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.

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.

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)

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 of strings; 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".
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
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_exec_groups
(Experimental) runs the action on the given exec group's execution platform. If none, uses the target's default execution platform.

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)

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 ; 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".
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
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_exec_groups
(Experimental) runs the action on the given exec group's execution platform. If none, uses the target's default execution platform.

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.