Merge pull request #512 from pcc/compilation-database

[platform/upstream/ninja.git] / doc / manual.asciidoc

Ninja
=====
Evan Martin <martine@danga.com>


Introduction
------------

Ninja is yet another build system.  It takes as input the
interdependencies of files (typically source code and output
executables) and orchestrates building them, _quickly_.

Ninja joins a sea of other build systems.  Its distinguishing goal is
to be fast.  It is born from
http://neugierig.org/software/chromium/notes/2011/02/ninja.html[my
work on the Chromium browser project], which has over 30,000 source
files and whose other build systems (including one built from custom
non-recursive Makefiles) can take ten seconds to start building after
changing one file.  Ninja is under a second.

Philosophical overview
~~~~~~~~~~~~~~~~~~~~~~

Where other build systems are high-level languages, Ninja aims to be
an assembler.

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.

Ninja contains the barest functionality necessary to describe
arbitrary dependency graphs.  Its lack of syntax makes it impossible
to express complex decisions.

Instead, Ninja is intended to be used with a separate program
generating its input files.  The generator program (like the
`./configure` 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 `.ninja` file
generator.

Design goals
~~~~~~~~~~~~

Here are the design goals of Ninja:

* very fast (i.e., instant) incremental builds, even for very large
  projects.

* 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 an "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.

* 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 `-M` flags for header
  dependencies).

* when convenience and speed are in conflict, prefer speed.

Some explicit _non-goals_:

* convenient syntax for writing build files by hand.  _You should
  generate your ninja files using another program_.  This is how we
  can sidestep many policy decisions.

* built-in rules. _Out of the box, Ninja has no rules for
  e.g. compiling C code._

* build-time customization of the build. _Options belong in
  the program that generates the ninja files_.

* build-time decision-making ability such as conditionals or search
  paths. _Making decisions is slow._

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 `.ninja` files.

Comparison to Make
~~~~~~~~~~~~~~~~~~

Ninja is closest in spirit and functionality to make, relying on
simple dependencies between file timestamps.

But fundamentally, make has a lot of _features_: 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.

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.

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.)

* A Ninja rule may point at a path for extra implicit dependency
  information.  This makes it easy to get header dependencies correct
  for C/C++ code.

* A build edge may have multiple outputs.

* 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.

* Output directories are always implicitly created before running the
  command that relies on them.

* Rules can provide shorter descriptions of the command being run, so
  you can print e.g. `CC foo.o` instead of a long command line while
  building.

* 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.

* 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.


Using Ninja for your project
----------------------------

Ninja currently works on Unix-like systems. It's seen the most testing
on Linux (and has the best performance there) but it runs fine on Mac
OS X and FreeBSD.  Ninja has some preliminary Windows support but the
full details of the implementation -- like how to get C header
interdependencies correct and fast when using MSVC's compiler -- is
not yet complete.

If your project is small, Ninja's speed impact is likely unnoticeable.
Some build timing numbers are included below.  (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.

There are many other build systems that are more user-friendly or
featureful than Ninja itself.  For some recommendations: the Ninja
author found http://gittup.org/tup/[the tup build system] influential
in Ninja's design, and thinks https://github.com/apenwarr/redo[redo]'s
design is quite clever.

Ninja's benefit comes from using it in conjunction with a smarter
meta-build system.

http://code.google.com/p/gyp/[gyp]:: The meta-build system used to
generate build files for Google Chrome.  gyp can generate Ninja files
for Linux and Mac and is used by many Chrome developers; support for
Windows is in progress.  See the
http://code.google.com/p/chromium/wiki/NinjaBuild[Chromium Ninja
documentation for more details].  gyp is relatively unpopular outside
of the Chrome and v8 world.

* For Chrome (~30k source files), Ninja reduced no-op builds from
  around 15 seconds to under one second.
* https://plus.google.com/108996039294665965197/posts/SfhrFAhRyyd[A
  Mozilla developer compares build systems]: "While chromium's full
  build is 2.15x slower than firefox's, a nop build is 78.2x faster!
  That is really noticeable during development. No incremental build
  of firefox can be faster than 57.9s, which means that in practice
  almost all of them will be over a minute."

http://www.cmake.org/[CMake]:: A widely used meta-build system that
can generate Ninja files on Linux as of CMake version 2.8.8.  (There
is some Mac and Windows support -- http://www.reactos.org[ReactOS]
uses Ninja on Windows for their buildbots, but those platforms are not
yet officially supported by CMake as the full test suite doesn't
pass.)

* For building Blender, one user reported "Single file rebuild is 0.97
  sec, same on makefiles was 3.7sec."
* For building LLVM on Windows, one user reported no-op build times:
  "ninja: 0.4s / MSBuild: 11s / jom: 53s".

others:: Ninja ought to fit perfectly into other meta-build software
like http://industriousone.com/premake[premake].  If you do this work,
please let us know!

Running Ninja
~~~~~~~~~~~~~

Run `ninja`.  By default, it looks for a file named `build.ninja` in
the current directory and builds all out-of-date targets.  You can
specify which targets (files) to build as command line arguments.

`ninja -h` prints help output.  Many of Ninja's flags intentionally
match those of Make; e.g `ninja -C build -j 20` changes into the
`build` 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 `-j`.)


Environment variables
~~~~~~~~~~~~~~~~~~~~~

Ninja supports one environment variable to control its behavior:
`NINJA_STATUS`, the progress status printed before the rule being run.

Several placeholders are available:

`%s`:: The number of started edges.
`%t`:: The total number of edges that must be run to complete the build.
`%p`:: The percentage of started edges.
`%r`:: The number of currently running edges.
`%u`:: The number of remaining edges to start.
`%f`:: The number of finished edges.
`%o`:: Overall rate of finished edges per second
`%c`:: Current rate of finished edges per second (average over builds specified by -j or its default)
`%%`:: A plain `%` character.

The default progress status is `"[%s/%t] "` (note the trailing space
to separate from the build rule). Another example of possible progress status
could be `"[%u/%r/%f] "`.

Extra tools
~~~~~~~~~~~

The `-t` flag on the Ninja command line runs some tools that we have
found useful during Ninja's development.  The current tools are:

[horizontal]
`query`:: dump the inputs and outputs of a given target.

`browse`:: 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.

`graph`:: output a file in the syntax used by `graphviz`, a automatic
graph layout tool.  Use it like:
+
----
ninja -t graph mytarget | dot -Tpng -ograph.png
----
+
In the Ninja source tree, `ninja graph.png`
generates an image for Ninja itself.  If no target is given generate a
graph for all root targets.

`targets`:: output a list of targets either by rule or by depth.  If used
like +ninja -t targets rule _name_+ 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
+ninja -t targets depth _digit_+ 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
+ninja -t targets depth 1+ is assumed. In this mode targets may be listed
several times. If used like this +ninja -t targets all+ it
prints all the targets available without indentation and it is faster
than the _depth_ mode.

`commands`:: 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.

`clean`:: remove built files. By default it removes all built files
except for those created by the generator.  Adding the `-g` flag also
removes built files created by the generator (see <<ref_rule,the rule
reference for the +generator+ attribute>>).  Additional arguments are
targets, which removes the given targets and recursively all files
built for them.
+
If used like +ninja -t clean -r _rules_+ it removes all files built using
the given rules.
+
Files created but not referenced in the graph are not removed. This
tool takes in account the +-v+ and the +-n+ options (note that +-n+
implies +-v+).

`compdb`:: 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
http://clang.llvm.org/docs/JSONCompilationDatabase.html[JSON format] expected
by the Clang tooling interface.
_Available since Ninja 1.2._


Writing your own Ninja files
----------------------------

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.

Conceptual overview
~~~~~~~~~~~~~~~~~~~

Ninja evaluates a graph of dependencies between files, and runs
whichever commands are necessary to make your build target up to date.
If you are familiar with Make, Ninja is very similar.

A build file (default name: `build.ninja`) provides a list of _rules_
-- short names for longer commands, like how to run the compiler --
along with a list of _build_ statements saying how to build files
using the rules -- which rule to apply to which inputs to produce
which outputs.

Conceptually, `build` statements describe the dependency graph of your
project, while `rule` statements describe how to generate the files
along a given edge of the graph.

Syntax example
~~~~~~~~~~~~~~

Here's a basic `.ninja` file that demonstrates most of the syntax.
It will be used as an example for the following sections.

---------------------------------
cflags = -Wall

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

build foo.o: cc foo.c
---------------------------------

Variables
~~~~~~~~~
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

----------------
cflags = -g
----------------

can be used on the right side of an equals sign, dereferencing it with
a dollar sign, like this:

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

Variables can also be referenced using curly braces like `${in}`.

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.

Rules
~~~~~

Rules declare a short name for a command line.  They begin with a line
consisting of the `rule` keyword and a name for the rule.  Then
follows an indented set of `variable = value` lines.

The basic example above declares a new rule named `cc`, along with the
command to run.  In the context of a rule, the `command` variable
defines the command to run, `$in` expands to the list of
input files (`foo.c`), and `$out` to the output files (`foo.o`) for the
command.  A full list of special variables is provided in
<<ref_rule,the reference>>.

Build statements
~~~~~~~~~~~~~~~~

Build statements declare a relationship between input and output
files.  They begin with the `build` keyword, and have the format
+build _outputs_: _rulename_ _inputs_+.  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.

The basic example above describes how to build `foo.o`, using the `cc`
rule.

In the scope of a `build` block (including in the evaluation of its
associated `rule`), the variable `$in` is the list of inputs and the
variable `$out` is the list of outputs.

A build statement may be followed by an indented set of `key = value`
pairs, much like a rule.  These variables will shadow any variables
when evaluating the variables in the command.  For example:

----------------
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

----------------

For more discussion of how scoping works, consult <<ref_scope,the
reference>>.

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 `cflags` is passed above.

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.

Pools
~~~~~

_Available since Ninja 1.1._

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.

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.

Each pool has a `depth` variable which is specified in the build file.
The pool is then referred to with the `pool` variable on either a rule
or a build statement.

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 -j).

----------------
# 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

----------------


Generating Ninja files from code
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`misc/ninja_syntax.py` in the Ninja distribution is a tiny Python
module to facilitate generating Ninja files.  It allows you to make
Python calls like `ninja.rule(name='foo', command='bar',
depfile='$out.d')` and it will generate the appropriate syntax.  Feel
free to just inline it into your project's build system if it's
useful.


More details
------------

The `phony` rule
~~~~~~~~~~~~~~~~

The special rule name `phony` can be used to create aliases for other
targets.  For example:

----------------
build foo: phony some/file/in/a/faraway/subdir/foo
----------------

This makes `ninja foo` build the longer path.  Semantically, the
`phony` rule is equivalent to a plain rule where the `command` 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.

`phony` 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.


Default target statements
~~~~~~~~~~~~~~~~~~~~~~~~~

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.

Default target statements begin with the `default` keyword, and have
the format +default _targets_+.  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:

----------------
default foo bar
default baz
----------------

This causes Ninja to build the `foo`, `bar` and `baz` targets by
default.


[[ref_log]]
The Ninja log
~~~~~~~~~~~~~

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.

The log file is kept in the build root in a file called `.ninja_log`.
If you provide a variable named `builddir` in the outermost scope,
`.ninja_log` will be kept in that directory instead.


[[ref_versioning]]
Version compatibility
~~~~~~~~~~~~~~~~~~~~~

_Available since Ninja 1.XXX._

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 `build.ninja` may declare a variable named
`ninja_required_version` that asserts the minimum Ninja version
required to use the generated file.  For example,

-----
ninja_required_version = 1.1
-----

declares that the build file relies on some feature that was
introduced in Ninja 1.1 (perhaps the `pool` 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.

Ninja always warns if the major versions of Ninja and the
`ninja_required_version` don't match; a major version change hasn't
come up yet so it's difficult to predict what behavior might be
required.


Ninja file reference
--------------------

A file is a series of declarations.  A declaration can be one of:

1. A rule declaration, which begins with +rule _rulename_+, and
   then has a series of indented lines defining variables.

2. A build edge, which looks like +build _output1_ _output2_:
   _rulename_ _input1_ _input2_+. +
   Implicit dependencies may be tacked on the end with +|
   _dependency1_ _dependency2_+. +
   Order-only dependencies may be tacked on the end with +||
   _dependency1_ _dependency2_+.  (See <<ref_dependencies,the reference on
   dependency types>>.)

3. Variable declarations, which look like +_variable_ = _value_+.

4. Default target statements, which look like +default _target1_ _target2_+.

5. References to more files, which look like +subninja _path_+ or
   +include _path_+.  The difference between these is explained below
   <<ref_scope,in the discussion about scoping>>.

Lexical syntax
~~~~~~~~~~~~~~

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.  (To simplify some code, tabs
and carriage returns are currently disallowed; this could be fixed if
it really mattered to you.)

Comments begin with `#` and extend to the end of the line.

Newlines are significant.  Statements like `build foo bar` are a set
of space-separated tokens that end at the newline.  Newlines and
spaces within a token must be escaped.

There is only one escape character, `$`, and it has the following
behaviors:

[horizontal]
`$` followed by a newline:: escape the newline (continue the current line
across a line break).

`$` followed by text:: a variable reference.

`${varname}`:: alternate syntax for `$varname`.

`$` followed by space:: a space.  (This is only necessary in lists of
paths, where a space would otherwise separate filenames.  See below.)

`$:` :: a colon.  (This is only necessary in `build` lines, where a colon
would otherwise terminate the list of outputs.)

`$$`:: a literal `$`.

A `build` or `default` 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.

----
spaced = foo bar
build $spaced/baz other$ file: ...
# The above build line has two outputs: "foo bar/baz" and "other file".
----

In a `name = value` 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.

----
two_words_with_one_space = foo $
    bar
one_word_with_no_space = foo$
    bar
----

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.

Top-level variables
~~~~~~~~~~~~~~~~~~~

Two variables are significant when declared in the outermost file scope.

`builddir`:: a directory for some Ninja output files.  See <<ref_log,the
  discussion of the build log>>.  (You can also store other build output
  in this directory.)

`ninja_required_version`:: the minimum version of Ninja required to process
  the build correctly.  See <<ref_versioning,the discussion of versioning>>.


[[ref_rule]]
Rule variables
~~~~~~~~~~~~~~

A `rule` block contains a list of `key = value` declarations that
affect the processing of the rule.  Here is a full list of special
keys.

`command` (_required_):: the command line to run.  This string (after
  $variables are expanded) is passed directly to `sh -c` without
  interpretation by Ninja. Each `rule` may have only one `command`
  declaration. To specify multiple commands use `&&` (or similar) to
  concatenate operations. 

`depfile`:: path to an optional `Makefile` that contains extra
  _implicit dependencies_ (see <<ref_dependencies,the reference on
  dependency types>>).  This is explicitly to support `gcc` and its `-M`
  family of flags, which output the list of headers a given `.c` file
  depends on.
+
Use it like in the following example:
+
----
rule cc
  depfile = $out.d
  command = gcc -MMD -MF $out.d [other gcc flags here]
----
+
When loading a `depfile`, Ninja implicitly adds edges such that it is
not an error if the listed dependency is missing.  This allows you to
delete a depfile-discovered header file and rebuild, without the build
aborting due to a missing input.

`description`:: a short description of the command, used to pretty-print
  the command as it's running.  The `-v` 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.

`generator`:: if present, specifies that this rule is used to
  re-invoke the generator program.  Files built using `generator`
  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.

`in`:: the shell-quoted space-separated list of files provided as
  inputs to the build line referencing this `rule`.  (`$in` 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.)

`in_newline`:: the same as `$in` except that multiple inputs are
  separated by newlines rather than spaces.  (For use with
  `$rspfile_content`; this works around a bug in the MSVC linker where
  it uses a fixed-size buffer for processing input.)

`out`:: the shell-quoted space-separated list of files provided as
  outputs to the build line referencing this `rule`.

`restat`:: 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.

`rspfile`, `rspfile_content`:: if present (both), Ninja will use a
  response file for the given command, i.e. write the selected string
  (`rspfile_content`) to the given file (`rspfile`) before calling the
  command and delete the file after successful execution of the
  command.
+
This is particularly useful on Windows OS, where the maximal length of
a command line is limited and response files must be used instead.
+
Use it like in the following example:
+
----
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]
----

[[ref_dependencies]]
Build dependencies
~~~~~~~~~~~~~~~~~~

There are three types of build dependencies which are subtly different.

1. _Explicit dependencies_, as listed in a build line.  These are
   available as the `$in` 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.
+
This is the standard form of dependency to be used for e.g. the
source file of a compile command.

2. _Implicit dependencies_, either as picked up from
   a `depfile` attribute on a rule or from the syntax +| _dep1_
   _dep2_+ 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 `$in` variable.
+
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.
+
Note that dependencies as loaded through depfiles have slightly different
semantics, as described in the <<ref_rule,rule reference>>.

3. _Order-only dependencies_, expressed with the syntax +|| _dep1_
   _dep2_+ 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.
+
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.)

Variable expansion
~~~~~~~~~~~~~~~~~~

Variables are expanded in paths (in a `build` or `default` statement)
and on the right side of a `name = value` statement.

When a `name = value` statement is evaluated, its right-hand side is
expanded immediately (according to the below scoping rules), and
from then on `$name` 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.

All variables are expanded immediately as they're encountered in parsing,
with one important exception: variables in `rule` blocks are expanded
when the rule is _used_, not when it is declared.  In the following
example, the `demo` rule prints "this is a demo of bar".

----
rule demo
  command = echo "this is a demo of $foo'

build out: demo
  foo = bar
----

[[ref_scope]]
Evaluation and scoping
~~~~~~~~~~~~~~~~~~~~~~

Top-level variable declarations are scoped to the file they occur in.

The `subninja` keyword, used to include another `.ninja` file,
introduces a new scope.  The included `subninja` file may use the
variables 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.

To include another `.ninja` file in the current scope, much like a C
`#include` statement, use `include` instead of `subninja`.

Variable declarations indented in a `build` block are scoped to the
`build` block.  The full lookup order for a variable expanded in a
`build` block (or the `rule` is uses) is:

1. Special built-in variables (`$in`, `$out`).

2. Build-level variables from the `build` block.

3. Rule-level variables from the `rule` block (i.e. `$command`).
   (Note from the above discussion on expansion that these are
   expanded "late", and may make use of in-scope bindings like `$in`.)

4. File-level variables from the file that the `build` line was in.

5. Variables from the file that included that file using the
   `subninja` keyword.