Ninja ===== Evan Martin 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) would 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 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. * 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.) * Ninja has special support for discovering extra dependencies at build time, making it easy to get <> 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 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. 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. 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 and related projects (v8, node.js). gyp can generate Ninja files for all platforms supported by Chrome. See the http://code.google.com/p/chromium/wiki/NinjaBuild[Chromium Ninja documentation for more details]. 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.) 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) `%e`:: Elapsed time in seconds. _(Available since Ninja 1.2.)_ `%%`:: 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 <>). 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 as determined by file modification times. 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 <>. 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 $cflags. build foo.o: cc foo.c # But you can shadow variables like cflags for a particular build. build special.o: cc special.c cflags = -Wall # The variable was only shadowed for the scope of special.o; # Subsequent build lines get the outer (original) cflags. build bar.o: cc bar.c ---------------- For more discussion of how scoping works, consult <>. If you need more complicated information passed from the build statement to the rule (for example, if the rule needs "the file extension of the first input"), pass that through as an extra variable, like how `cflags` is passed above. If the top-level Ninja file is specified as an output of any build statement and it is out of date, Ninja will rebuild and reload it before building the targets requested by the user. Generating Ninja files from code ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ `misc/ninja_syntax.py` in the Ninja distribution is a tiny Python module to facilitate generating Ninja files. It allows you to make Python calls like `ninja.rule(name='foo', command='bar', depfile='$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 1.2._ 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. [[ref_headers]] C/C++ header dependencies ~~~~~~~~~~~~~~~~~~~~~~~~~ To get C/C++ header dependencies (or any other build dependency that works in a similar way) correct Ninja has some extra functionality. 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. 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. 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. depfile ^^^^^^^ `gcc` (and other compilers like `clang`) support emitting dependency information in the syntax of a Makefile. (Any command that can write dependencies in this form can be used, not just `gcc`.) To bring this information into Ninja requires cooperation. On the Ninja side, the `depfile` attribute on the `build` 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 `depfile` path. Use it like in the following example: ---- rule cc depfile = $out.d command = gcc -MMD -MF $out.d [other gcc flags here] ---- The `-MMD` flag to `gcc` tells it to output header dependencies, and the `-MF` flag tells it where to write them. deps ^^^^ _(Available since Ninja 1.3.)_ 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. 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. Ninja supports this processing in two forms. 1. `deps = gcc` specifies that the tool outputs `gcc`-style dependencies in the form of Makefiles. Adding this to the above example will cause Ninja to process the `depfile` immediately after the compilation finishes, then delete the `.d` file (which is only used as a temporary). 2. `deps = msvc` specifies that the tool outputs header dependencies in the form produced by Visual Studio's compiler's http://msdn.microsoft.com/en-us/library/hdkef6tk(v=vs.90).aspx[`/showIncludes` flag]. Briefly, this means the tool outputs specially-formatted lines to its stdout. Ninja then filters these lines from the displayed output. No `depfile` 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. + ---- msvc_deps_prefix = Note: including file: rule cc deps = msvc command = cl /showIncludes -c $in /Fo$out ---- [[ref_pool]] 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 ---------------- The `console` pool ^^^^^^^^^^^^^^^^^^ _Available since Ninja 1.5._ There exists a pre-defined pool named `console` 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). While a task in the `console` pool is running, Ninja's regular output (such as progress status and output from concurrent tasks) is buffered until it completes. 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 <>.) 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 <>. 6. A pool declaration, which looks like +pool _poolname_+. Pools are explained <>. 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. [[ref_toplevel]] Top-level variables ~~~~~~~~~~~~~~~~~~~ Two variables are significant when declared in the outermost file scope. `builddir`:: a directory for some Ninja output files. See <>. (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_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 $variables are expanded) is passed directly to `sh -c` without interpretation by Ninja. Each `rule` may have only one `command` declaration. To specify multiple commands use `&&` (or similar) to concatenate operations. `depfile`:: path to an optional `Makefile` that contains extra _implicit dependencies_ (see <>). This is explicitly to support C/C++ header dependencies; see <>. `deps`:: _(Available since Ninja 1.3.)_ if present, must be one of `gcc` or `msvc` to specify special dependency processing. See <>. The generated database is stored as `.ninja_deps` in the `builddir`, see <>. `msvc_deps_prefix`:: _(Available since Ninja 1.5.)_ defines the string which should be stripped from msvc's /showIncludes output. Only needed when `deps = msvc` and no English Visual Studio version is used. `description`:: a short description of the command, used to pretty-print the command as it's running. The `-v` flag controls whether to print the full command or its description; if a command fails, the full command line will always be printed before the command's output. `generator`:: if present, specifies that this rule is used to re-invoke the generator program. Files built using `generator` rules are treated specially in two ways: firstly, they will not be rebuilt if the command line changes; and secondly, they are not cleaned by default. `in`:: the shell-quoted space-separated list of files provided as inputs to the build line referencing this `rule`. (`$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 `$in` variable. + This is for expressing dependencies that don't show up on the command line of the command; for example, for a rule that runs a script, the script itself should be an implicit dependency, as changes to the script should cause the output to rebuild. + Note that dependencies as loaded through depfiles have slightly different semantics, as described in the <>. 3. _Order-only dependencies_, expressed with the syntax +|| _dep1_ _dep2_+ on the end of a build line. When these are out of date, the output is not rebuilt until they are built, but changes in order-only dependencies alone do not cause the output to be rebuilt. + Order-only dependencies can be useful for bootstrapping dependencies that are only discovered during build time: for example, to generate a header file before starting a subsequent compilation step. (Once the header is used in compilation, a generated dependency file will then express the implicit dependency.) Variable expansion ~~~~~~~~~~~~~~~~~~ Variables are expanded in paths (in a `build` or `default` statement) and on the right side of a `name = value` statement. When a `name = value` statement is evaluated, its right-hand side is expanded immediately (according to the below scoping rules), and from then on `$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.