8 Ninja is yet another build system. It takes as input the
9 interdependencies of files (typically source code and output
10 executables) and orchestrates building them, _quickly_.
12 Ninja joins a sea of other build systems. Its distinguishing goal is
13 to be fast. It is born from
14 http://neugierig.org/software/chromium/notes/2011/02/ninja.html[my
15 work on the Chromium browser project], which has over 30,000 source
16 files and whose other build systems (including one built from custom
17 non-recursive Makefiles) would take ten seconds to start building
18 after changing one file. Ninja is under a second.
20 Philosophical overview
21 ~~~~~~~~~~~~~~~~~~~~~~
23 Where other build systems are high-level languages, Ninja aims to be
26 Build systems get slow when they need to make decisions. When you are
27 in a edit-compile cycle you want it to be as fast as possible -- you
28 want the build system to do the minimum work necessary to figure out
29 what needs to be built immediately.
31 Ninja contains the barest functionality necessary to describe
32 arbitrary dependency graphs. Its lack of syntax makes it impossible
33 to express complex decisions.
35 Instead, Ninja is intended to be used with a separate program
36 generating its input files. The generator program (like the
37 `./configure` found in autotools projects) can analyze system
38 dependencies and make as many decisions as possible up front so that
39 incremental builds stay fast. Going beyond autotools, even build-time
40 decisions like "which compiler flags should I use?" or "should I
41 build a debug or release-mode binary?" belong in the `.ninja` file
47 Here are the design goals of Ninja:
49 * very fast (i.e., instant) incremental builds, even for very large
52 * very little policy about how code is built. Different projects and
53 higher-level build systems have different opinions about how code
54 should be built; for example, should built objects live alongside
55 the sources or should all build output go into a separate directory?
56 Is there a "package" rule that builds a distributable package of
57 the project? Sidestep these decisions by trying to allow either to
58 be implemented, rather than choosing, even if that results in
61 * get dependencies correct, and in particular situations that are
62 difficult to get right with Makefiles (e.g. outputs need an implicit
63 dependency on the command line used to generate them; to build C
64 source code you need to use gcc's `-M` flags for header
67 * when convenience and speed are in conflict, prefer speed.
69 Some explicit _non-goals_:
71 * convenient syntax for writing build files by hand. _You should
72 generate your ninja files using another program_. This is how we
73 can sidestep many policy decisions.
75 * built-in rules. _Out of the box, Ninja has no rules for
76 e.g. compiling C code._
78 * build-time customization of the build. _Options belong in
79 the program that generates the ninja files_.
81 * build-time decision-making ability such as conditionals or search
82 paths. _Making decisions is slow._
84 To restate, Ninja is faster than other build systems because it is
85 painfully simple. You must tell Ninja exactly what to do when you
86 create your project's `.ninja` files.
91 Ninja is closest in spirit and functionality to Make, relying on
92 simple dependencies between file timestamps.
94 But fundamentally, make has a lot of _features_: suffix rules,
95 functions, built-in rules that e.g. search for RCS files when building
96 source. Make's language was designed to be written by humans. Many
97 projects find make alone adequate for their build problems.
99 In contrast, Ninja has almost no features; just those necessary to get
100 builds correct while punting most complexity to generation of the
101 ninja input files. Ninja by itself is unlikely to be useful for most
104 Here are some of the features Ninja adds to Make. (These sorts of
105 features can often be implemented using more complicated Makefiles,
106 but they are not part of make itself.)
108 * Ninja has special support for discovering extra dependencies at build
109 time, making it easy to get <<ref_headers,header dependencies>>
110 correct for C/C++ code.
112 * A build edge may have multiple outputs.
114 * Outputs implicitly depend on the command line that was used to generate
115 them, which means that changing e.g. compilation flags will cause
116 the outputs to rebuild.
118 * Output directories are always implicitly created before running the
119 command that relies on them.
121 * Rules can provide shorter descriptions of the command being run, so
122 you can print e.g. `CC foo.o` instead of a long command line while
125 * Builds are always run in parallel, based by default on the number of
126 CPUs your system has. Underspecified build dependencies will result
129 * Command output is always buffered. This means commands running in
130 parallel don't interleave their output, and when a command fails we
131 can print its failure output next to the full command line that
132 produced the failure.
135 Using Ninja for your project
136 ----------------------------
138 Ninja currently works on Unix-like systems and Windows. It's seen the
139 most testing on Linux (and has the best performance there) but it runs
140 fine on Mac OS X and FreeBSD.
142 If your project is small, Ninja's speed impact is likely unnoticeable.
143 (However, even for small projects it sometimes turns out that Ninja's
144 limited syntax forces simpler build rules that result in faster
145 builds.) Another way to say this is that if you're happy with the
146 edit-compile cycle time of your project already then Ninja won't help.
148 There are many other build systems that are more user-friendly or
149 featureful than Ninja itself. For some recommendations: the Ninja
150 author found http://gittup.org/tup/[the tup build system] influential
151 in Ninja's design, and thinks https://github.com/apenwarr/redo[redo]'s
152 design is quite clever.
154 Ninja's benefit comes from using it in conjunction with a smarter
157 http://code.google.com/p/gyp/[gyp]:: The meta-build system used to
158 generate build files for Google Chrome and related projects (v8,
159 node.js). gyp can generate Ninja files for all platforms supported by
161 https://chromium.googlesource.com/chromium/src/+/master/docs/ninja_build.md[Chromium Ninja documentation for more details].
163 https://cmake.org/[CMake]:: A widely used meta-build system that
164 can generate Ninja files on Linux as of CMake version 2.8.8. Newer versions
165 of CMake support generating Ninja files on Windows and Mac OS X too.
167 https://github.com/ninja-build/ninja/wiki/List-of-generators-producing-ninja-build-files[others]:: Ninja ought to fit perfectly into other meta-build software
168 like http://industriousone.com/premake[premake]. If you do this work,
174 Run `ninja`. By default, it looks for a file named `build.ninja` in
175 the current directory and builds all out-of-date targets. You can
176 specify which targets (files) to build as command line arguments.
178 There is also a special syntax `target^` for specifying a target
179 as the first output of some rule containing the source you put in
180 the command line, if one exists. For example, if you specify target as
181 `foo.c^` then `foo.o` will get built (assuming you have those targets
182 in your build files).
184 `ninja -h` prints help output. Many of Ninja's flags intentionally
185 match those of Make; e.g `ninja -C build -j 20` changes into the
186 `build` directory and runs 20 build commands in parallel. (Note that
187 Ninja defaults to running commands in parallel anyway, so typically
188 you don't need to pass `-j`.)
191 Environment variables
192 ~~~~~~~~~~~~~~~~~~~~~
194 Ninja supports one environment variable to control its behavior:
195 `NINJA_STATUS`, the progress status printed before the rule being run.
197 Several placeholders are available:
199 `%s`:: The number of started edges.
200 `%t`:: The total number of edges that must be run to complete the build.
201 `%p`:: The percentage of started edges.
202 `%r`:: The number of currently running edges.
203 `%u`:: The number of remaining edges to start.
204 `%f`:: The number of finished edges.
205 `%o`:: Overall rate of finished edges per second
206 `%c`:: Current rate of finished edges per second (average over builds
207 specified by `-j` or its default)
208 `%e`:: Elapsed time in seconds. _(Available since Ninja 1.2.)_
209 `%%`:: A plain `%` character.
211 The default progress status is `"[%s/%t] "` (note the trailing space
212 to separate from the build rule). Another example of possible progress status
213 could be `"[%u/%r/%f] "`.
218 The `-t` flag on the Ninja command line runs some tools that we have
219 found useful during Ninja's development. The current tools are:
222 `query`:: dump the inputs and outputs of a given target.
224 `browse`:: browse the dependency graph in a web browser. Clicking a
225 file focuses the view on that file, showing inputs and outputs. This
226 feature requires a Python installation.
228 `graph`:: output a file in the syntax used by `graphviz`, a automatic
229 graph layout tool. Use it like:
232 ninja -t graph mytarget | dot -Tpng -ograph.png
235 In the Ninja source tree, `ninja graph.png`
236 generates an image for Ninja itself. If no target is given generate a
237 graph for all root targets.
239 `targets`:: output a list of targets either by rule or by depth. If used
240 like +ninja -t targets rule _name_+ it prints the list of targets
241 using the given rule to be built. If no rule is given, it prints the source
242 files (the leaves of the graph). If used like
243 +ninja -t targets depth _digit_+ it
244 prints the list of targets in a depth-first manner starting by the root
245 targets (the ones with no outputs). Indentation is used to mark dependencies.
246 If the depth is zero it prints all targets. If no arguments are provided
247 +ninja -t targets depth 1+ is assumed. In this mode targets may be listed
248 several times. If used like this +ninja -t targets all+ it
249 prints all the targets available without indentation and it is faster
250 than the _depth_ mode.
252 `commands`:: given a list of targets, print a list of commands which, if
253 executed in order, may be used to rebuild those targets, assuming that all
254 output files are out of date.
256 `clean`:: remove built files. By default it removes all built files
257 except for those created by the generator. Adding the `-g` flag also
258 removes built files created by the generator (see <<ref_rule,the rule
259 reference for the +generator+ attribute>>). Additional arguments are
260 targets, which removes the given targets and recursively all files
263 If used like +ninja -t clean -r _rules_+ it removes all files built using
266 Files created but not referenced in the graph are not removed. This
267 tool takes in account the +-v+ and the +-n+ options (note that +-n+
270 `compdb`:: given a list of rules, each of which is expected to be a
271 C family language compiler rule whose first input is the name of the
272 source file, prints on standard output a compilation database in the
273 http://clang.llvm.org/docs/JSONCompilationDatabase.html[JSON format] expected
274 by the Clang tooling interface.
275 _Available since Ninja 1.2._
278 Writing your own Ninja files
279 ----------------------------
281 The remainder of this manual is only useful if you are constructing
282 Ninja files yourself: for example, if you're writing a meta-build
283 system or supporting a new language.
288 Ninja evaluates a graph of dependencies between files, and runs
289 whichever commands are necessary to make your build target up to date
290 as determined by file modification times. If you are familiar with
291 Make, Ninja is very similar.
293 A build file (default name: `build.ninja`) provides a list of _rules_
294 -- short names for longer commands, like how to run the compiler --
295 along with a list of _build_ statements saying how to build files
296 using the rules -- which rule to apply to which inputs to produce
299 Conceptually, `build` statements describe the dependency graph of your
300 project, while `rule` statements describe how to generate the files
301 along a given edge of the graph.
306 Here's a basic `.ninja` file that demonstrates most of the syntax.
307 It will be used as an example for the following sections.
309 ---------------------------------
313 command = gcc $cflags -c $in -o $out
315 build foo.o: cc foo.c
316 ---------------------------------
320 Despite the non-goal of being convenient to write by hand, to keep
321 build files readable (debuggable), Ninja supports declaring shorter
322 reusable names for strings. A declaration like the following
328 can be used on the right side of an equals sign, dereferencing it with
329 a dollar sign, like this:
333 command = gcc $cflags -c $in -o $out
336 Variables can also be referenced using curly braces like `${in}`.
338 Variables might better be called "bindings", in that a given variable
339 cannot be changed, only shadowed. There is more on how shadowing works
340 later in this document.
345 Rules declare a short name for a command line. They begin with a line
346 consisting of the `rule` keyword and a name for the rule. Then
347 follows an indented set of `variable = value` lines.
349 The basic example above declares a new rule named `cc`, along with the
350 command to run. In the context of a rule, the `command` variable
351 defines the command to run, `$in` expands to the list of
352 input files (`foo.c`), and `$out` to the output files (`foo.o`) for the
353 command. A full list of special variables is provided in
354 <<ref_rule,the reference>>.
359 Build statements declare a relationship between input and output
360 files. They begin with the `build` keyword, and have the format
361 +build _outputs_: _rulename_ _inputs_+. Such a declaration says that
362 all of the output files are derived from the input files. When the
363 output files are missing or when the inputs change, Ninja will run the
364 rule to regenerate the outputs.
366 The basic example above describes how to build `foo.o`, using the `cc`
369 In the scope of a `build` block (including in the evaluation of its
370 associated `rule`), the variable `$in` is the list of inputs and the
371 variable `$out` is the list of outputs.
373 A build statement may be followed by an indented set of `key = value`
374 pairs, much like a rule. These variables will shadow any variables
375 when evaluating the variables in the command. For example:
378 cflags = -Wall -Werror
380 command = gcc $cflags -c $in -o $out
382 # If left unspecified, builds get the outer $cflags.
383 build foo.o: cc foo.c
385 # But you can shadow variables like cflags for a particular build.
386 build special.o: cc special.c
389 # The variable was only shadowed for the scope of special.o;
390 # Subsequent build lines get the outer (original) cflags.
391 build bar.o: cc bar.c
395 For more discussion of how scoping works, consult <<ref_scope,the
398 If you need more complicated information passed from the build
399 statement to the rule (for example, if the rule needs "the file
400 extension of the first input"), pass that through as an extra
401 variable, like how `cflags` is passed above.
403 If the top-level Ninja file is specified as an output of any build
404 statement and it is out of date, Ninja will rebuild and reload it
405 before building the targets requested by the user.
407 Generating Ninja files from code
408 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
410 `misc/ninja_syntax.py` in the Ninja distribution is a tiny Python
411 module to facilitate generating Ninja files. It allows you to make
412 Python calls like `ninja.rule(name='foo', command='bar',
413 depfile='$out.d')` and it will generate the appropriate syntax. Feel
414 free to just inline it into your project's build system if it's
424 The special rule name `phony` can be used to create aliases for other
425 targets. For example:
428 build foo: phony some/file/in/a/faraway/subdir/foo
431 This makes `ninja foo` build the longer path. Semantically, the
432 `phony` rule is equivalent to a plain rule where the `command` does
433 nothing, but phony rules are handled specially in that they aren't
434 printed when run, logged (see below), nor do they contribute to the
435 command count printed as part of the build process.
437 `phony` can also be used to create dummy targets for files which
438 may not exist at build time. If a phony build statement is written
439 without any dependencies, the target will be considered out of date if
440 it does not exist. Without a phony build statement, Ninja will report
441 an error if the file does not exist and is required by the build.
444 Default target statements
445 ~~~~~~~~~~~~~~~~~~~~~~~~~
447 By default, if no targets are specified on the command line, Ninja
448 will build every output that is not named as an input elsewhere.
449 You can override this behavior using a default target statement.
450 A default target statement causes Ninja to build only a given subset
451 of output files if none are specified on the command line.
453 Default target statements begin with the `default` keyword, and have
454 the format +default _targets_+. A default target statement must appear
455 after the build statement that declares the target as an output file.
456 They are cumulative, so multiple statements may be used to extend
457 the list of default targets. For example:
464 This causes Ninja to build the `foo`, `bar` and `baz` targets by
472 For each built file, Ninja keeps a log of the command used to build
473 it. Using this log Ninja can know when an existing output was built
474 with a different command line than the build files specify (i.e., the
475 command line changed) and knows to rebuild the file.
477 The log file is kept in the build root in a file called `.ninja_log`.
478 If you provide a variable named `builddir` in the outermost scope,
479 `.ninja_log` will be kept in that directory instead.
483 Version compatibility
484 ~~~~~~~~~~~~~~~~~~~~~
486 _Available since Ninja 1.2._
488 Ninja version labels follow the standard major.minor.patch format,
489 where the major version is increased on backwards-incompatible
490 syntax/behavioral changes and the minor version is increased on new
491 behaviors. Your `build.ninja` may declare a variable named
492 `ninja_required_version` that asserts the minimum Ninja version
493 required to use the generated file. For example,
496 ninja_required_version = 1.1
499 declares that the build file relies on some feature that was
500 introduced in Ninja 1.1 (perhaps the `pool` syntax), and that
501 Ninja 1.1 or greater must be used to build. Unlike other Ninja
502 variables, this version requirement is checked immediately when
503 the variable is encountered in parsing, so it's best to put it
504 at the top of the build file.
506 Ninja always warns if the major versions of Ninja and the
507 `ninja_required_version` don't match; a major version change hasn't
508 come up yet so it's difficult to predict what behavior might be
512 C/C++ header dependencies
513 ~~~~~~~~~~~~~~~~~~~~~~~~~
515 To get C/C++ header dependencies (or any other build dependency that
516 works in a similar way) correct Ninja has some extra functionality.
518 The problem with headers is that the full list of files that a given
519 source file depends on can only be discovered by the compiler:
520 different preprocessor defines and include paths cause different files
521 to be used. Some compilers can emit this information while building,
522 and Ninja can use that to get its dependencies perfect.
524 Consider: if the file has never been compiled, it must be built anyway,
525 generating the header dependencies as a side effect. If any file is
526 later modified (even in a way that changes which headers it depends
527 on) the modification will cause a rebuild as well, keeping the
528 dependencies up to date.
530 When loading these special dependencies, Ninja implicitly adds extra
531 build edges such that it is not an error if the listed dependency is
532 missing. This allows you to delete a header file and rebuild without
533 the build aborting due to a missing input.
538 `gcc` (and other compilers like `clang`) support emitting dependency
539 information in the syntax of a Makefile. (Any command that can write
540 dependencies in this form can be used, not just `gcc`.)
542 To bring this information into Ninja requires cooperation. On the
543 Ninja side, the `depfile` attribute on the `build` must point to a
544 path where this data is written. (Ninja only supports the limited
545 subset of the Makefile syntax emitted by compilers.) Then the command
546 must know to write dependencies into the `depfile` path.
547 Use it like in the following example:
552 command = gcc -MMD -MF $out.d [other gcc flags here]
555 The `-MMD` flag to `gcc` tells it to output header dependencies, and
556 the `-MF` flag tells it where to write them.
561 _(Available since Ninja 1.3.)_
563 It turns out that for large projects (and particularly on Windows,
564 where the file system is slow) loading these dependency files on
567 Ninja 1.3 can instead process dependencies just after they're generated
568 and save a compacted form of the same information in a Ninja-internal
571 Ninja supports this processing in two forms.
573 1. `deps = gcc` specifies that the tool outputs `gcc`-style dependencies
574 in the form of Makefiles. Adding this to the above example will
575 cause Ninja to process the `depfile` immediately after the
576 compilation finishes, then delete the `.d` file (which is only used
579 2. `deps = msvc` specifies that the tool outputs header dependencies
580 in the form produced by Visual Studio's compiler's
581 http://msdn.microsoft.com/en-us/library/hdkef6tk(v=vs.90).aspx[`/showIncludes`
582 flag]. Briefly, this means the tool outputs specially-formatted lines
583 to its stdout. Ninja then filters these lines from the displayed
584 output. No `depfile` attribute is necessary, but the localized string
585 in front of the the header file path. For instance
586 `msvc_deps_prefix = Note: including file: `
587 for a English Visual Studio (the default). Should be globally defined.
590 msvc_deps_prefix = Note: including file:
593 command = cl /showIncludes -c $in /Fo$out
596 If the include directory directives are using absolute paths, your depfile
597 may result in a mixture of relative and absolute paths. Paths used by other
598 build rules need to match exactly. Therefore, it is recommended to use
599 relative paths in these cases.
605 _Available since Ninja 1.1._
607 Pools allow you to allocate one or more rules or edges a finite number
608 of concurrent jobs which is more tightly restricted than the default
611 This can be useful, for example, to restrict a particular expensive rule
612 (like link steps for huge executables), or to restrict particular build
613 statements which you know perform poorly when run concurrently.
615 Each pool has a `depth` variable which is specified in the build file.
616 The pool is then referred to with the `pool` variable on either a rule
617 or a build statement.
619 No matter what pools you specify, ninja will never run more concurrent jobs
620 than the default parallelism, or the number of jobs specified on the command
624 # No more than 4 links at a time.
628 # No more than 1 heavy object at a time.
629 pool heavy_object_pool
639 # The link_pool is used here. Only 4 links will run concurrently.
640 build foo.exe: link input.obj
642 # A build statement can be exempted from its rule's pool by setting an
643 # empty pool. This effectively puts the build statement back into the default
644 # pool, which has infinite depth.
645 build other.exe: link input.obj
648 # A build statement can specify a pool directly.
649 # Only one of these builds will run at a time.
650 build heavy_object1.obj: cc heavy_obj1.cc
651 pool = heavy_object_pool
652 build heavy_object2.obj: cc heavy_obj2.cc
653 pool = heavy_object_pool
660 _Available since Ninja 1.5._
662 There exists a pre-defined pool named `console` with a depth of 1. It has
663 the special property that any task in the pool has direct access to the
664 standard input, output and error streams provided to Ninja, which are
665 normally connected to the user's console (hence the name) but could be
666 redirected. This can be useful for interactive tasks or long-running tasks
667 which produce status updates on the console (such as test suites).
669 While a task in the `console` pool is running, Ninja's regular output (such
670 as progress status and output from concurrent tasks) is buffered until
676 A file is a series of declarations. A declaration can be one of:
678 1. A rule declaration, which begins with +rule _rulename_+, and
679 then has a series of indented lines defining variables.
681 2. A build edge, which looks like +build _output1_ _output2_:
682 _rulename_ _input1_ _input2_+. +
683 Implicit dependencies may be tacked on the end with +|
684 _dependency1_ _dependency2_+. +
685 Order-only dependencies may be tacked on the end with +||
686 _dependency1_ _dependency2_+. (See <<ref_dependencies,the reference on
689 Implicit outputs _(available since Ninja 1.7)_ may be added before
690 the `:` with +| _output1_ _output2_+ and do not appear in `$out`.
691 (See <<ref_outputs,the reference on output types>>.)
693 3. Variable declarations, which look like +_variable_ = _value_+.
695 4. Default target statements, which look like +default _target1_ _target2_+.
697 5. References to more files, which look like +subninja _path_+ or
698 +include _path_+. The difference between these is explained below
699 <<ref_scope,in the discussion about scoping>>.
701 6. A pool declaration, which looks like +pool _poolname_+. Pools are explained
702 <<ref_pool, in the section on pools>>.
707 Ninja is mostly encoding agnostic, as long as the bytes Ninja cares
708 about (like slashes in paths) are ASCII. This means e.g. UTF-8 or
709 ISO-8859-1 input files ought to work.
711 Comments begin with `#` and extend to the end of the line.
713 Newlines are significant. Statements like `build foo bar` are a set
714 of space-separated tokens that end at the newline. Newlines and
715 spaces within a token must be escaped.
717 There is only one escape character, `$`, and it has the following
720 `$` followed by a newline:: escape the newline (continue the current line
721 across a line break).
723 `$` followed by text:: a variable reference.
725 `${varname}`:: alternate syntax for `$varname`.
727 `$` followed by space:: a space. (This is only necessary in lists of
728 paths, where a space would otherwise separate filenames. See below.)
730 `$:` :: a colon. (This is only necessary in `build` lines, where a colon
731 would otherwise terminate the list of outputs.)
733 `$$`:: a literal `$`.
735 A `build` or `default` statement is first parsed as a space-separated
736 list of filenames and then each name is expanded. This means that
737 spaces within a variable will result in spaces in the expanded
742 build $spaced/baz other$ file: ...
743 # The above build line has two outputs: "foo bar/baz" and "other file".
746 In a `name = value` statement, whitespace at the beginning of a value
747 is always stripped. Whitespace at the beginning of a line after a
748 line continuation is also stripped.
751 two_words_with_one_space = foo $
753 one_word_with_no_space = foo$
757 Other whitespace is only significant if it's at the beginning of a
758 line. If a line is indented more than the previous one, it's
759 considered part of its parent's scope; if it is indented less than the
760 previous one, it closes the previous scope.
766 Two variables are significant when declared in the outermost file scope.
768 `builddir`:: a directory for some Ninja output files. See <<ref_log,the
769 discussion of the build log>>. (You can also store other build output
772 `ninja_required_version`:: the minimum version of Ninja required to process
773 the build correctly. See <<ref_versioning,the discussion of versioning>>.
780 A `rule` block contains a list of `key = value` declarations that
781 affect the processing of the rule. Here is a full list of special
784 `command` (_required_):: the command line to run. Each `rule` may
785 have only one `command` declaration. See <<ref_rule_command,the next
786 section>> for more details on quoting and executing multiple commands.
788 `depfile`:: path to an optional `Makefile` that contains extra
789 _implicit dependencies_ (see <<ref_dependencies,the reference on
790 dependency types>>). This is explicitly to support C/C++ header
791 dependencies; see <<ref_headers,the full discussion>>.
793 `deps`:: _(Available since Ninja 1.3.)_ if present, must be one of
794 `gcc` or `msvc` to specify special dependency processing. See
795 <<ref_headers,the full discussion>>. The generated database is
796 stored as `.ninja_deps` in the `builddir`, see <<ref_toplevel,the
797 discussion of `builddir`>>.
799 `msvc_deps_prefix`:: _(Available since Ninja 1.5.)_ defines the string
800 which should be stripped from msvc's /showIncludes output. Only
801 needed when `deps = msvc` and no English Visual Studio version is used.
803 `description`:: a short description of the command, used to pretty-print
804 the command as it's running. The `-v` flag controls whether to print
805 the full command or its description; if a command fails, the full command
806 line will always be printed before the command's output.
808 `generator`:: if present, specifies that this rule is used to
809 re-invoke the generator program. Files built using `generator`
810 rules are treated specially in two ways: firstly, they will not be
811 rebuilt if the command line changes; and secondly, they are not
814 `in`:: the space-separated list of files provided as inputs to the build line
815 referencing this `rule`, shell-quoted if it appears in commands. (`$in` is
816 provided solely for convenience; if you need some subset or variant of this
817 list of files, just construct a new variable with that list and use
820 `in_newline`:: the same as `$in` except that multiple inputs are
821 separated by newlines rather than spaces. (For use with
822 `$rspfile_content`; this works around a bug in the MSVC linker where
823 it uses a fixed-size buffer for processing input.)
825 `out`:: the space-separated list of files provided as outputs to the build line
826 referencing this `rule`, shell-quoted if it appears in commands.
828 `restat`:: if present, causes Ninja to re-stat the command's outputs
829 after execution of the command. Each output whose modification time
830 the command did not change will be treated as though it had never
831 needed to be built. This may cause the output's reverse
832 dependencies to be removed from the list of pending build actions.
834 `rspfile`, `rspfile_content`:: if present (both), Ninja will use a
835 response file for the given command, i.e. write the selected string
836 (`rspfile_content`) to the given file (`rspfile`) before calling the
837 command and delete the file after successful execution of the
840 This is particularly useful on Windows OS, where the maximal length of
841 a command line is limited and response files must be used instead.
843 Use it like in the following example:
847 command = link.exe /OUT$out [usual link flags here] @$out.rsp
849 rspfile_content = $in
851 build myapp.exe: link a.obj b.obj [possibly many other .obj files]
855 Interpretation of the `command` variable
856 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
857 Fundamentally, command lines behave differently on Unixes and Windows.
859 On Unixes, commands are arrays of arguments. The Ninja `command`
860 variable is passed directly to `sh -c`, which is then responsible for
861 interpreting that string into an argv array. Therefore the quoting
862 rules are those of the shell, and you can use all the normal shell
863 operators, like `&&` to chain multiple commands, or `VAR=value cmd` to
864 set environment variables.
866 On Windows, commands are strings, so Ninja passes the `command` string
867 directly to `CreateProcess`. (In the common case of simply executing
868 a compiler this means there is less overhead.) Consequently the
869 quoting rules are deterimined by the called program, which on Windows
870 are usually provided by the C library. If you need shell
871 interpretation of the command (such as the use of `&&` to chain
872 multiple commands), make the command execute the Windows shell by
873 prefixing the command with `cmd /c`.
879 There are two types of build outputs which are subtly different.
881 1. _Explicit outputs_, as listed in a build line. These are
882 available as the `$out` variable in the rule.
884 This is the standard form of output to be used for e.g. the
885 object file of a compile command.
887 2. _Implicit outputs_, as listed in a build line with the syntax +|
888 _out1_ _out2_+ + before the `:` of a build line _(available since
889 Ninja 1.7)_. The semantics are identical to explicit outputs,
890 the only difference is that implicit outputs don't show up in the
893 This is for expressing outputs that don't show up on the
894 command line of the command.
900 There are three types of build dependencies which are subtly different.
902 1. _Explicit dependencies_, as listed in a build line. These are
903 available as the `$in` variable in the rule. Changes in these files
904 cause the output to be rebuilt; if these file are missing and
905 Ninja doesn't know how to build them, the build is aborted.
907 This is the standard form of dependency to be used e.g. for the
908 source file of a compile command.
910 2. _Implicit dependencies_, either as picked up from
911 a `depfile` attribute on a rule or from the syntax +| _dep1_
912 _dep2_+ on the end of a build line. The semantics are identical to
913 explicit dependencies, the only difference is that implicit dependencies
914 don't show up in the `$in` variable.
916 This is for expressing dependencies that don't show up on the
917 command line of the command; for example, for a rule that runs a
918 script, the script itself should be an implicit dependency, as
919 changes to the script should cause the output to rebuild.
921 Note that dependencies as loaded through depfiles have slightly different
922 semantics, as described in the <<ref_rule,rule reference>>.
924 3. _Order-only dependencies_, expressed with the syntax +|| _dep1_
925 _dep2_+ on the end of a build line. When these are out of date, the
926 output is not rebuilt until they are built, but changes in order-only
927 dependencies alone do not cause the output to be rebuilt.
929 Order-only dependencies can be useful for bootstrapping dependencies
930 that are only discovered during build time: for example, to generate a
931 header file before starting a subsequent compilation step. (Once the
932 header is used in compilation, a generated dependency file will then
933 express the implicit dependency.)
935 File paths are compared as is, which means that an absolute path and a
936 relative path, pointing to the same file, are considered different by Ninja.
941 Variables are expanded in paths (in a `build` or `default` statement)
942 and on the right side of a `name = value` statement.
944 When a `name = value` statement is evaluated, its right-hand side is
945 expanded immediately (according to the below scoping rules), and
946 from then on `$name` expands to the static string as the result of the
947 expansion. It is never the case that you'll need to "double-escape" a
948 value to prevent it from getting expanded twice.
950 All variables are expanded immediately as they're encountered in parsing,
951 with one important exception: variables in `rule` blocks are expanded
952 when the rule is _used_, not when it is declared. In the following
953 example, the `demo` rule prints "this is a demo of bar".
957 command = echo "this is a demo of $foo"
964 Evaluation and scoping
965 ~~~~~~~~~~~~~~~~~~~~~~
967 Top-level variable declarations are scoped to the file they occur in.
969 Rule declarations are also scoped to the file they occur in.
970 _(Available since Ninja 1.6)_
972 The `subninja` keyword, used to include another `.ninja` file,
973 introduces a new scope. The included `subninja` file may use the
974 variables and rules from the parent file, and shadow their values for the file's
975 scope, but it won't affect values of the variables in the parent.
977 To include another `.ninja` file in the current scope, much like a C
978 `#include` statement, use `include` instead of `subninja`.
980 Variable declarations indented in a `build` block are scoped to the
981 `build` block. The full lookup order for a variable expanded in a
982 `build` block (or the `rule` is uses) is:
984 1. Special built-in variables (`$in`, `$out`).
986 2. Build-level variables from the `build` block.
988 3. Rule-level variables from the `rule` block (i.e. `$command`).
989 (Note from the above discussion on expansion that these are
990 expanded "late", and may make use of in-scope bindings like `$in`.)
992 4. File-level variables from the file that the `build` line was in.
994 5. Variables from the file that included that file using the