Extensions > API reference > actions

actions

Module providing functions to create actions. Access this module using ctx.actions.

args

Args actions.args()

Returns an Args object that can be used to build memory-efficient command lines.

declare_directory

File actions.declare_directory(filename, *, sibling=None)

Declares that the rule or aspect creates a directory with the given name, in the current package. You must create an action that generates the directory. The contents of the directory are not directly accessible from Starlark, but can be expanded in an action command with Args.add_all().

Parameters

Parameter	Description
filename	required If no 'sibling' provided, path of the new directory, relative to the current package. Otherwise a base name for a file ('sibling' defines a directory).
sibling	File; or None; default = None A file that lives in the same directory as the newly declared directory. The file must be in the current package.

declare_file

File actions.declare_file(filename, *, sibling=None)

Declares that the rule or aspect creates a file with the given filename. If sibling is not specified, the file name is relative to the package directory, otherwise the file is in the same directory as sibling. Files cannot be created outside of the current package.

Remember that in addition to declaring a file, you must separately create an action that emits the file. Creating that action will require passing the returned File object to the action's construction function.

Note that predeclared output files do not need to be (and cannot be) declared using this function. You can obtain their File objects from ctx.outputs instead. See example of use.

Parameters

Parameter	Description
filename	required If no 'sibling' provided, path of the new file, relative to the current package. Otherwise a base name for a file ('sibling' determines a directory).
sibling	File; or None; default = None A file that lives in the same directory as the newly created file. The file must be in the current package.

declare_symlink

File actions.declare_symlink(filename, *, sibling=None)

Experimental. This parameter is experimental and may change at any time. Please do not depend on it. It may be enabled on an experimental basis by setting --experimental_allow_unresolved_symlinks

Declares that the rule or aspect creates a symlink with the given name in the current package. You must create an action that generates this symlink. Bazel will never dereference this symlink and will transfer it verbatim to sandboxes or remote executors.

Parameters

Parameter	Description
filename	required If no 'sibling' provided, path of the new symlink, relative to the current package. Otherwise a base name for a file ('sibling' defines a directory).
sibling	File; or None; default = None A file that lives in the same directory as the newly declared symlink.

do_nothing

None actions.do_nothing(mnemonic, inputs=[])

Creates an empty action that neither executes a command nor produces any output, but that is useful for inserting 'extra actions'.

Parameters

Parameter	Description
mnemonic	required A one-word description of the action, for example, CppCompile or GoLink.
inputs	sequence of Files; or depset; default = [] List of the input files of the action.

expand_template

None actions.expand_template(template, output, substitutions, is_executable=False)

Creates a template expansion action. When the action is executed, it will generate a file based on a template. Parts of the template will be replaced using the substitutions dictionary, in the order the substitutions are specified. Whenever a key of the dictionary appears in the template (or a result of a previous substitution), it is replaced with the associated value. There is no special syntax for the keys. You may, for example, use curly braces to avoid conflicts (for example, {KEY}). See example of use.

Parameters

Parameter	Description
template	required The template file, which is a UTF-8 encoded text file.
output	required The output file, which is a UTF-8 encoded text file.
substitutions	required Substitutions to make when expanding the template.
is_executable	default = False Whether the output file should be executable.

run

None actions.run(outputs, inputs=[], unused_inputs_list=None, executable, tools=unbound, arguments=[], mnemonic=None, progress_message=None, use_default_shell_env=False, env=None, execution_requirements=None, input_manifests=None, exec_group=None, shadowed_action=None, resource_set=None)

Creates an action that runs an executable. See example of use.

Parameters

Parameter	Description
outputs	sequence of Files; required List of the output files of the action.
inputs	sequence of Files; or depset; default = [] List or depset of the input files of the action.
unused_inputs_list	File; or None; default = None File containing list of inputs unused by the action. The content of this file (generally one of the outputs of the action) corresponds to the list of input files that were not used during the whole action execution. Any change in those files must not affect in any way the outputs of the action.
executable	File; or string; or FilesToRunProvider; required The executable file to be called by the action.
tools	sequence; or depset; default = unbound List or depset of any tools needed by the action. Tools are inputs with additional runfiles that are automatically made available to the action.
arguments	sequence; default = [] Command line arguments of the action. Must be a list of strings or actions.args() objects.
mnemonic	string; or None; default = None A one-word description of the action, for example, CppCompile or GoLink.
progress_message	string; or None; default = None Progress message to show to the user during the build, for example, "Compiling foo.cc to create foo.o". The message may contain %{label}, %{input}, or %{output} patterns, which are substituted with label string, first input, or output's path, respectively. Prefer to use patterns instead of static strings, because the former are more efficient.
use_default_shell_env	default = False Whether the action should use the built in shell environment or not.
env	dict; or None; default = None Sets the dictionary of environment variables.
execution_requirements	dict; or None; default = None Information for scheduling the action. See tags for useful keys.
input_manifests	sequence; or None; default = None (Experimental) sets the input runfiles metadata; they are typically generated by resolve_command.
exec_group	string; or None; default = None Runs the action on the given exec group's execution platform. If none, uses the target's default execution platform.
shadowed_action	Action; default = None Runs the action using the given shadowed action's inputs and environment added to the action's inputs list and environment. The action environment can overwrite any of the shadowed action's environment variables. If none, uses only the action's inputs and given environment.
resource_set	callable; or None; default = None A callback function that returns a resource set dictionary, used to estimate resource usage at execution time if this action is run locally. The function accepts two positional arguments: a string representing an OS name (e.g. "osx"), and an integer representing the number of inputs to the action. The returned dictionary may contain the following entries, each of which may be a float or an int: "cpu": number of CPUs; default 1 "memory": in MB; default 250 "local_test": number of local tests; default 1 If this parameter is set to None or if --experimental_action_resource_set is false, the default values are used. The callback must be top-level (lambda and nested functions aren't allowed).

run_shell

None actions.run_shell(outputs, inputs=[], tools=unbound, arguments=[], mnemonic=None, command, progress_message=None, use_default_shell_env=False, env=None, execution_requirements=None, input_manifests=None, exec_group=None, shadowed_action=None, resource_set=None)

Creates an action that runs a shell command. See example of use.

Parameters

Parameter	Description
outputs	sequence of Files; required List of the output files of the action.
inputs	sequence of Files; or depset; default = [] List or depset of the input files of the action.
tools	sequence of Files; or depset; default = unbound List or depset of any tools needed by the action. Tools are inputs with additional runfiles that are automatically made available to the action. The list can contain Files or FilesToRunProvider instances.
arguments	sequence; default = [] Command line arguments of the action. Must be a list of strings or actions.args() objects. Bazel passes the elements in this attribute as arguments to the command.The command can access these arguments using shell variable substitutions such as $1, $2, etc. Note that since Args objects are flattened before indexing, if there is an Args object of unknown size then all subsequent strings will be at unpredictable indices. It may be useful to use $@ (to retrieve all arguments) in conjunction with Args objects of indeterminate size. In the case where command is a list of strings, this parameter may not be used.
mnemonic	string; or None; default = None A one-word description of the action, for example, CppCompile or GoLink.
command	string; or sequence of strings; required Shell command to execute. This may either be a string (preferred) or a sequence of strings (deprecated). If command is a string, then it is executed as if by sh -c <command> "" <arguments> -- that is, the elements in arguments are made available to the command as $1, $2 (or %1, %2 if using Windows batch), etc. If arguments contains any actions.args() objects, their contents are appended one by one to the command line, so $i can refer to individual strings within an Args object. Note that if an Args object of unknown size is passed as part of arguments, then the strings will be at unknown indices; in this case the $@ shell substitution (retrieve all arguments) may be useful. (Deprecated) If command is a sequence of strings, the first item is the executable to run and the remaining items are its arguments. If this form is used, the arguments parameter must not be supplied. Note that this form is deprecated and will soon be removed. It is disabled with `--incompatible_run_shell_command_string`. Use this flag to verify your code is compatible. Bazel uses the same shell to execute the command as it does for genrules.
progress_message	string; or None; default = None Progress message to show to the user during the build, for example, "Compiling foo.cc to create foo.o". The message may contain %{label}, %{input}, or %{output} patterns, which are substituted with label string, first input, or output's path, respectively. Prefer to use patterns instead of static strings, because the former are more efficient.
use_default_shell_env	default = False Whether the action should use the built in shell environment or not.
env	dict; or None; default = None Sets the dictionary of environment variables.
execution_requirements	dict; or None; default = None Information for scheduling the action. See tags for useful keys.
input_manifests	sequence; or None; default = None (Experimental) sets the input runfiles metadata; they are typically generated by resolve_command.
exec_group	string; or None; default = None Runs the action on the given exec group's execution platform. If none, uses the target's default execution platform.
shadowed_action	Action; default = None Runs the action using the given shadowed action's discovered inputs added to the action's inputs list. If none, uses only the action's inputs.
resource_set	callable; or None; default = None A callback function for estimating resource usage if run locally. Seectx.actions.run().

symlink

None actions.symlink(output, target_file=None, target_path=None, is_executable=False, progress_message=None)

Creates an action that writes a symlink in the file system.

This function must be called with exactly one of target_file and target_path specified.

If target_file is used, then output must be declared as a regular file (such as by using declare_file()). It will actually be created as a symlink that points to the path of target_file.

If target_path is used instead, then output must be declared as a symlink (such as by using declare_symlink()). In this case, the symlink will point to whatever the content of target_path is. This can be used to create a dangling symlink.

Parameters

Parameter	Description
output	required The output of this action.
target_file	File; or None; default = None The File that the output symlink will point to.
target_path	string; or None; default = None (Experimental) The exact path that the output symlink will point to. No normalization or other processing is applied. Access to this feature requires setting --experimental_allow_unresolved_symlinks.
is_executable	default = False May only be used with target_file, not target_path. If true, when the action is executed, the target_file's path is checked to confirm that it is executable, and an error is reported if it is not. Setting is_executable to False does not mean the target is not executable, just that no verification is done. This feature does not make sense for target_path because dangling symlinks might not exist at build time.
progress_message	string; or None; default = None Progress message to show to the user during the build.

write

None actions.write(output, content, is_executable=False)

Creates a file write action. When the action is executed, it will write the given content to a file. This is used to generate files using information available in the analysis phase. If the file is large and with a lot of static content, consider using expand_template.

Parameters

Parameter	Description
output	required The output file.
content	string; or Args; required the contents of the file. May be a either a string or an actions.args() object.
is_executable	default = False Whether the output file should be executable.