Rscons is a software construction framework inspired by SCons and implemented in Ruby.

Gem Version


$ gem install rscons



Rscons provides a standalone executable ("rscons") with a command-line interface. The rscons executable will read a build script (by default named Rsconsfile or Rsconsfile.rb) and execute its contents.

With Rake

Rscons can be used with rake as well. The same content that would be written in Rsconsfile can be placed in a Rakefile. It could be placed within a rake task block or split among multiple tasks.

Example Build Scripts

Example: Building a C Program do |env|
  env["CFLAGS"] << "-Wall"
  env.Program("program", Dir["src/**/*.c"])

Example: Building a D Program do |env|
  env["DFLAGS"] << "-Wall"
  env.Program("program", Dir["src/**/*.d"])

Example: Cloning an Environment

main_env = do |env|
  # Store object files from sources under "src" in "build/main"
  env.build_dir("src", "build/main")
  env["CFLAGS"] = ["-DSOME_DEFINE", "-O3"]
  env["LIBS"] = ["SDL"]
  env.Program("program", Dir["src/**/*.cc"])

debug_env = main_env.clone do |env|
  # Store object files from sources under "src" in "build/debug"
  env.build_dir("src", "build/debug")
  env["CFLAGS"] -= ["-O3"]
  env["CFLAGS"] += ["-g", "-O0"]
  env.Program("program-debug", Dir["src/**/*.cc"])

Example: Custom Builder

Custom builders are implemented as classes which extend from Rscons::Builder. The builder must have a run method which is called to invoke the builder. The run method should return the name of the target built on success, and false on failure.

class GenerateFoo < Rscons::Builder
  def run(target, sources, cache, env, vars)
    cache.mkdir_p(File.dirname(target)), "w") do |fh|
      fh.puts <<EOF
#define GENERATED 42
end do |env|
  env.GenerateFoo("foo.h", [])
  env.Program("a.out", Dir["*.c"])

Example: Custom Builder That Only Regenerates When Necessary

class CmdBuilder < Rscons::Builder
  def run(target, sources, cache, env, vars)
    cmd = ["cmd", "-i", sources.first, "-o", target]
    unless cache.up_to_date?(target, cmd, sources, env)
      cache.register_build(target, cmd, sources, env)
end do |env|
  env.CmdBuilder("foo.gen", "foo_gen.cfg")

Example: Custom Builder That Generates Multiple Output Files

class CModuleGenerator < Rscons::Builder
  def run(target, sources, cache, env, vars)
    c_fname = target
    h_fname = target.sub(/\.c$/, ".h")
    cmd = ["generate_c_and_h", sources.first, c_fname, h_fname]
    unless cache.up_to_date?([c_fname, h_fname], cmd, sources, env)
      cache.register_build([c_fname, h_fname], cmd, sources, env)
end do |env|
  env.CModuleGenerator("build/foo.c", "foo_gen.cfg")

Example: Custom Builder Using Builder#standard_build()

The standard_build method from the Rscons::Builder base class can be used when the builder needs to execute a system command to produce the target file. The standard_build method will return the correct value so its return value can be used as the return value from the run method.

class CmdBuilder < Rscons::Builder
  def run(target, sources, cache, env, vars)
    cmd = ["cmd", "-i", sources.first, "-o", target]
    standard_build("CmdBld #{target}", target, cmd, sources, env, cache)
end do |env|
  env.CmdBuilder("foo.gen", "foo_gen.cfg")

Example: Custom Builder Using Environment#add_builder()

The add_builder method of the Rscons::Environment class optionally allows you to define and register a builder by providing a name and action block. This can be useful if the builder you are trying to define is easily expressed as a short ruby procedure. When add_builder is called in this manner a new builder will be registered with the environment with the given name. When this builder is used it will call the provided block in order to build the target. do |env|
  env.add_builder(:JsonToYaml) do |target, sources, cache, env, vars|
    unless cache.up_to_date?(target, :JsonToYaml, sources, env)
      cache.mkdir_p(File.dirname(target)), 'w') do |f|
      cache.register_build(target, :JsonToYaml, sources, env)

Example: Using different compilation flags for some sources do |env|
  env["CFLAGS"] = ["-O3", "-Wall", "-DDEFINE"]
  env.add_build_hook do |build_op|
    if build_op[:target] =~ %r{build/third-party}
      build_op[:vars]["CFLAGS"] -= ["-Wall"]
  env.build_dir("src", "build")
  env.Program("program", Dir["**/*.cc"])

Example: Creating a static library do |env|
  env.Library("mylib.a", Dir["src/**/*.c"])

Example: Creating a C++ parser source from a Yacc/Bison input file do |env|
  env.CFile("#{env.build_root}/", "parser.yy")



The Environment is the main top-level object that Rscons operates with. An Environment must be created by the user in order to build anything. All build targets are registered within an Environment. In many cases only a single Environment will be needed, but more than one can be created (either from scratch or by cloning another existing Environment) if needed.

An Environment consists of:

  • a collection of builders
  • a collection of construction variables used by those builders
  • a mapping of build directories from source directories
  • a default build root to apply if no specific build directories are matched
  • a collection of user-defined build targets
  • a collection of user-defined build hooks

When cloning an environment, by default the construction variables, builders, build hooks, build directories, and build root are cloned, but the new environment does not inherit any of the registered build targets.

The set of environment attributes that are cloned is controllable via the :clone option to the #clone method. For example, env.clone(clone: [:variables, :builders]) will include construction variables, and builders but not build hooks, build directories, or the build root.

The set of pending targets is never cloned.

Cloned environments contain "deep copies" of construction variables. For example, in:

base_env =
base_env["CPPPATH"] = ["one", "two"]
cloned_env = base_env.clone
cloned_env["CPPPATH"] << "three"

base_env["CPPPATH"] will not include "three".


Builders are the workhorses that Rscons uses to execute build operations. Each builder is specialized to perform a particular operation.

Rscons ships with a number of builders:

  • Command, which executes a user-defined command to produce the target.
  • Copy, which is identical to Install.
  • CFile, which builds a C or C++ source file from a lex or yacc input file.
  • Disassemble, which disassembles an object file to a disassembly listing.
  • Install, which installs files or directories to a specified destination.
  • Library, which collects object files into a static library archive file.
  • Object, which compiles source files to produce an object file.
  • Preprocess, which invokes the C/C++ preprocessor on a source file.
  • Program, which links object files to produce an executable.
  • SharedLibrary, which links object files to produce a dynamically loadable library.
  • SharedObject, which compiles source files to produce an object file, in a way that is able to be used to create a shared library.

If you want to create an Environment that does not contain any builders, you can use the :exclude_builders key to the Environment constructor.


env.Command(target, sources, "CMD" => command)
# Example
env.Command("docs.html", "",
    "CMD" => ["pandoc", "-fmarkdown", "-thtml", "-o${_TARGET}", "${_SOURCES}"],
    "CMD_DESC" => "PANDOC")

The command builder executes a user-defined command in order to produce the desired target file based on the provided source files.


env.CFile(target, source)
# Example
env.CFile("parser.c", "parser.y")

The CFile builder will generate a C or C++ source file from a lex (.l, .ll) or yacc (.y, .yy) input file.


env.Disassemble(target, source)
# Example
env.Disassemble("module.dis", "module.o")

The Disassemble builder generates a disassembly listing using objdump from and object file.


env.Install(destination, sources)
# Example
env.Install("dist/bin", "app.exe")
env.Install("dist/share", "share")


env.Library(target, sources)
# Example
env.Library("lib.a", Dir["src/**/*.c"])

The Library builder creates a static library archive from the given source files.


env.Object(target, sources)
# Example
env.Object("module.o", "module.c")

The Object builder compiles the given sources to an object file. Although it can be called explicitly, it is more commonly implicitly called by the Program builder.


env.Preprocess(target, source)
# Example
env.Preprocess("", "")

The Preprocess builder invokes either ${CC} or ${CXX} (depending on if the source contains an extension in ${CXXSUFFIX} or not) and writes the preprocessed output to the target file.


env.Program(target, sources)
# Example
env.Program("myprog", Dir["src/**/*.cc"])

The Program builder compiles and links the given sources to an executable file. Object files or source files can be given as sources. A platform-dependent program suffix will be appended to the target name if one is not specified. This can be controlled with the PROGSUFFIX construction variable.


env.SharedLibrary(target, sources)
# Example
env.SharedLibrary("mydll", Dir["src/**/*.cc"])

The SharedLibrary builder compiles and links the given sources to a dynamically loadable library. Object files or source files can be given as sources. A platform-dependent prefix and suffix will be appended to the target name if they are not specified by the user. These values can be controlled by overriding the SHLIBPREFIX and SHLIBSUFFIX construction variables.


env.SharedObject(target, sources)
# Example
env.SharedObject("lib_module.o", "lib_module.c")

The SharedObject builder compiles the given sources to an object file. Any compilation flags necessary to build the object file in a manner that allows it to be used to create a shared library are added. Although it can be called explicitly, it is more commonly implicitly called by the SharedLibrary builder.

Construction Variables

Construction variables are used to define the toolset and any build options that Rscons will use to build a project. The default construction variable values are configured to build applications using gcc. However, all construction variables can be overridden by the user.

Name Type Description Default
AR String Static library archiver executable "ar"
ARCMD Array Static library archiver command line ["$AR", "$ARFLAGS", "$_TARGET", "$_SOURCES"]
ARFLAGS Array Static library archiver flags ["rcs"]
AS String Assembler executable "$CC"
ASCMD Array Assembler command line ["$AS", "-c", "-o", "$_TARGET", "$ASDEPGEN", "$INCPREFIX$ASPPPATH", "$ASPPFLAGS", "$ASFLAGS", "$_SOURCES"]
ASDEPGEN Array Assembly dependency generation flags ["-MMD", "-MF", "$_DEPFILE"]
ASFLAGS Array Assembler flags []
ASPPFLAGS Array Assembler preprocessor flags ["$CPPFLAGS"]
ASPPPATH Array Assembler preprocessor path ["$CPPPATH"]
ASSUFFIX Array Assembly file suffixes [".S"]
CC String C compiler executable "gcc"
CCCMD Array C compiler command line ["$CC", "-c", "-o", "$_TARGET", "$CCDEPGEN", "$INCPREFIX$CPPPATH", "$CPPFLAGS", "$CFLAGS", "$CCFLAGS", "$_SOURCES"]
CCDEPGEN Array C compiler dependency generation flags ["-MMD", "-MF", "$_DEPFILE"]
CCFLAGS Array Common flags for both C and C++ compiler []
CFLAGS Array C compiler flags []
CPP_CMD Array Preprocess command line ["$_PREPROCESS_CC", "-E", "$_PREPROCESS_DEPGEN", "-o", "$_TARGET", "$INCPREFIX$CPPPATH", "$CPPFLAGS", "$_SOURCES"]
CPP_TARGET_SUFFIX String Suffix used for crt:preprocess target filename. ".c"
CPPDEFINES Array C preprocessor defines []
CPPDEFPREFIX String Prefix used for C preprocessor to introduce a define "-D"
CPPFLAGS Array C preprocessor flags ["$CPPDEFPREFIX$CPPDEFINES"]
CPPPATH Array C preprocessor path []
CSUFFIX Array C source file suffixes [".c"]
CXX String C++ compiler executable "g++"
CXXCMD Array C++ compiler command line ["$CXX", "-c", "-o", "$_TARGET", "$CXXDEPGEN", "$INCPREFIX$CPPPATH", "$CPPFLAGS", "$CXXFLAGS", "$CCFLAGS", "$_SOURCES"]
CXXDEPGEN Array C++ compiler dependency generation flags ["-MMD", "-MF", "$_DEPFILE"]
CXXFLAGS Array C++ compiler flags []
CXXSUFFIX Array C++ source file suffixes [".cc", ".cpp", ".cxx", ".C"]
D_IMPORT_PATH Array D compiler import path []
DC String D compiler executable "gdc"
DCCMD Array D compiler command line ["$DC", "-c", "-o", "$_TARGET", "$INCPREFIX$D_IMPORT_PATH", "$DFLAGS", "$_SOURCES"]
DEPFILESUFFIX String Dependency file suffix for Makefile-style dependency rules emitted by the compiler (used internally for temporary dependency files used to determine a source file's dependencies) ".mf"
DFLAGS Array D compiler flags []
DISASM_CMD Array Disassemble command line ["$OBJDUMP", "$DISASM_FLAGS", "$_SOURCES"]
DISASM_FLAGS Array Disassemble flags ["--disassemble", "--source"]
DSUFFIX String/Array Default D source file suffix ".d"
INCPREFIX String Prefix used for C preprocessor to add an include path "-I"
LD String nil Linker executable (automatically determined when nil)
LDFLAGS Array Linker flags []
LEX String Lex executable "flex"
LEX_CMD Array Lex command line ["$LEX", "$LEX_FLAGS", "-o", "$_TARGET", "$_SOURCES"]
LEX_FLAGS Array Lex flags []
LEXSUFFIX Array Lex input file suffixes [".l", ".ll"]
LIBDIRPREFIX String Prefix given to linker to add a library search path "-L"
LIBLINKPREFIX String Prefix given to linker to add a library to link with "-l"
LIBPATH Array Library load path []
LIBS Array Libraries to link with []
LIBSUFFIX String/Array Default static library file suffix ".a"
OBJDUMP String Objdump executable "objdump"
OBJSUFFIX String/Array Default object file suffix ".o"
PROGSUFFIX String Default program suffix. Windows: ".exe", POSIX: ""
SHCC String Shared object C compiler "$CC"
SHCCCMD Array Shared object C compiler command line ["$SHCC", "-c", "-o", "$_TARGET", "$CCDEPGEN", "$INCPREFIX$CPPPATH", "$CPPFLAGS", "$SHCFLAGS", "$SHCCFLAGS", "$_SOURCES"]
SHCCFLAGS Array Shared object C and C++ compiler flags Windows: ["$CCFLAGS"], POSIX: ["$CCFLAGS", -fPIC"]
SHCFLAGS Array Shared object C compiler flags ["$CFLAGS"]
SHCXX String Shared object C++ compiler "$CXX"
SHCXXCMD Array Shared object C++ compiler command line ["$SHCXX", "-c", "-o", "$_TARGET", "$CXXDEPGEN", "$INCPREFIX$CPPPATH", "$CPPFLAGS", "$SHCXXFLAGS", "$SHCCFLAGS", "$_SOURCES"]
SHCXXFLAGS Array Shared object C++ compiler flags ["$CXXFLAGS"]
SHDC String Shared object D compiler "gdc"
SHDCCMD Array Shared object D compiler command line ["$SHDC", "-c", "-o", "$_TARGET", "$INCPREFIX$D_IMPORT_PATH", "$SHDFLAGS", "$_SOURCES"]
SHDFLAGS Array Shared object D compiler flags Windows: ["$DFLAGS"], POSIX: ["$DFLAGS", "-fPIC"]
SHLD String Shared library linker nil (if nil, $SHCC, $SHCXX, or $SHDC is used depending on the sources being linked)
SHLDCMD Array Shared library linker command line ["$SHLD", "-o", "$_TARGET", "$SHLDFLAGS", "$_SOURCES", "$SHLIBDIRPREFIX$LIBPATH", "$SHLIBLINKPREFIX$LIBS"]
SHLDFLAGS Array Shared library linker flags ["$LDFLAGS", "-shared"]
SHLIBDIRPREFIX String Prefix given to shared library linker to add a library search path "-L"
SHLIBLINKPREFIX String Prefix given to shared library linker to add a library to link with "-l"
SHLIBPREFIX String Shared library file name prefix Windows: "", POSIX: "lib"
SHLIBSUFFIX String Shared library file name suffix Windows: ".dll", POSIX: ".so"
SIZE String Size executable. "size"
YACC String Yacc executable "bison"
YACC_CMD Array Yacc command line ["$YACC", "$YACC_FLAGS", "-o", "$_TARGET", "$_SOURCES"]
YACC_FLAGS Array Yacc flags ["-d"]
YACCSUFFIX Array Yacc input file suffixes [".y", ".yy"]

Build Hooks

Environments can have build hooks which are added with env.add_build_hook(). Build hooks are invoked immediately before a builder executes. Build hooks can modify the construction variables in use for the build operation. They can also register new build targets.

Environments can also have post-build hooks added with env.add_post_build_hook(). Post-build hooks are only invoked if the build operation was a success. Post-build hooks can invoke commands using the newly-built files, or register new build targets.

Each build hook block will be invoked for every build operation, so the block should test the target or sources if its action should only apply to some subset of build targets or source files.

Example build hook: do |env|
  # Build third party sources without -Wall
  env.add_build_hook do |build_op|
    if build_op[:builder].name == "Object" and
      build_op[:sources].first =~ %r{src/third-party}
      build_op[:vars]["CFLAGS"] -= ["-Wall"]

The build_op parameter to the build hook block is a Hash describing the build operation with the following keys:

  • :builder - Builder instance in use
  • :env - Environment calling the build hook; note that this may be different from the Environment that the build hook was added to in the case that the original Environment was cloned with build hooks!
  • :target - String name of the target file
  • :sources - Array of the source files
  • :vars - Rscons::VarSet containing the construction variables to use. The build hook can overwrite entries in build_op[:vars] to alter the construction variables in use for this specific build operation.

Phony Targets

A build target name given as a Symbol instead of a String is interpreted as a "phony" target. Phony targets operate similarly to normal build targets, except that a file is not expected to be produced by the builder. Phony targets will still be "rebuilt" if any source or the command is out of date.

Explicit Dependencies

A target can be marked as depending on another file that Rscons would not otherwise know about via the Environment#depends function. For example, to force the linker to re-link a Program output when a linker script changes: do |env|
  env.Program("a.out", "foo.c", "LDFLAGS" => %w[-T linker_script.ld])
  env.depends("a.out", "linker_script.ld")

You can pass multiple dependency files to Environment#depends:

env.depends("my_app", "config/link.ld", "README.txt", *Dir.glob("assets/**/*"))

Construction Variable Naming

  • uppercase strings - the default construction variables that Rscons uses
  • strings beginning with "_" - set and used internally by builders
  • symbols, lowercase strings - reserved as user-defined construction variables

API documentation

Documentation for the complete Rscons API can be found at

Release Notes


New Features

  • #43 - Add ability to record side-effect file production


New Features

  • #40 - env.depends should imply env.build_after
  • #41 - be more colorful


  • #39 - wait for in-progress subcommands to complete on build failure
  • #42 - cloned Environments should inherit n_threads



  • fix the circular build dependency detection logic


New Features

  • Change default Environment :clone option to :all to clone all attributes
  • #38 - raise error when circular dependencies are found
  • #34 - Allow overriding n_threads on a per-Environment level


  • #35 - env.build_after should expand paths
  • #36 - SHCFLAGS and SHCXXFLAGS should inherit non-SH flags by default
  • #37 - Fix non-blocking thread-wait if Rscons.n_threads is set to 0


New Features

  • #23 - add parallelization - builds are now parallelized by default
  • #31 - add LEXSUFFIX, YACCSUFFIX construction variables
  • #30 - place object files for absolute source paths under build_root
  • #28 - support redirecting standard output using the Command builder
  • Always use a build root and default it to "build"
  • Add builder features
  • #8 - add SharedObject and SharedLibrary builders


  • expand target and source paths before calling Builder#create_build_target
  • #29 - fix PROGSUFFIX handling
  • #32 - Pre-build hooks do not respect modified key values


  • Environment#parse_flags should put -std=XXX flags in CCFLAGS, not CFLAGS


  • allow phony targets in conjunction with build roots


  • change *SUFFIX defaults to arrays
  • add various C++ file suffixes
  • use $INCPREFIX instead of hard-coded "-I" in Preprocess builder


New Features

  • #6 - add Install and Copy builders
  • #22 - allow overriding Command builder short description with CMD_DESC variable
  • #24 - add "rscons" executable
  • #25 - add support for phony targets given as Symbols instead of Strings
  • #26 - support registering multiple build targets with the same target name
  • #27 - add Directory builder


  • #20 - fix variable references that expand to arrays in build target sources
  • #21 - rework Preprocess builder to consider deep dependencies
  • fix Rscons.set_suffix to append the given suffix if the filename has none
  • remove $CFLAGS from default CPP_CMD


  • fix Environment#dump when construction variables are symbols


  • new Command builder to execute arbitrary user commands
  • new SimpleBuilder class
    • create new builders quickly by passing a block to Environment#add_builder
  • improved YARD documentation
  • add Environment#dump to debug Environment construction variables


  • allow build hooks to register new build targets
  • add post-build hooks (register with Environment#add_post_build_hook)
  • clear all build targets after processing an Environment
  • allow trailing slashes in arguments to Environment#build_dir


  • add DEPFILESUFFIX construction variable to override dependency file suffix
  • fix Environment#depends to expand its arguments for construction variables


  • support lambdas as construction variable values


  • add "json" as a runtime dependency
  • update construction variables to match SCons more closely
  • add Environment#shell
  • add Environment#parse_flags, #parse_flags!, #merge_flags
  • unbuffer $stdout by default
  • add PROGSUFFIX construction variable (defaults to .exe on MinGW/Cygwin)
  • add Rscons::BuildTarget and Builder#create_build_target
  • update specs to RSpec 3.x and fix to run on MinGW/Cygwin/Linux
  • add YARD documentation to get to 100% coverage


  • fix builders properly using construction variable overrides
  • expand nil construction variables to empty strings


  • add Environment#expand_path
  • expand construction variable references in builder targets and sources before invoking builder


  • fix invoking a builder with no sources while a build root defined


  • add CFile builder
  • add Disassemble builder
  • add Preprocess builder
  • pass the Environment object to build hooks in the :env key of the build_op parameter
  • expand target/source paths beginning with "^/" to be relative to the Environment's build root
  • many performance improvements, including:
    • use JSON instead of YAML for the cache to improve loading speed (Issue #7)
    • store a hash of the build command instead of the full command contents in the cache
    • implement copy-on-write semantics for construction variables when cloning Environments
    • only load the cache once instead of on each Environment#process
    • only write the cache when something has changed
  • fix Cache#mkdir_p to handle relative paths (Issue #5)
  • flush the cache to disk if a builder raises an exception (Issue #4)


  • change Environment#execute() options parameter to accept the following options keys:
    • :env to pass an environment Hash to Kernel#system
    • :options to pass an options Hash to Kernel#system


  • add :clone option to Environment#clone to control exactly which Environment attributes are cloned
  • allow nil to be passed in to Environment#build_root=


  • Change Cache#up_to_date?() and #register_build() to accept a single target file or an array of target file names


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request