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This section defines various terms and concepts that are common to many functions or build rules.

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Bourne shell tokenization

Certain string attributes of some rules are split into multiple words according to the tokenization rules of the Bourne shell: unquoted spaces delimit separate words, and single- and double-quotes characters and backslashes are used to prevent tokenization. Those attributes that are subject to this tokenization are explicitly indicated as such in their definitions in this document. Attributes subject to “Make” variable expansion and Bourne shell tokenization are typically used for passing arbitrary options to compilers and other tools. Examples of such attributes are cc_library.copts and java_library.javacopts. Together these substitutions allow a single string variable to expand into a configuration-specific list of option words.

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Label expansion

Some string attributes of a very few rules are subject to label expansion: if those strings contain a valid label as a substring, such as //mypkg:target, and that label is a declared prerequisite of the current rule, it is expanded into the pathname of the file represented by the target //mypkg:target. Example attributes include genrule.cmd and cc_binary.linkopts. The details may vary significantly in each case, over such issues as: whether relative labels are expanded; how labels that expand to multiple files are treated, etc. Consult the rule attribute documentation for specifics.

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Typical attributes defined by most build rules

This section describes attributes that are defined by many build rules, but not all. AttributeDescriptiondata List of labels; default is [] Files needed by this rule at runtime. May list file or rule targets. Generally allows any target. The default outputs and runfiles of targets in the data attribute should appear in the *.runfiles area of any executable which is output by or has a runtime dependency on this target. This may include data files or binaries used when this target’s srcs are executed. See the data dependencies section for more information about how to depend on and use data files. New rules should define a data attribute if they process inputs which might use other inputs at runtime. Rules’ implementation functions must also populate the target’s
runfiles from the outputs and runfiles of any data attribute, as well as runfiles from any dependency attribute which provides either source code or runtime dependencies. deps List of labels; default is [] Dependencies for this target. Generally should only list rule targets. (Though some rules permit files to be listed directly in deps, this should be avoided when possible.) Language-specific rules generally limit the listed targets to those with specific providers. The precise semantics of what it means for a target to depend on another using deps are specific to the kind of rule, and the rule-specific documentation goes into more detail. For rules which process source code, deps generally specifies code dependencies used by the code in srcs. Most often, a deps dependency is used to allow one module to use symbols defined in another module written in the same programming language and separately compiled. Cross-language dependencies are also permitted in many cases: For example, a java_library target may depend on C++ code in a cc_library target, by listing the latter in the deps attribute. See the definition of dependencies for more information. licenses List of strings; nonconfigurable; default is ["none"] A list of license-type strings to be used for this particular target. This is part of a deprecated licensing API that Bazel no longer uses. Don’t use this. srcs List of labels; default is [] Files processed or included by this rule. Generally lists files directly, but may list rule targets (like filegroup or genrule) to include their default outputs. Language-specific rules often require that the listed files have particular file extensions.

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Attributes common to all build rules

This section describes attributes that are implicitly added to all build rules. AttributeDescriptionaspect_hints List of labels; default is [] A list of arbitrary labels which is exposed to aspects (in particular - aspects invoked by this rule’s reverse dependencies), but isn’t exposed to this rule’s own implementation. Consult documentation for language-specific rule sets for details about what effect a particular aspect hint would have. You could think of an aspect hint as a richer alternative to a tag: while a tag conveys only a boolean state (the tag is either present or absent in the tags list), an aspect hint can convey arbitrary structured information in its providers. In practice, aspect hints are used for interoperability between different language-specific rule sets. For example, imagine you have a mylang_binary target which needs to depend on an otherlang_library target. The MyLang-specific logic needs some additional information about the OtherLang target in order to use it, but otherlang_library doesn’t provide this information because it knows nothing about MyLang. One solution might be for the MyLang rule set to define a mylang_hint rule which can be used to encode that additional information; the user can add the hint to their otherlang_library’s aspect_hints, and mylang_binary can use an aspect to collect the additional information from a MyLang-specific provider in the mylang_hint. For a concrete example, see swift_interop_hint and swift_overlay in rules_swift. Best practices:
  • Targets listed in aspect_hints should be lightweight and minimal.
  • Language-specific logic should consider only aspect hints having providers relevant to that language, and should ignore any other aspect hints.
compatible_with List of labels; nonconfigurable; default is [] The list of environments this target can be built for, in addition to default-supported environments. This is part of Bazel’s constraint system, which lets users declare which targets can and cannot depend on each other. For example, externally deployable binaries shouldn’t depend on libraries with company-secret code. See ConstraintSemantics for details. deprecation String; nonconfigurable; default is None An explanatory warning message associated with this target. Typically this is used to notify users that a target has become obsolete, or has become superseded by another rule, is private to a package, or is perhaps considered harmful for some reason. It is a good idea to include some reference (like a webpage, a bug number or example migration CLs) so that one can easily find out what changes are required to avoid the message. If there is a new target that can be used as a drop in replacement, it is a good idea to just migrate all users of the old target. This attribute has no effect on the way things are built, but it may affect a build tool’s diagnostic output. The build tool issues a warning when a rule with a deprecation attribute is depended upon by a target in another package. Intra-package dependencies are exempt from this warning, so that, for example, building the tests of a deprecated rule does not encounter a warning. If a deprecated target depends on another deprecated target, no warning message is issued. Once people have stopped using it, the target can be removed. exec_compatible_with List of labels; nonconfigurable; default is [] A list of constraint_values that must be present in the execution platform of this target’s default exec group. This is in addition to any constraints already set by the rule type. Constraints are used to restrict the list of available execution platforms. For more details, see the description of toolchain resolution. and exec groups exec_group_compatible_with Dictionary of strings to lists of labels; nonconfigurable; default is {} A dictionary of exec group names to lists of constraint_values that must be present in the execution platform for the given exec group. This is in addition to any constraints already set on the exec group’s definition. Constraints are used to restrict the list of available execution platforms. For more details, see the description of toolchain resolution. and exec groups exec_properties Dictionary of strings; default is {} A dictionary of strings that will be added to the exec_properties of a platform selected for this target. See exec_properties of the platform rule. If a key is present in both the platform and target-level properties, the value will be taken from the target. Keys can be prefixed with the name of an execution group followed by a . to apply them only to that particular exec group. features List of feature strings; default is [] A feature is string tag that can be enabled or disabled on a target. The meaning of a feature depends on the rule itself. This features attribute is combined with the package level features attribute. For example, if the features [“a”, “b”] are enabled on the package level, and a target’s features attribute contains [“-a”, “c”], the features enabled for the rule will be “b” and “c”. See example. package_metadata List of labels; nonconfigurable; default is the package’s default_package_metadata A list of labels that are associated metadata about this target. Typically, the labels are simple rules that return a provider of constant values. Rules and aspects may use these labels to perform some additional analysis on the build graph. The canonical use case is that of rules_license. For that use case, package_metadata and default_package_metadata is used to attach information about a package’s licence or version to targets. An aspect applied to a top-level binary can be used to gather those and produce compliance reports. restricted_to List of labels; nonconfigurable; default is [] The list of environments this target can be built for, instead of default-supported environments. This is part of Bazel’s constraint system. See compatible_with for details. tags List of strings; nonconfigurable; default is [] Tags can be used on any rule. Tags on test and test_suite rules are useful for categorizing the tests. Tags on non-test targets are used to control sandboxed execution of genrule s and Starlark actions, and for parsing by humans and/or external tools. Bazel modifies the behavior of its sandboxing code if it finds the following keywords in the tags attribute of any test or genrule target, or the keys of execution_requirements for any Starlark action.
  • no-sandbox keyword results in the action or test never being sandboxed; it can still be cached or run remotely - use no-cache or no-remote to prevent either or both of those.
  • no-cache keyword results in the action or test never being cached (locally or remotely). Note: for the purposes of this tag, the disk cache is considered a local cache, whereas the HTTP and gRPC caches are considered remote. Other caches, such as Skyframe or the persistent action cache, are not affected.
  • no-remote-cache keyword results in the action or test never being cached remotely (but it may be cached locally; it may also be executed remotely). Note: for the purposes of this tag, the disk cache is considered a local cache, whereas the HTTP and gRPC caches are considered remote. Other caches, such as Skyframe or the persistent action cache, are not affected. If a combination of local disk cache and remote cache are used (combined cache), it’s treated as a remote cache and disabled entirely unless --incompatible_remote_results_ignore_disk is set in which case the local components will be used.
  • no-remote-exec keyword results in the action or test never being executed remotely (but it may be cached remotely).
  • no-remote keyword prevents the action or test from being executed remotely or cached remotely. This is equivalent to using both no-remote-cache and no-remote-exec.
  • no-remote-cache-upload keyword disables upload part of remote caching of a spawn. it does not disable remote execution.
  • local keyword precludes the action or test from being remotely cached, remotely executed, or run inside the sandbox. For genrules and tests, marking the rule with the local = True attribute has the same effect.
  • requires-network keyword allows access to the external network from inside the sandbox. This tag only has an effect if sandboxing is enabled.
  • block-network keyword blocks access to the external network from inside the sandbox. In this case, only communication with localhost is allowed. This tag only has an effect if sandboxing is enabled.
  • requires-fakeroot runs the test or action as uid and gid 0 (i.e., the root user). This is only supported on Linux. This tag takes precedence over the --sandbox_fake_username command-line option.
Tags on tests are generally used to annotate a test’s role in your debug and release process. Typically, tags are most useful for C++ and Python tests, which lack any runtime annotation ability. The use of tags and size elements gives flexibility in assembling suites of tests based around codebase check-in policy. Bazel modifies test running behavior if it finds the following keywords in the tags attribute of the test rule:
  • exclusive will force the test to be run in the “exclusive” mode, ensuring that no other tests are running at the same time. Such tests will be executed in serial fashion after all build activity and non-exclusive tests have been completed. Remote execution is disabled for such tests because Bazel doesn’t have control over what’s running on a remote machine.
  • exclusive-if-local will force the test to be run in the “exclusive” mode if it is executed locally, but will run the test in parallel if it’s executed remotely.
  • manual keyword will exclude the target from expansion of target pattern wildcards ( ..., :*, :all, etc.) and test_suite rules which do not list the test explicitly when computing the set of top-level targets to build/run for the build, test, and coverage commands. It does not affect target wildcard or test suite expansion in other contexts, including the query command. Note that manual does not imply that a target should not be built/run automatically by continuous build/test systems. For example, it may be desirable to exclude a target from bazel test ... because it requires specific Bazel flags, but still have it included in properly-configured presubmit or continuous test runs.
  • external keyword will force test to be unconditionally executed (regardless of --cache_test_results value).
See Tag Conventions in the Test Encyclopedia for more conventions on tags attached to test targets. target_compatible_with List of labels; default is [] A list of constraint_value s that must be present in the target platform for this target to be considered compatible. This is in addition to any constraints already set by the rule type. If the target platform does not satisfy all listed constraints then the target is considered incompatible. Incompatible targets are skipped for building and testing when the target pattern is expanded (e.g. //..., :all). When explicitly specified on the command line, incompatible targets cause Bazel to print an error and cause a build or test failure. Targets that transitively depend on incompatible targets are themselves considered incompatible. They are also skipped for building and testing. An empty list (which is the default) signifies that the target is compatible with all platforms. All rules other than Workspace Rules support this attribute. For some rules this attribute has no effect. For example, specifying target_compatible_with for a cc_toolchain is not useful. See the Platforms page for more information about incompatible target skipping. testonly Boolean; nonconfigurable; default is False except for test and test suite targets If True, only testonly targets (such as tests) can depend on this target. Equivalently, a rule that is not testonly is not allowed to depend on any rule that is testonly. Tests ( *_test rules) and test suites ( test_suite rules) are testonly by default. This attribute is intended to mean that the target should not be contained in binaries that are released to production. Because testonly is enforced at build time, not run time, and propagates virally through the dependency tree, it should be applied judiciously. For example, stubs and fakes that are useful for unit tests may also be useful for integration tests involving the same binaries that will be released to production, and therefore should probably not be marked testonly. Conversely, rules that are dangerous to even link in, perhaps because they unconditionally override normal behavior, should definitely be marked testonly. toolchains List of labels; nonconfigurable; default is [] The set of targets whose Make variables this target is allowed to access. These targets are either instances of rules that provide TemplateVariableInfo or special targets for toolchain types built into Bazel. These include:
  • @bazel_tools//tools/cpp:toolchain_type
  • @rules_java//toolchains:current_java_runtime
Note that this is distinct from the concept of toolchain resolution that is used by rule implementations for platform-dependent configuration. You cannot use this attribute to determine which specific cc_toolchain or java_toolchain a target will use. visibility List of labels; nonconfigurable; default varies The visibility attribute controls whether the target can be depended on by targets in other locations. See the documentation for visibility. For targets declared directly in a BUILD file or in legacy macros called from a BUILD file, the default value is the package’s default_visibility if specified, or else ["//visibility:private"]. For targets declared in one or more symbolic macros, the default value is always just ["//visibility:private"] (which makes it useable only within the package containing the macro’s code).

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Attributes common to all test rules (*_test)

This section describes attributes that are common to all test rules. AttributeDescriptionargs List of strings; subject to $(location) and “Make variable” substitution, and Bourne shell tokenization; default is [] Command line arguments that Bazel passes to the target when it is executed with bazel test. These arguments are passed before any --test_arg values specified on the bazel test command line. env Dictionary of strings; values are subject to $(location) and “Make variable” substitution; default is {} Specifies additional environment variables to set when the test is executed by bazel test. This attribute only applies to native rules, like cc_test, py_test, and sh_test. It does not apply to Starlark-defined test rules. For your own Starlark rules, you can add an “env” attribute and use it to populate a RunEnvironmentInfo Provider. TestEnvironment Provider. env_inherit List of strings; default is [] Specifies additional environment variables to inherit from the external environment when the test is executed by bazel test. This attribute only applies to native rules, like cc_test, py_test, and sh_test. It does not apply to Starlark-defined test rules. size String "enormous", "large", "medium", or "small"; nonconfigurable; default is "medium" Specifies a test target’s “heaviness”: how much time/resources it needs to run. Unit tests are considered “small”, integration tests “medium”, and end-to-end tests “large” or “enormous”. Bazel uses the size to determine a default timeout, which can be overridden using the timeout attribute. The timeout is for all tests in the BUILD target, not for each individual test. When the test is run locally, the size is additionally used for scheduling purposes: Bazel tries to respect --local_{ram,cpu}_resources and not overwhelm the local machine by running lots of heavy tests at the same time. Test sizes correspond to the following default timeouts and assumed peak local resource usages: SizeRAM (in MB)CPU (in CPU cores)Default timeoutsmall201short (1 minute)medium1001moderate (5 minutes)large3001long (15 minutes)enormous8001eternal (60 minutes) The environment variable TEST_SIZE will be set to the value of this attribute when spawning the test. timeout String "short", "moderate", "long", or "eternal"; nonconfigurable; default is derived from the test’s size attribute How long the test is expected to run before returning. While a test’s size attribute controls resource estimation, a test’s timeout may be set independently. If not explicitly specified, the timeout is based on the test’s size. The test timeout can be overridden with the --test_timeout flag, e.g. for running under certain conditions which are known to be slow. Test timeout values correspond to the following time periods: Timeout ValueTime Periodshort1 minutemoderate5 minuteslong15 minuteseternal60 minutes For times other than the above, the test timeout can be overridden with the --test_timeout bazel flag, e.g. for manually running under conditions which are known to be slow. The --test_timeout values are in seconds. For example --test_timeout=120 will set the test timeout to two minutes. The environment variable TEST_TIMEOUT will be set to the test timeout (in seconds) when spawning the test. flaky Boolean; nonconfigurable; default is False Marks test as flaky. If set, executes the test up to three times, marking it as failed only if it fails each time. By default, this attribute is set to False and the test is executed only once. Note, that use of this attribute is generally discouraged - tests should pass reliably when their assertions are upheld. shard_count Non-negative integer less than or equal to 50; default is -1 Specifies the number of parallel shards to use to run the test. If set, this value will override any heuristics used to determine the number of parallel shards with which to run the test. Note that for some test rules, this parameter may be required to enable sharding in the first place. Also see --test_sharding_strategy. If test sharding is enabled, the environment variable TEST_TOTAL_SHARDS will be set to this value when spawning the test. Sharding requires the test runner to support the test sharding protocol. If it does not, then it will most likely run every test in every shard, which is not what you want. See Test Sharding in the Test Encyclopedia for details on sharding. local Boolean; nonconfigurable; default is False Forces the test to be run locally, without sandboxing. Setting this to True is equivalent to providing “local” as a tag ( tags=["local"]).

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Attributes common to all binary rules (*_binary)

This section describes attributes that are common to all binary rules. AttributeDescriptionargs List of strings; subject to $(location) and “Make variable” substitution, and Bourne shell tokenization; nonconfigurable; default is [] Command line arguments that Bazel will pass to the target when it is executed either by the run command or as a test. These arguments are passed before the ones that are specified on the bazel run or bazel test command line. NOTE: The arguments are not passed when you run the target outside of Bazel (for example, by manually executing the binary in bazel-bin/). env Dictionary of strings; values are subject to $(location) and “Make variable” substitution; default is {} Specifies additional environment variables to set when the target is executed by bazel run. This attribute only applies to native rules, like cc_binary, py_binary, and sh_binary. It does not apply to Starlark-defined executable rules. For your own Starlark rules, you can add an “env” attribute and use it to populate a RunEnvironmentInfo Provider. NOTE: The environment variables are not set when you run the target outside of Bazel (for example, by manually executing the binary in bazel-bin/). output_licenses List of strings; default is [] The licenses of the output files that this binary generates. This is part of a deprecated licensing API that Bazel no longer uses. Don’t use this.

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Configurable attributes

Most attributes are “configurable”, meaning that their values may change when the target is built in different ways. Specifically, configurable attributes may vary based on the flags passed to the Bazel command line, or what downstream dependency is requesting the target. This can be used, for instance, to customize the target for multiple platforms or compilation modes. The following example declares different sources for different target architectures. Running bazel build :multiplatform_lib --cpu x86 will build the target using x86_impl.cc, while substituting --cpu arm will instead cause it to use arm_impl.cc. 
cc_library(
    name = "multiplatform_lib",
    srcs = select({
        ":x86_mode": ["x86_impl.cc"],
        ":arm_mode": ["arm_impl.cc"]
    })
)
config_setting(
    name = "x86_mode",
    values = { "cpu": "x86" }
)
config_setting(
    name = "arm_mode",
    values = { "cpu": "arm" }
)
The select() function chooses among different alternative values for a configurable attribute based on which config_setting or constraint_value criteria the target’s configuration satisfies. Bazel evaluates configurable attributes after processing macros and before processing rules (technically, between the loading and analysis phases). Any processing before select() evaluation doesn’t know which branch the select() chooses. Macros, for example, can’t change their behavior based on the chosen branch, and bazel query can only make conservative guesses about a target’s configurable dependencies. See this FAQ for more on using select() with rules and macros. Attributes marked nonconfigurable in their documentation cannot use this feature. Usually an attribute is nonconfigurable because Bazel internally needs to know its value before it can determine how to resolve a select(). See Configurable Build Attributes for a detailed overview.

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Implicit output targets

Implicit outputs in C++ are deprecated. Please refrain from using it in other languages where possible. We don’t have a deprecation path yet but they will eventually be deprecated too. When you define a build rule in a BUILD file, you are explicitly declaring a new, named rule target in a package. Many build rule functions also implicitly entail one or more output file targets, whose contents and meaning are rule-specific. For example, when you explicitly declare a java_binary(name='foo', ...) rule, you are also implicitly declaring an output file target foo_deploy.jar as a member of the same package. (This particular target is a self-contained Java archive suitable for deployment.) Implicit output targets are first-class members of the global target graph. Just like other targets, they are built on demand, either when specified in the top-level built command, or when they are necessary prerequisites for other build targets. They can be referenced as dependencies in BUILD files, and can be observed in the output of analysis tools such as bazel query. For each kind of build rule, the rule’s documentation contains a special section detailing the names and contents of any implicit outputs entailed by a declaration of that kind of rule. An important but somewhat subtle distinction between the two namespaces used by the build system: labels identify targets, which may be rules or files, and file targets may be divided into either source (or input) file targets and derived (or output) file targets. These are the things you can mention in BUILD files, build from the command-line, or examine using bazel query; this is the target namespace. Each file target corresponds to one actual file on disk (the “file system namespace”); each rule target may correspond to zero, one or more actual files on disk. There may be files on disk that have no corresponding target; for example, .o object files produced during C++ compilation cannot be referenced from within BUILD files or from the command line. In this way, the build tool may hide certain implementation details of how it does its job. This is explained more fully in the BUILD Concept Reference.