Buck: genrule()


A genrule() is used to generate files from a shell command. It must produce a single output file or folder.


  • name (required) #

    The short name for this build target.

  • srcs (defaults to []) #

    Either a list or a map of the source files which Buck makes available to the shell command at the path in the SRCDIR environment variable. If you specify a list, the source files are the names in the list. If you specify a map, the source files are made available as the names in the keys of the map, where the values of the map are the original source file names.

  • cmd (defaults to None) #

    The shell command to run to generate the output file. It is the fallback for bash and cmd_exe arguments. The following environment variables are populated by Buck and available to the shell command. They are accessed using the syntax:





    A string expansion of the srcs argument delimited by the environment_expansion_separator argument where each element of srcs will be translated into an absolute path.


    The absolute path to a directory to which sources are copied prior to running the command.


    The output file or directory for the genrule(). This variable will have whatever value is specified by the out argument. The value should be a valid filepath. The semantics of the shell command determine whether this filepath is treated as a file or a directory. If the filepath is a directory, then the shell command needs to create it.

    The file or directory specified by this variable must always be written by this command. If not, the execution of this rule will be considered a failure, halting the build process.


    A temporary directory which can be used for intermediate results and will not be bundled into the output.

    String parameter macros

    It is also possible to expand references to other rules within the cmd, using builtin string parameter macros. All build rules expanded in the command are automatically considered to be dependencies of the genrule().

    Note that the paths returned by these macros are absolute paths. You should convert these paths to be relative paths before embedding them in, for example, a shell script or batch file. Using relative paths ensures that your builds are hermetic, that is, they are reproducible across different machine environments.

    Additionally, if you embed these paths in a shell script, you should execute that script using the sh_binary rule and include the targets for these paths in the resources argument of that sh_binary rule. These are the same targets that you pass to the string parameter macros.

    $(classpath //path/to:target)

    Expands to the transitive classpath of the specified build rule, provided that the rule has a Java classpath. If the rule does not have (or contribute to) a classpath, then an exception is thrown and the build breaks.

    $(exe //path/to:target)

    Expands a build rule that results in an executable to the commands necessary to run that executable. For example, a java_binary() might expand to a call to java -jar path/to/target.jar . Files that are executable (perhaps generated by a genrule()) are also expanded. If the build rule does not generate an executable output, then an exception is thrown and the build breaks.

    $(location //path/to:target)

    Expands to the location of the output of the specified build rule. This means that you can refer to the output without needing to be aware of how Buck is storing data on the disk mid-build.

    $(maven_coords //path/to:target)

    Expands to the Maven coordinates for the specified build rule. This allows you to access the Maven coordinates for Maven-aware build rules. The format of the expansion is:


  • bash (defaults to None) #

    A platform-specific version of the shell command parameter cmd. It runs on Linux and UNIX systems—including OSX—on which bash is installed. It has a higher priority than cmd. The bash argument is run with /bin/bash -c. It has access to the same set of macros and variables as the cmd argument.

  • cmd_exe (defaults to None) #

    A platform-specific version of the shell command parameter cmd. It runs on Windows and has a higher priority than cmd. The cmd_exe argument is run with cmd.exe /c. It has access to the same set of macros and variables as the cmd argument.

  • type (defaults to None) #

    Specifies the type of this genrule. This is used for logging and is particularly useful for grouping genrules that share an underlying logical "type".

    For example, if you have the following cxx_genrule defined in the root directory of your Buck project

      name = 'cxx_gen',
      type = 'epilog',
      cmd  = 'touch finish.txt; cp finish.txt $OUT',
      out  = 'finish.txt'

    then the following buck query command

    buck query "attrfilter( type, 'epilog', '//...' )"



  • out (required) #

    The name of the output file or directory. The complete path to this argument is provided to the shell command through the OUT environment variable.

  • environment_expansion_separator (defaults to " ") #

    The delimiter between paths in environment variables, such as SRCS, that can contain multiple paths. It can be useful to specify this parameter if the paths could contain spaces.

  • enable_sandbox (defaults to False) #

    Whether this target should be executed in a sandbox or not.

  • executable (defaults to False) #

    Whether the output of the genrule is itself executable. Marking an output as executable makes buck run and $(exe ...) macro expansion work with this target.

  • remote (defaults to False) #

    Opts this genrule in to remote execution. Note that it is only safe to execute a genrule remotely if it is completely hermetic and completely and correctly describes its dependencies. Defaults to false. This parameter is unstable. It is subject to removal, default reversal, and other arbitrary changes in the future.

  • tests (defaults to []) #

    List of build targets that identify tests that exercise this target.

  • visibility (defaults to []) #

    List of build target patterns that identify the build rules that can include this rule as a dependency, for example, by listing it in their deps or exported_deps attributes. For more information, see visibility.

  • licenses (defaults to []) #

    Set of license files for this library. To get the list of license files for a given build rule and all of its dependencies, you can use buck query.

  • labels (defaults to []) #

    Set of arbitrary strings which allow you to annotate a build rule with tags that can be searched for over an entire dependency tree using buck query attrfilter().


This genrule() uses a Python script to derive a new AndroidManifest.xml from an AndroidManifest.xml in the source tree. Note you don't need to prepend execution commands with python: Buck knows how to execute different kinds of binaries using $(exe) command.
  name = 'generate_manifest',
  srcs = [
  bash = '$(exe //python/android:basic_to_full) ' \
      '$SRCDIR/AndroidManifest.xml > $OUT',
  cmd_exe = '$(exe //python/android:basic_to_full) ' \
      '%SRCDIR%\\AndroidManifest.xml > %OUT%',
  out = 'AndroidManifest.xml',