bazel-lib/docs/expand_make_vars.md at 9059f8fa5ee1dc3a1c95102ff2a5a95fbe7fb8e6

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Public API for expanding variables

expand_template

expand_template(name, data, is_executable, out, substitutions, template)

Template expansion

This performs a simple search over the template file for the keys in substitutions, and replaces them with the corresponding values.

Values may also use location templates as documented in expand_locations as well as configuration variables such as $(BINDIR), $(TARGET_CPU), and $(COMPILATION_MODE) as documented in expand_variables.

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this target.	Name	required
data	List of targets for additional lookup information.	List of labels	optional	[]
is_executable	Whether to mark the output file as executable.	Boolean	optional	False
out	Where to write the expanded file.	Label	required
substitutions	Mapping of strings to substitutions.	Dictionary: String -> String	required
template	The template file to expand.	Label	required

expand_locations

expand_locations(ctx, input, targets)

Expand location templates.

Expands all $(execpath ...), $(rootpath ...) and deprecated $(location ...) templates in the given string by replacing with the expanded path. Expansion only works for labels that point to direct dependencies of this rule or that are explicitly listed in the optional argument targets.

See https://docs.bazel.build/versions/main/be/make-variables.html#predefined_label_variables.

Use $(rootpath) and $(rootpaths) to expand labels to the runfiles path that a built binary can use to find its dependencies. This path is of the format:

./file
path/to/file
../external_repo/path/to/file

Use $(execpath) and $(execpaths) to expand labels to the execroot (where Bazel runs build actions). This is of the format:

./file
path/to/file
external/external_repo/path/to/file
<bin_dir>/path/to/file
<bin_dir>/external/external_repo/path/to/file

The deprecated $(location) and $(locations) expansions returns either the execpath or rootpath depending on the context.

PARAMETERS

Name	Description	Default Value
ctx	context	none
input	String to be expanded	none
targets	List of targets for additional lookup information.	`[]`

RETURNS

The expanded path or the original path

expand_variables

expand_variables(ctx, s, outs, output_dir, attribute_name)

Expand make variables and substitute like genrule does.

This function is the same as ctx.expand_make_variables with the additional genrule-like substitutions of:

$@: The output file if it is a single file. Else triggers a build error.
$(@D): The output directory.

If there is only one file name in outs, this expands to the directory containing that file.

If there is only one directory in outs, this expands to the single output directory.

If there are multiple files, this instead expands to the package's root directory in the bin tree, even if all generated files belong to the same subdirectory!
$(RULEDIR): The output directory of the rule, that is, the directory corresponding to the name of the package containing the rule under the bin tree.
$(BUILD_FILE_PATH): ctx.build_file_path
$(VERSION_FILE): ctx.version_file.path
$(INFO_FILE): ctx.info_file.path
$(TARGET): ctx.label
$(WORKSPACE): ctx.workspace_name

See https://docs.bazel.build/versions/main/be/general.html#genrule.cmd and https://docs.bazel.build/versions/main/be/make-variables.html#predefined_genrule_variables for more information of how these special variables are expanded.