one of these builds will run at a time.
build heavy_object1.obj: cc heavy_obj1.cc
  pool = heavy_object_pool
build heavy_object2.obj: cc heavy_obj2.cc
  pool = heavy_object_pool</pre><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_the_literal_console_literal_pool"></a>The <code class="literal">console</code> pool</h3></div></div></div><p><span class="emphasis"><em>Available since Ninja 1.5.</em></span></p><p>There exists a pre-defined pool named <code class="literal">console</code> with a depth of 1. It has
the special property that any task in the pool has direct access to the
standard input, output and error streams provided to Ninja, which are
normally connected to the user’s console (hence the name) but could be
redirected. This can be useful for interactive tasks or long-running tasks
which produce status updates on the console (such as test suites).</p><p>While a task in the <code class="literal">console</code> pool is running, Ninja’s regular output (such
as progress status and output from concurrent tasks) is buffered until
it completes.</p></div></div></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_ninja_file_reference"></a>Ninja file reference</h1></div></div></div><p>A file is a series of declarations.  A declaration can be one of:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
A rule declaration, which begins with <code class="literal">rule <span class="emphasis"><em>rulename</em></span></code>, and
   then has a series of indented lines defining variables.
</li><li class="listitem"><p class="simpara">
A build edge, which looks like <code class="literal">build <span class="emphasis"><em>output1</em></span> <span class="emphasis"><em>output2</em></span>:
   <span class="emphasis"><em>rulename</em></span> <span class="emphasis"><em>input1</em></span> <span class="emphasis"><em>input2</em></span></code>.
   Implicit dependencies may be tacked on the end with <code class="literal">|
   <span class="emphasis"><em>dependency1</em></span> <span class="emphasis"><em>dependency2</em></span></code>.
   Order-only dependencies may be tacked on the end with <code class="literal">||
   <span class="emphasis"><em>dependency1</em></span> <span class="emphasis"><em>dependency2</em></span></code>.  (See <a class="link" href="#ref_dependencies" title="Build dependencies">the reference on    dependency types</a>.)
</p><p class="simpara">Implicit outputs <span class="emphasis"><em>(available since Ninja 1.7)</em></span> may be added before
the <code class="literal">:</code> with <code class="literal">| <span class="emphasis"><em>output1</em></span> <span class="emphasis"><em>output2</em></span></code> and do not appear in <code class="literal">$out</code>.
(See <a class="link" href="#ref_outputs" title="Build outputs">the reference on output types</a>.)</p></li><li class="listitem">
Variable declarations, which look like <code class="literal"><span class="emphasis"><em>variable</em></span> = <span class="emphasis"><em>value</em></span></code>.
</li><li class="listitem">
Default target statements, which look like <code class="literal">default <span class="emphasis"><em>target1</em></span> <span class="emphasis"><em>target2</em></span></code>.
</li><li class="listitem">
References to more files, which look like <code class="literal">subninja <span class="emphasis"><em>path</em></span></code> or
   <code class="literal">include <span class="emphasis"><em>path</em></span></code>.  The difference between these is explained below
   <a class="link" href="#ref_scope" title="Evaluation and scoping">in the discussion about scoping</a>.
</li><li class="listitem">
A pool declaration, which looks like <code class="literal">pool <span class="emphasis"><em>poolname</em></span></code>. Pools are explained
   <a class="link" href="#ref_pool" title="Pools">in the section on pools</a>.
</li></ol></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_lexical_syntax"></a>Lexical syntax</h2></div></div></div><p>Ninja is mostly encoding agnostic, as long as the bytes Ninja cares
about (like slashes in paths) are ASCII.  This means e.g. UTF-8 or
ISO-8859-1 input files ought to work.</p><p>Comments begin with <code class="literal">#</code> and extend to the end of the line.</p><p>Newlines are significant.  Statements like <code class="literal">build foo bar</code> are a set
of space-separated tokens that end at the newline.  Newlines and
spaces within a token must be escaped.</p><p>There is only one escape character, <code class="literal">$</code>, and it has the following
behaviors:</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="literal">$</code> followed by a newline
</span></dt><dd>
escape the newline (continue the current line
across a line break).
</dd><dt><span class="term">
<code class="literal">$</code> followed by text
</span></dt><dd>
a variable reference.
</dd><dt><span class="term">
<code class="literal">${varname}</code>
</span></dt><dd>
alternate syntax for <code class="literal">$varname</code>.
</dd><dt><span class="term">
<code class="literal">$</code> followed by space
</span></dt><dd>
a space.  (This is only necessary in lists of
paths, where a space would otherwise separate filenames.  See below.)
</dd><dt><span class="term">
<code class="literal">$:</code> 
</span></dt><dd>
a colon.  (This is only necessary in <code class="literal">build</code> lines, where a colon
would otherwise terminate the list of outputs.)
</dd><dt><span class="term">
<code class="literal">$$</code>
</span></dt><dd>
a literal <code class="literal">$</code>.
</dd></dl></div><p>A <code class="literal">build</code> or <code class="literal">default</code> statement is first parsed as a space-separated
list of filenames and then each name is expanded.  This means that
spaces within a variable will result in spaces in the expanded
filename.</p><pre class="screen">spaced = foo bar
build $spaced/baz other$ file: ...
# The above build line has two outputs: "foo bar/baz" and "other file".</pre><p>In a <code class="literal">name = value</code> statement, whitespace at the beginning of a value
is always stripped.  Whitespace at the beginning of a line after a
line continuation is also stripped.</p><pre class="screen">two_words_with_one_space = foo $
    bar
one_word_with_no_space = foo$
    bar</pre><p>Other whitespace is only significant if it’s at the beginning of a
line.  If a line is indented more than the previous one, it’s
considered part of its parent’s scope; if it is indented less than the
previous one, it closes the previous scope.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_toplevel"></a>Top-level variables</h2></div></div></div><p>Two variables are significant when declared in the outermost file scope.</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="literal">builddir</code>
</span></dt><dd>
a directory for some Ninja output files.  See <a class="link" href="#ref_log" title="The Ninja log">the   discussion of the build log</a>.  (You can also store other build output
  in this directory.)
</dd><dt><span class="term">
<code class="literal">ninja_required_version</code>
</span></dt><dd>
the minimum version of Ninja required to process
  the build correctly.  See <a class="link" href="#ref_versioning" title="Version compatibility">the discussion of versioning</a>.
</dd></dl></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_rule"></a>Rule variables</h2></div></div></div><p>A <code class="literal">rule</code> block contains a list of <code class="literal">key = value</code> declarations that
affect the processing of the rule.  Here is a full list of special
keys.</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="literal">command</code> (<span class="emphasis"><em>required</em></span>)
</span></dt><dd>
the command line to run.  Each <code class="literal">rule</code> may
  have only one <code class="literal">command</code> declaration. See <a class="link" href="#ref_rule_command" title="Interpretation of the command variable">the next   section</a> for more details on quoting and executing multiple commands.
</dd><dt><span class="term">
<code class="literal">depfile</code>
</span></dt><dd>
path to an optional <code class="literal">Makefile</code> that contains extra
  <span class="emphasis"><em>implicit dependencies</em></span> (see <a class="link" href="#ref_dependencies" title="Build dependencies">the reference on   dependency types</a>).  This is explicitly to support C/C++ header
  dependencies; see <a class="link" href="#ref_headers" title="C/C++ header dependencies">the full discussion</a>.
</dd><dt><span class="term">
<code class="literal">deps</code>
</span></dt><dd>
<span class="emphasis"><em>(Available since Ninja 1.3.)</em></span> if present, must be one of
  <code class="literal">gcc</code> or <code class="literal">msvc</code> to specify special dependency processing.  See
   <a class="link" href="#ref_headers" title="C/C++ header dependencies">the full discussion</a>.  The generated database is
   stored as <code class="literal">.ninja_deps</code> in the <code class="literal">builddir</code>, see <a class="link" href="#ref_toplevel" title="Top-level variables">the    discussion of <code class="literal">builddir</code></a>.
</dd><dt><span class="term">
<code class="literal">msvc_deps_prefix</code>
</span></dt><dd>
<span class="emphasis"><em>(Available since Ninja 1.5.)</em></span> defines the string
  which should be stripped from msvc’s /showIncludes output. Only
  needed when <code class="literal">deps = msvc</code> and no English Visual Studio version is used.
</dd><dt><span class="term">
<code class="literal">description</code>
</span></dt><dd>
a short description of the command, used to pretty-print
  the command as it’s running.  The <code class="literal">-v</code> flag controls whether to print
  the full command or its description; if a command fails, the full command
  line will always be printed before the command’s output.
</dd><dt><span class="term">
<code class="literal">generator</code>
</span></dt><dd>
if present, specifies that this rule is used to
  re-invoke the generator program.  Files built using <code class="literal">generator</code>
  rules are treated specially in two ways: firstly, they will not be
  rebuilt if the command line changes; and secondly, they are not
  cleaned by default.
</dd><dt><span class="term">
<code class="literal">in</code>
</span></dt><dd>
the space-separated list of files provided as inputs to the build line
  referencing this <code class="literal">rule</code>, shell-quoted if it appears in commands.  (<code class="literal">$in</code> is
  provided solely for convenience; if you need some subset or variant of this
  list of files, just construct a new variable with that list and use
  that instead.)
</dd><dt><span class="term">
<code class="literal">in_newline</code>
</span></dt><dd>
the same as <code class="literal">$in</code> except that multiple inputs are
  separated by newlines rather than spaces.  (For use with
  <code class="literal">$rspfile_content</code>; this works around a bug in the MSVC linker where
  it uses a fixed-size buffer for processing input.)
</dd><dt><span class="term">
<code class="literal">out</code>
</span></dt><dd>
the space-separated list of files provided as outputs to the build line
  referencing this <code class="literal">rule</code>, shell-quoted if it appears in commands.
</dd><dt><span class="term">
<code class="literal">restat</code>
</span></dt><dd>
if present, causes Ninja to re-stat the command’s outputs
  after execution of the command.  Each output whose modification time
  the command did not change will be treated as though it had never
  needed to be built.  This may cause the output’s reverse
  dependencies to be removed from the list of pending build actions.
</dd><dt><span class="term">
<code class="literal">rspfile</code>, <code class="literal">rspfile_content</code>
</span></dt><dd><p class="simpara">
if present (both), Ninja will use a
  response file for the given command, i.e. write the selected string
  (<code class="literal">rspfile_content</code>) to the given file (<code class="literal">rspfile</code>) before calling the
  command and delete the file after successful execution of the
  command.
</p><p class="simpara">This is particularly useful on Windows OS, where the maximal length of
a command line is limited and response files must be used instead.</p><p class="simpara">Use it like in the following example:</p><pre class="screen">rule link
  command = link.exe /OUT$out [usual link flags here] @$out.rsp
  rspfile = $out.rsp
  rspfile_content = $in

build myapp.exe: link a.obj b.obj [possibly many other .obj files]</pre></dd></dl></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="ref_rule_command"></a>Interpretation of the <code class="literal">command</code> variable</h3></div></div></div><p>Fundamentally, command lines behave differently on Unixes and Windows.</p><p>On Unixes, commands are arrays of arguments.  The Ninja <code class="literal">command</code>
variable is passed directly to <code class="literal">sh -c</code>, which is then responsible for
interpreting that string into an argv array.  Therefore the quoting
rules are those of the shell, and you can use all the normal shell
operators, like <code class="literal">&amp;&amp;</code> to chain multiple commands, or <code class="literal">VAR=value cmd</code> to
set environment variables.</p><p>On Windows, commands are strings, so Ninja passes the <code class="literal">command</code> string
directly to <code class="literal">CreateProcess</code>.  (In the common case of simply executing
a compiler this means there is less overhead.)  Consequently the
quoting rules are deterimined by the called program, which on Windows
are usually provided by the C library.  If you need shell
interpretation of the command (such as the use of <code class="literal">&amp;&amp;</code> to chain
multiple commands), make the command execute the Windows shell by
prefixing the command with <code class="literal">cmd /c</code>.</p></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_outputs"></a>Build outputs</h2></div></div></div><p>There are two types of build outputs which are subtly different.</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p class="simpara">
<span class="emphasis"><em>Explicit outputs</em></span>, as listed in a build line.  These are
   available as the <code class="literal">$out</code> variable in the rule.
</p><p class="simpara">This is the standard form of output to be used for e.g. the
object file of a compile command.</p></li><li class="listitem"><p class="simpara">
<span class="emphasis"><em>Implicit outputs</em></span>, as listed in a build line with the syntax <code class="literal">|
   <span class="emphasis"><em>out1</em></span> <span class="emphasis"><em>out2</em></span></code> + before the <code class="literal">:</code> of a build line <span class="emphasis"><em>(available since
   Ninja 1.7)</em></span>.  The semantics are identical to explicit outputs,
  the only difference is that implicit outputs don’t show up in the
  <code class="literal">$out</code> variable.
</p><p class="simpara">This is for expressing outputs that don’t show up on the
command line of the command.</p></li></ol></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_dependencies"></a>Build dependencies</h2></div></div></div><p>There are three types of build dependencies which are subtly different.</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p class="simpara">
<span class="emphasis"><em>Explicit dependencies</em></span>, as listed in a build line.  These are
   available as the <code class="literal">$in</code> variable in the rule.  Changes in these files
   cause the output to be rebuilt; if these file are missing and
   Ninja doesn’t know how to build them, the build is aborted.
</p><p class="simpara">This is the standard form of dependency to be used e.g. for the
source file of a compile command.</p></li><li class="listitem"><p class="simpara">
<span class="emphasis"><em>Implicit dependencies</em></span>, either as picked up from
   a <code class="literal">depfile</code> attribute on a rule or from the syntax <code class="literal">| <span class="emphasis"><em>dep1</em></span>
   <span class="emphasis"><em>dep2</em></span></code> on the end of a build line.  The semantics are identical to
   explicit dependencies, the only difference is that implicit dependencies
   don’t show up in the <code class="literal">$in</code> variable.
</p><p class="simpara">This is for expressing dependencies that don’t show up on the
command line of the command; for example, for a rule that runs a
script, the script itself should be an implicit dependency, as
changes to the script should cause the output to rebuild.</p><p class="simpara">Note that dependencies as loaded through depfiles have slightly different
semantics, as described in the <a class="link" href="#ref_rule" title="Rule variables">rule reference</a>.</p></li><li class="listitem"><p class="simpara">
<span class="emphasis"><em>Order-only dependencies</em></span>, expressed with the syntax <code class="literal">|| <span class="emphasis"><em>dep1</em></span>
   <span class="emphasis"><em>dep2</em></span></code> on the end of a build line.  When these are out of date, the
   output is not rebuilt until they are built, but changes in order-only
   dependencies alone do not cause the output to be rebuilt.
</p><p class="simpara">Order-only dependencies can be useful for bootstrapping dependencies
that are only discovered during build time: for example, to generate a
header file before starting a subsequent compilation step.  (Once the
header is used in compilation, a generated dependency file will then
express the implicit dependency.)</p></li></ol></div><p>File paths are compared as is, which means that an absolute path and a
relative path, pointing to the same file, are considered different by Ninja.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_variable_expansion"></a>Variable expansion</h2></div></div></div><p>Variables are expanded in paths (in a <code class="literal">build</code> or <code class="literal">default</code> statement)
and on the right side of a <code class="literal">name = value</code> statement.</p><p>When a <code class="literal">name = value</code> statement is evaluated, its right-hand side is
expanded immediately (according to the below scoping rules), and
from then on <code class="literal">$name</code> expands to the static string as the result of the
expansion.  It is never the case that you’ll need to "double-escape" a
value to prevent it from getting expanded twice.</p><p>All variables are expanded immediately as they’re encountered in parsing,
with one important exception: variables in <code class="literal">rule</code> blocks are expanded
when the rule is <span class="emphasis"><em>used</em></span>, not when it is declared.  In the following
example, the <code class="literal">demo</code> rule prints "this is a demo of bar".</p><pre class="screen">rule demo
  command = echo "this is a demo of $foo"

build out: demo
  foo = bar</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ref_scope"></a>Evaluation and scoping</h2></div></div></div><p>Top-level variable declarations are scoped to the file they occur in.</p><p>Rule declarations are also scoped to the file they occur in.
<span class="emphasis"><em>(Available since Ninja 1.6)</em></span></p><p>The <code class="literal">subninja</code> keyword, used to include another <code class="literal">.ninja</code> file,
introduces a new scope.  The included <code class="literal">subninja</code> file may use the
variables and rules from the parent file, and shadow their values for the file’s
scope, but it won’t affect values of the variables in the parent.</p><p>To include another <code class="literal">.ninja</code> file in the current scope, much like a C
<code class="literal">#include</code> statement, use <code class="literal">include</code> instead of <code class="literal">subninja</code>.</p><p>Variable declarations indented in a <code class="literal">build</code> block are scoped to the
<code class="literal">build</code> block.  The full lookup order for a variable expanded in a
<code class="literal">build</code> block (or the <code class="literal">rule</code> is uses) is:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
Special built-in variables (<code class="literal">$in</code>, <code class="literal">$out</code>).
</li><li class="listitem">
Build-level variables from the <code class="literal">build</code> block.
</li><li class="listitem">
Rule-level variables from the <code class="literal">rule</code> block (i.e. <code class="literal">$command</code>).
   (Note from the above discussion on expansion that these are
   expanded "late", and may make use of in-scope bindings like <code class="literal">$in</code>.)
</li><li class="listitem">
File-level variables from the file that the <code class="literal">build</code> line was in.
</li><li class="listitem">
Variables from the file that included that file using the
   <code class="literal">subninja</code> keyword.
</li></ol></div></div></div></div></body></html>