Rules
alias
View rule sourcealias(name, actual, compatible_with, deprecation, features, restricted_to, tags, target_compatible_with, testonly, visibility)
The alias rule creates another name a rule can be referred to as.
Aliasing only works for "regular" targets. In particular, package_group
and test_suite cannot be aliased.
Aliasing may be of help in large repositories where renaming a target would require making changes to lots of files. You can also use alias rule to store a select function call if you want to reuse that logic for multiple targets.
The alias rule has its own visibility declaration. In all other respects, it behaves like the rule it references (e.g. testonly on the alias is ignored; the testonly-ness of the referenced rule is used instead) with some minor exceptions:
-
Tests are not run if their alias is mentioned on the command line. To define an alias
that runs the referenced test, use a
test_suiterule with a single target in itstestsattribute. -
When defining environment groups, the aliases to
environmentrules are not supported. They are not supported in the--target_environmentcommand line option, either.
Examples
filegroup(
name = "data",
srcs = ["data.txt"],
)
alias(
name = "other",
actual = ":data",
)
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
actual
|
Label; required The target this alias refers to. It does not need to be a rule, it can also be an input file. |
config_setting
View rule sourceconfig_setting(name, constraint_values, define_values, deprecation, distribs, features, flag_values, licenses, tags, testonly, values, visibility)
Matches an expected configuration state (expressed as build flags or platform constraints) for the purpose of triggering configurable attributes. See select for how to consume this rule and Configurable attributes for an overview of the general feature.
Examples
The following matches any build that sets --compilation_mode=opt or
-c opt (either explicitly at the command line or implicitly from .bazelrc files):
config_setting(
name = "simple",
values = {"compilation_mode": "opt"}
)
The following matches any build that targets ARM and applies the custom define
FOO=bar (for instance, bazel build --cpu=arm --define FOO=bar ...):
config_setting(
name = "two_conditions",
values = {
"cpu": "arm",
"define": "FOO=bar"
}
)
The following matches any build that sets
user-defined flag
--//custom_flags:foo=1 (either explicitly at the command line or implicitly from
.bazelrc files):
config_setting(
name = "my_custom_flag_is_set",
flag_values = { "//custom_flags:foo": "1" },
)
The following matches any build that targets a platform with an x86_64 architecture and glibc
version 2.25, assuming the existence of a constraint_value with label
//example:glibc_2_25. Note that a platform still matches if it defines additional
constraint values beyond these two.
config_setting(
name = "64bit_glibc_2_25",
constraint_values = [
"@platforms//cpu:x86_64",
"//example:glibc_2_25",
]
)
config_setting doesn't match the top-level command-line flags, it may still match
some build targets.
Notes
- See select for what happens when multiple
config_settings match the current configuration state. - For flags that support shorthand forms (e.g.
--compilation_modevs.-c),valuesdefinitions must use the full form. These automatically match invocations using either form. -
If a flag takes multiple values (like
--copt=-Da --copt=-Dbor a list-typed Starlark flag),values = { "flag": "a" }matches if"a"is present anywhere in the actual list.values = { "myflag": "a,b" }works the same way: this matches--myflag=a --myflag=b,--myflag=a --myflag=b --myflag=c,--myflag=a,b, and--myflag=c,b,a. Exact semantics vary between flags. For example,--coptdoesn't support multiple values in the same instance:--copt=a,bproduces["a,b"]while--copt=a --copt=bproduces["a", "b"](sovalues = { "copt": "a,b" }matches the former but not the latter). But--ios_multi_cpus(for Apple rules) does:-ios_multi_cpus=a,bandios_multi_cpus=a --ios_multi_cpus=bboth produce["a", "b"]. Check flag definitions and test your conditions carefully to verify exact expectations. - If you need to define conditions that aren't modeled by built-in build flags, use
Starlark-defined flags. You can also use
--define, but this offers weaker support and is not recommended. See here for more discussion. - Avoid repeating identical
config_settingdefinitions in different packages. Instead, reference a commonconfig_settingthat defined in a canonical package. values,define_values, andconstraint_valuescan be used in any combination in the sameconfig_settingbut at least one must be set for any givenconfig_setting.
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
constraint_values
|
List of labels; nonconfigurable; default is constraint_values that the target platform must specify
in order to match this config_setting. (The execution platform is not
considered here.) Any additional constraint values that the platform has are ignored. See
Configurable Build Attributes for details.
If two If two |
define_values
|
Dictionary: String -> String; nonconfigurable; default is values but
specifically for the --define flag.
That means:
config_setting(
name = "a_and_b",
values = {
"define": "a=1",
"define": "b=2",
})
doesn't work because the same key (
config_setting(
name = "a_and_b",
define_values = {
"a": "1",
"b": "2",
})
correctly matches
|
flag_values
|
Dictionary: label -> String; nonconfigurable; default is values but
for
user-defined build flags.
This is a distinct attribute because user-defined flags are referenced as labels while built-in flags are referenced as arbitrary strings. |
values
|
Dictionary: String -> String; nonconfigurable; default is This rule inherits the configuration of the configured target that
references it in a For convenience's sake, configuration values are specified as build flags (without
the preceding If a flag is not explicitly set at the command line, its default value is used.
If a key appears multiple times in the dictionary, only the last instance is used.
If a key references a flag that can be set multiple times on the command line (e.g.
|
filegroup
View rule sourcefilegroup(name, srcs, data, compatible_with, deprecation, distribs, features, licenses, output_group, restricted_to, tags, target_compatible_with, testonly, visibility)
Use filegroup to gather the outputs of a set of targets under a single
label.
filegroup is not a substitute for listing targets on the command line or
in an attribute of another rule, because targets have many properties other than their
outputs, which are not collected in the same way. However, it's still useful in quite
a few cases, for example, in the srcs attribute of a genrule, or
the data attribute of a *_binary rule.
Using filegroup is encouraged instead of referencing directories directly.
Directly referencing directories is discouraged because the build system does not have
full knowledge of all files below the directory, so it may not rebuild when these files change.
When combined with glob, filegroup can ensure that all
files are explicitly known to the build system.
Examples
To create a filegroup consisting of two source files, do
filegroup(
name = "mygroup",
srcs = [
"a_file.txt",
"//a/library:target",
"//a/binary:target",
],
)
Or, use a glob to fully crawl a testdata directory:
filegroup(
name = "exported_testdata",
srcs = glob([
"testdata/*.dat",
"testdata/logs/**/*.log",
]),
)
To make use of these definitions, reference the filegroup with a label from any rule:
cc_library(
name = "my_library",
srcs = ["foo.cc"],
data = [
"//my_package:exported_testdata",
"//my_package:mygroup",
],
)
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
srcs
|
List of labels; default is
It is common to use the result of a glob expression for
the value of the |
data
|
List of labels; default is
Targets named in the |
output_group
|
String; default is An "output group" is a category of output artifacts of a target, specified in that rule's implementation. |
genquery
View rule sourcegenquery(name, deps, data, compatible_with, compressed_output, deprecation, distribs, exec_compatible_with, exec_properties, expression, features, licenses, opts, restricted_to, scope, strict, tags, target_compatible_with, testonly, visibility)
genquery() runs a query specified in the
Bazel query language and dumps the result
into a file.
In order to keep the build consistent, the query is allowed only to visit
the transitive closure of the targets specified in the scope
attribute. Queries violating this rule will fail during execution if
strict is unspecified or true (if strict is false,
the out of scope targets will simply be skipped with a warning). The
easiest way to make sure this does not happen is to mention the same labels
in the scope as in the query expression.
The only difference between the queries allowed here and on the command
line is that queries containing wildcard target specifications (e.g.
//pkg:* or //pkg:all) are not allowed here.
The reasons for this are two-fold: first, because genquery has
to specify a scope to prevent targets outside the transitive closure of the
query to influence its output; and, second, because BUILD files
do not support wildcard dependencies (e.g. deps=["//a/..."]
is not allowed).
The genquery's output is ordered lexicographically in order to enforce deterministic output,
with the exception of --output=graph|minrank|maxrank or when somepath
is used as the top-level function.
The name of the output file is the name of the rule.
Examples
This example writes the list of the labels in the transitive closure of the specified target to a file.
genquery(
name = "kiwi-deps",
expression = "deps(//kiwi:kiwi_lib)",
scope = ["//kiwi:kiwi_lib"],
)
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
compressed_output
|
Boolean; default is True, query output is written in GZIP file format. This setting can be used
to avoid spikes in Bazel's memory use when the query output is expected to be large. Bazel
already internally compresses query outputs greater than 220 bytes regardless of
the value of this setting, so setting this to True may not reduce retained
heap. However, it allows Bazel to skip decompression when writing the output file,
which can be memory-intensive.
|
expression
|
String; required The query to be executed. In contrast to the command line and other places in BUILD files, labels here are resolved relative to the root directory of the workspace. For example, the label:b in this attribute in the file a/BUILD will refer to the
target //:b.
|
opts
|
List of strings; default is bazel query. Some query options are not allowed
here: --keep_going, --query_file, --universe_scope,
--order_results and --order_output. Options not specified here
will have their default values just like on the command line of bazel query.
|
scope
|
List of labels; required The scope of the query. The query is not allowed to touch targets outside the transitive closure of these targets. |
strict
|
Boolean; default is |
genrule
View rule sourcegenrule(name, srcs, outs, cmd, cmd_bash, cmd_bat, cmd_ps, compatible_with, deprecation, distribs, exec_compatible_with, exec_properties, executable, features, licenses, local, message, output_licenses, output_to_bindir, restricted_to, tags, target_compatible_with, testonly, toolchains, tools, visibility)
A genrule generates one or more files using a user-defined Bash command.
Genrules are generic build rules that you can use if there's no specific rule for the task.
For example, you could run a Bash one-liner. If however you need to compile C++ files, stick
to the existing cc_* rules, because all the heavy lifting has already been done
for you.
Note that genrule requires a shell to interpret the command argument. It is also easy to reference arbitrary programs available on the PATH, however this makes the command non-hermetic and may not be reproducible. If you only need to run a single tool, consider using run_binary instead.
Like every other action, the action created by genrules should not assume anything about their
working directory; all Bazel guarantees is that their declared inputs will be available at the
path that $(location) returns for their label. For example, if the action is run in a
sandbox or remotely, the implementation of the sandbox or the remote execution will determine the
working directory. If run directly (using the standalone strategy), the working
directory will be the execution root, i.e. the result of bazel info execution_root.
Do not use a genrule for running tests. There are special dispensations for tests and test
results, including caching policies and environment variables. Tests generally need to be run
after the build is complete and on the target architecture, whereas genrules are executed during
the build and on the exec architecture (the two may be different). If you need a general purpose
testing rule, use sh_test.
Cross-compilation Considerations
See the user manual for more info about cross-compilation.
While genrules run during a build, their outputs are often used after the build, for deployment or testing. Consider the example of compiling C code for a microcontroller: the compiler accepts C source files and generates code that runs on a microcontroller. The generated code obviously cannot run on the CPU that was used for building it, but the C compiler (if compiled from source) itself has to.
The build system uses the exec configuration to describe the machine(s) on which the build runs and the target configuration to describe the machine(s) on which the output of the build is supposed to run. It provides options to configure each of these and it segregates the corresponding files into separate directories to avoid conflicts.
For genrules, the build system ensures that dependencies are built appropriately:
srcs are built (if necessary) for the target configuration,
tools are built for the exec configuration, and the output is considered to
be for the target configuration. It also provides
"Make" variables that genrule commands can pass to the corresponding tools.
It is intentional that genrule defines no deps attribute: other built-in rules use
language-dependent meta information passed between the rules to automatically determine how to
handle dependent rules, but this level of automation is not possible for genrules. Genrules work
purely at the file and runfiles level.
Special Cases
Exec-exec compilation: in some cases, the build system needs to run genrules such that the
output can also be executed during the build. If for example a genrule builds some custom compiler
which is subsequently used by another genrule, the first one has to produce its output for the
exec configuration, because that's where the compiler will run in the other genrule. In this case,
the build system does the right thing automatically: it builds the srcs and
outs of the first genrule for the exec configuration instead of the target
configuration. See the user manual for more
info.
JDK & C++ Tooling: to use a tool from the JDK or the C++ compiler suite, the build system provides a set of variables to use. See "Make" variable for details.
Genrule Environment
The genrule command is executed by a Bash shell that is configured to fail when a command
or a pipeline fails, using set -e -o pipefail.
The build tool executes the Bash command in a sanitized process environment that
defines only core variables such as PATH, PWD,
TMPDIR, and a few others.
To ensure that builds are reproducible, most variables defined in the user's shell
environment are not passed though to the genrule's command. However, Bazel (but not
Blaze) passes through the value of the user's PATH environment variable.
Any change to the value of PATH will cause Bazel to re-execute the command
on the next build.
A genrule command should not access the network except to connect processes that are children of the command itself, though this is not currently enforced.
The build system automatically deletes any existing output files, but creates any necessary parent directories before it runs a genrule. It also removes any output files in case of a failure.
General Advice
- Do ensure that tools run by a genrule are deterministic and hermetic. They should not write timestamps to their output, and they should use stable ordering for sets and maps, as well as write only relative file paths to the output, no absolute paths. Not following this rule will lead to unexpected build behavior (Bazel not rebuilding a genrule you thought it would) and degrade cache performance.
- Do use
$(location)extensively, for outputs, tools and sources. Due to the segregation of output files for different configurations, genrules cannot rely on hard-coded and/or absolute paths. - Do write a common Starlark macro in case the same or very similar genrules are used in multiple places. If the genrule is complex, consider implementing it in a script or as a Starlark rule. This improves readability as well as testability.
- Do make sure that the exit code correctly indicates success or failure of the genrule.
- Do not write informational messages to stdout or stderr. While useful for debugging, this can easily become noise; a successful genrule should be silent. On the other hand, a failing genrule should emit good error messages.
$$evaluates to a$, a literal dollar-sign, so in order to invoke a shell command containing dollar-signs such asls $(dirname $x), one must escape it thus:ls $$(dirname $$x).- Avoid creating symlinks and directories. Bazel doesn't copy over the directory/symlink structure created by genrules and its dependency checking of directories is unsound.
- When referencing the genrule in other rules, you can use either the genrule's label or the
labels of individual output files. Sometimes the one approach is more readable, sometimes the
other: referencing outputs by name in a consuming rule's
srcswill avoid unintentionally picking up other outputs of the genrule, but can be tedious if the genrule produces many outputs.
Examples
This example generates foo.h. There are no sources, because the command doesn't take
any input. The "binary" run by the command is a perl script in the same package as the genrule.
genrule(
name = "foo",
srcs = [],
outs = ["foo.h"],
cmd = "./$(location create_foo.pl) > \"$@\"",
tools = ["create_foo.pl"],
)
The following example shows how to use a filegroup
and the outputs of another genrule. Note that using $(SRCS) instead
of explicit $(location) directives would also work; this example uses the latter for
sake of demonstration.
genrule(
name = "concat_all_files",
srcs = [
"//some:files", # a filegroup with multiple files in it ==> $(locations)
"//other:gen", # a genrule with a single output ==> $(location)
],
outs = ["concatenated.txt"],
cmd = "cat $(locations //some:files) $(location //other:gen) > $@",
)
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. You may refer to this rule by name in the srcs or deps section of other BUILD
rules. If the rule generates source files, you should use the
srcs attribute.
|
srcs
|
List of labels; default is
This attributes is not suitable to list tools executed by the
The build system ensures these prerequisites are built before running the genrule
command; they are built using the same configuration as the original build request. The
names of the files of these prerequisites are available to the command as a
space-separated list in |
outs
|
List of filenames; nonconfigurable; required A list of files generated by this rule.Output files must not cross package boundaries. Output filenames are interpreted as relative to the package.
If the
The genrule command is expected to create each output file at a predetermined location.
The location is available in |
cmd
|
String; default is $(location)
and "Make" variable substitution.
cmd_bash, cmd_ps and cmd_bat,
if none of them are applicable.
If the command line length exceeds the platform limit (64K on Linux/macOS, 8K on Windows),
then genrule will write the command to a script and execute that script to work around. This
applies to all cmd attributes ( |
cmd_bash
|
String; default is This attribute has higher priority than |
cmd_bat
|
String; default is This attribute has higher priority than
|
cmd_ps
|
String; default is This attribute has higher priority than
To make Powershell easier to use and less error-prone, we run the following commands to set up the environment before executing Powershell command in genrule.
|
executable
|
Boolean; nonconfigurable; default is
Setting this flag to True means the output is an executable file and can be run using the
Declaring data dependencies for the generated executable is not supported. |
local
|
Boolean; default is
If set to True, this option forces this
This is equivalent to providing 'local' as a tag ( |
message
|
String; default is
A progress message that will be printed as this build step is executed. By default, the
message is "Generating output" (or something equally bland) but you may provide a
more specific one. Use this attribute instead of |
output_licenses
|
Licence type; default is common attributes
|
output_to_bindir
|
Boolean; nonconfigurable; default is
If set to True, this option causes output files to be written into the |
tools
|
List of labels; default is
The build system ensures these prerequisites are built before running the genrule command;
they are built using the exec
configuration, since these tools are executed as part of the build. The path of an
individual
Any |
starlark_doc_extract
View rule sourcestarlark_doc_extract(name, deps, src, data, compatible_with, deprecation, distribs, exec_compatible_with, exec_properties, features, licenses, render_main_repo_name, restricted_to, symbol_names, tags, target_compatible_with, testonly, visibility)
starlark_doc_extract() extracts documentation for rules, functions (including
macros), aspects, and providers defined or re-exported in a given .bzl or
.scl file. The output of this rule is a ModuleInfo binary proto as defined
in
stardoc_output.proto
in the Bazel source tree.
Implicit output targets
name.binaryproto(the default output): AModuleInfobinary proto.name.textproto(only built if explicitly requested): the text proto version ofname.binaryproto.
Warning: the output format of this rule is not guaranteed to be stable. It is intended mainly for internal use by Stardoc.
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
deps
|
List of labels; default is load()-ed by
src. These targets should under normal usage be
bzl_library
targets, but the starlark_doc_extract rule does not enforce that, and accepts
any target which provides Starlark files in its DefaultInfo.
Note that the wrapped Starlark files must be files in the source tree; Bazel cannot
|
src
|
Label; required A Starlark file from which to extract documentation.Note that this must be a file in the source tree; Bazel cannot |
render_main_repo_name
|
Boolean; default is //foo:bar.bzl will be emitted as
@main_repo_name//foo:bar.bzl).
The name to use for the main repository is obtained from This attribute should be set to |
symbol_names
|
List of strings; default is
|
test_suite
View rule sourcetest_suite(name, compatible_with, deprecation, distribs, features, licenses, restricted_to, tags, target_compatible_with, testonly, tests, visibility)
A test_suite defines a set of tests that are considered "useful" to humans. This
allows projects to define sets of tests, such as "tests you must run before checkin", "our
project's stress tests" or "all small tests." The bazel test command respects this sort
of organization: For an invocation like bazel test //some/test:suite, Bazel first
enumerates all test targets transitively included by the //some/test:suite target (we
call this "test_suite expansion"), then Bazel builds and tests those targets.
Examples
A test suite to run all of the small tests in the current package.
test_suite(
name = "small_tests",
tags = ["small"],
)
A test suite that runs a specified set of tests:
test_suite(
name = "smoke_tests",
tests = [
"system_unittest",
"public_api_unittest",
],
)
A test suite to run all tests in the current package which are not flaky.
test_suite(
name = "non_flaky_test",
tags = ["-flaky"],
)
Arguments
| Attributes | |
|---|---|
name |
Name; required A unique name for this target. |
tags
|
List of strings; nonconfigurable; default is Tags which begin with a "-" character are considered negative tags. The preceding "-" character is not considered part of the tag, so a suite tag of "-small" matches a test's "small" size. All other tags are considered positive tags. Optionally, to make positive tags more explicit, tags may also begin with the "+" character, which will not be evaluated as part of the text of the tag. It merely makes the positive and negative distinction easier to read. Only test rules that match all of the positive tags and none of the negative tags will be included in the test suite. Note that this does not mean that error checking for dependencies on tests that are filtered out is skipped; the dependencies on skipped tests still need to be legal (e.g. not blocked by visibility constraints).
The
Note that a test's
If you need a |
tests
|
List of labels; nonconfigurable; default is
Any
If the |