The Ninja build system
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 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.
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
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
-Mflags 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
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 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.oinstead 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 the tup build system influential in Ninja’s design, and thinks redo's design is quite clever.
Ninja’s benefit comes from using it in conjunction with a smarter meta-build system.
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 Chromium Ninja documentation for more details.
A widely used meta-build system that can generate Ninja files on Linux as of CMake version 2.8.8. Newer versions of CMake support generating Ninja files on Windows and Mac OS X too.
Ninja ought to fit perfectly into other meta-build software like premake. If you do this work, please let us know!
ninja. By default, it looks for a file named
the current directory and builds all out-of-date targets. You can
specify which targets (files) to build as command line arguments.
There is also a special syntax
target^ for specifying a target
as the first output of some rule containing the source you put in
the command line, if one exists. For example, if you specify target as
foo.o will get built (assuming you have those targets
in your build files).
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
Ninja supports one environment variable to control its behavior:
NINJA_STATUS, the progress status printed before the rule being run.
Several placeholders are available:
The number of started edges.
The total number of edges that must be run to complete the build.
The percentage of started edges.
The number of currently running edges.
The number of remaining edges to start.
The number of finished edges.
Overall rate of finished edges per second
Current rate of finished edges per second (average over builds specified by
-jor its default)
Elapsed time in seconds. (Available since Ninja 1.2.)
The default progress status is
"[%f/%t] " (note the trailing space
to separate from the build rule). Another example of possible progress status
-t flag on the Ninja command line runs some tools that we have
found useful during Ninja’s development. The current tools are:
dump the inputs and outputs of a given target.
browse the dependency graph in a web browser. Clicking a file focuses the view on that file, showing inputs and outputs. This feature requires a Python installation. By default port 8000 is used and a web browser will be opened. This can be changed as follows:
ninja -t browse --port=8000 --no-browser mytarget
output a file in the syntax used by
ninja -t graph mytarget | dot -Tpng -ograph.png
In the Ninja source tree,
output a list of targets either by rule or by depth. If used
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.
remove built files. By default it removes all built files
except for those created by the generator. Adding the
If used like
Files created but not referenced in the graph are not removed. This
tool takes in account the
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 JSON format expected by the Clang tooling interface. Available since Ninja 1.2.
show all dependencies stored in the
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.
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
build statements describe the dependency graph of your
rule statements describe how to generate the files
along a given edge of the graph.
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
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
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 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
defines the command to run,
$in expands to the list of
input files (
$out to the output files (
foo.o) for the
command. A full list of special variables is provided in
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
In the scope of a
build block (including in the evaluation of its
rule), the variable
$in is the list of inputs and the
$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 $cflags. build foo.o: cc foo.c # But you can shadow variables like cflags for a particular build. build special.o: cc special.c cflags = -Wall # The variable was only shadowed for the scope of special.o; # Subsequent build lines get the outer (original) cflags. build bar.o: cc bar.c
For more discussion of how scoping works, consult the reference.
If you need more complicated information passed from the build
statement to the rule (for example, if the rule needs "the file
extension of the first input"), pass that through as an extra
variable, like how
cflags is passed above.
If the top-level Ninja file is specified as an output of any build statement and it is out of date, Ninja will rebuild and reload it before building the targets requested by the user.
Generating Ninja files from code
misc/ninja_syntax.py in the Ninja distribution is a tiny Python
module to facilitate generating Ninja files. It allows you to make
Python calls like
depfile='$out.d') and it will generate the appropriate syntax. Feel
free to just inline it into your project’s build system if it’s
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
ninja foo build the longer path. Semantically, the
phony rule is equivalent to a plain rule where the
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
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
baz targets by
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
If you provide a variable named
builddir in the outermost scope,
.ninja_log will be kept in that directory instead.
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
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
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.
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
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
Use it like in the following example:
rule cc depfile = $out.d command = gcc -MMD -MF $out.d [other gcc flags here]
-MMD flag to
gcc tells it to output header dependencies, and
-MF flag tells it where to write them.
(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.
deps = gccspecifies that the tool outputs
gcc-style dependencies in the form of Makefiles. Adding this to the above example will cause Ninja to process the
depfileimmediately after the compilation finishes, then delete the
.dfile (which is only used as a temporary).
deps = msvcspecifies that the tool outputs header dependencies in the form produced by Visual Studio’s compiler’s
/showIncludesflag. Briefly, this means the tool outputs specially-formatted lines to its stdout. Ninja then filters these lines from the displayed output. No
depfileattribute 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
If the include directory directives are using absolute paths, your depfile may result in a mixture of relative and absolute paths. Paths used by other build rules need to match exactly. Therefore, it is recommended to use relative paths in these cases.
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
# 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
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
Ninja file reference
A file is a series of declarations. A declaration can be one of:
A rule declaration, which begins with
rule rulename, and then has a series of indented lines defining variables.
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 the reference on dependency types.)
Implicit outputs (available since Ninja 1.7) may be added before the
| output1 output2and do not appear in
$out. (See the reference on output types.)
Variable declarations, which look like
variable = value.
Default target statements, which look like
default target1 target2.
References to more files, which look like
include path. The difference between these is explained below in the discussion about scoping.
A pool declaration, which looks like
pool poolname. Pools are explained in the section on pools.
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.
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
$followed by a newline
escape the newline (continue the current line across a line break).
$followed by text
a variable reference.
alternate syntax for
$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
buildlines, where a colon would otherwise terminate the list of outputs.)
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
spaced = foo bar build $spaced/baz other$ file: ... # The above build line has two outputs: "foo bar/baz" and "other file".
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.
Two variables are significant when declared in the outermost file scope.
rule block contains a list of
key = value declarations that
affect the processing of the rule. Here is a full list of special
the command line to run. Each
rulemay have only one
commanddeclaration. See the next section for more details on quoting and executing multiple commands.
(Available since Ninja 1.3.) if present, must be one of
msvcto specify special dependency processing. See the full discussion. The generated database is stored as
builddir, see the discussion of
(Available since Ninja 1.5.) defines the string which should be stripped from msvc’s /showIncludes output. Only needed when
deps = msvcand no English Visual Studio version is used.
a short description of the command, used to pretty-print the command as it’s running. The
-vflag 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.
if present, specifies that this rule is used to re-invoke the generator program. Files built using
generatorrules 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.
the space-separated list of files provided as inputs to the build line referencing this
rule, shell-quoted if it appears in commands. (
$inis 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.)
the same as
$inexcept that multiple inputs are separated by newlines rather than spaces. (For use with
$rspfile_content; this works around a bug in the MSVC linker where it uses a fixed-size buffer for processing input.)
the space-separated list of files provided as outputs to the build line referencing this
rule, shell-quoted if it appears in commands.
if present, causes Ninja to re-stat the command’s outputs after execution of the command. Each output whose modification time the command did not change will be treated as though it had never needed to be built. This may cause the output’s reverse dependencies to be removed from the list of pending build actions.
if present (both), Ninja will use a response file for the given command, i.e. write the selected string (
rspfile_content) to the given file (
rspfile) before calling the command and delete the file after successful execution of the command.
This is particularly useful on Windows OS, where the maximal length of a command line is limited and response files must be used instead.
Use it like in the following example:
rule link command = link.exe /OUT$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]
Interpretation of the
Fundamentally, command lines behave differently on Unixes and Windows.
On Unixes, commands are arrays of arguments. The Ninja
variable is passed directly to
sh -c, which is then responsible for
interpreting that string into an argv array. Therefore the quoting
rules are those of the shell, and you can use all the normal shell
&& to chain multiple commands, or
VAR=value cmd to
set environment variables.
On Windows, commands are strings, so Ninja passes the
CreateProcess. (In the common case of simply executing
a compiler this means there is less overhead.) Consequently the
quoting rules are deterimined by the called program, which on Windows
are usually provided by the C library. If you need shell
interpretation of the command (such as the use of
&& to chain
multiple commands), make the command execute the Windows shell by
prefixing the command with
There are two types of build outputs which are subtly different.
Explicit outputs, as listed in a build line. These are available as the
$outvariable in the rule.
This is the standard form of output to be used for e.g. the object file of a compile command.
Implicit outputs, as listed in a build line with the syntax
| out1 out2+ before the
:of a build line (available since Ninja 1.7). The semantics are identical to explicit outputs, the only difference is that implicit outputs don’t show up in the
This is for expressing outputs that don’t show up on the command line of the command.
There are three types of build dependencies which are subtly different.
Explicit dependencies, as listed in a build line. These are available as the
$invariable 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 e.g. for the source file of a compile command.
Implicit dependencies, either as picked up from a
depfileattribute on a rule or from the syntax
| dep1 dep2on 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
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 rule reference.
Order-only dependencies, expressed with the syntax
|| dep1 dep2on 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.)
File paths are compared as is, which means that an absolute path and a relative path, pointing to the same file, are considered different by Ninja.
Variables are expanded in paths (in a
and on the right side of a
name = value statement.
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
demo rule prints "this is a demo of bar".
rule demo command = echo "this is a demo of $foo" build out: demo foo = bar
Evaluation and scoping
Top-level variable declarations are scoped to the file they occur in.
Rule declarations are also scoped to the file they occur in. (Available since Ninja 1.6)
subninja keyword, used to include another
introduces a new scope. The included
subninja file may use the
variables and rules from the parent file, and shadow their values for the file’s
scope, but it won’t affect values of the variables in the parent.
To include another
.ninja file in the current scope, much like a C
#include statement, use
include instead of
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:
Special built-in variables (
Build-level variables from the
Rule-level variables from the
$command). (Note from the above discussion on expansion that these are expanded "late", and may make use of in-scope bindings like
File-level variables from the file that the
buildline was in.
Variables from the file that included that file using the