A User's Guide to Bazel
To run Bazel, go to
your base workspace directory
or any of its subdirectories and type
% bazel help [Bazel release bazel-<version>] Usage: bazel <command> <options> ... Available commands: analyze-profile Analyzes build profile data. aquery Executes a query on the post-analysis action graph. build Builds the specified targets. canonicalize-flags Canonicalize Bazel flags. clean Removes output files and optionally stops the server. cquery Executes a post-analysis dependency graph query. dump Dumps the internal state of the Bazel server process. help Prints help for commands, or the index. info Displays runtime info about the bazel server. fetch Fetches all external dependencies of a target. mobile-install Installs apps on mobile devices. query Executes a dependency graph query. run Runs the specified target. shutdown Stops the Bazel server. test Builds and runs the specified test targets. version Prints version information for Bazel. Getting more help: bazel help <command> Prints help and options for <command>. bazel help startup_options Options for the JVM hosting Bazel. bazel help target-syntax Explains the syntax for specifying targets. bazel help info-keys Displays a list of keys used by the info command.
bazel tool performs many functions, called
commands; users of CVS and Subversion will be familiar
with this "Swiss army knife" arrangement. The most commonly used one is of
bazel build. You can browse the online help
The Bazel system is implemented as a long-lived server process.
This allows it to perform many optimizations not possible with a
batch-oriented implementation, such as caching of BUILD files,
dependency graphs, and other metadata from one build to the
next. This improves the speed of incremental builds, and allows
different commands, such as
query to share the same cache of loaded packages,
making queries very fast.
When you run
bazel, you're running the client. The
client finds the server based on the output base, which by default is
determined by the path of the base workspace directory and your
userid, so if you build in multiple workspaces, you'll have multiple
output bases and thus multiple Bazel server processes. Multiple
users on the same workstation can build concurrently in the same
workspace because their output bases will differ (different userids).
If the client cannot find a running server instance, it starts a new
one. The server process will stop after a period of inactivity (3 hours,
by default, which can be modified using the startup option
For the most part, the fact that there is a server running is invisible to the user, but sometimes it helps to bear this in mind. For example, if you're running scripts that perform a lot of automated builds in different directories, it's important to ensure that you don't accumulate a lot of idle servers; you can do this by explicitly shutting them down when you're finished with them, or by specifying a short timeout period.
The name of a Bazel server process appears in the output of
ps -e f as
bazel(dirname), where dirname is the
basename of the directory enclosing the root of your workspace directory.
% ps -e f 16143 ? Sl 3:00 bazel(src-johndoe2) -server -Djava.library.path=...
This makes it easier to find out which server process belongs to a
given workspace. (Beware that with certain other options
ps, Bazel server processes may be named just
java.) Bazel servers can be stopped using
the shutdown command.
bazel, the client first checks that the
server is the appropriate version; if not, the server is stopped and
a new one started. This ensures that the use of a long-running
server process doesn't interfere with proper versioning.
.bazelrc, the Bazel configuration file,
--bazelrc=file option, and
Bazel accepts many options. Typically, some of these are varied
frequently (for example,
--subcommands) while others stay the
same across several builds (e.g.
To avoid having to specify these unchanged options for every build (and other commands)
Bazel allows you to specify options in a configuration file.
Bazel looks for an optional configuration file in the following locations, in order. It will stop searching once it has successfully found a file.
The path specified by the
--bazelrc=filestartup option. If specified, this option must appear before the command name (e.g.
A file named
.bazelrcin your base workspace directory
A file named
.bazelrcin your home directory
--bazelrc=/dev/null effectively disables the
use of a configuration file. We strongly recommend that you use
this option when performing release builds, or automated tests that
Aside from the optional configuration file described above, Bazel also looks
for a master rc file named
bazel.bazelrc next to the binary, in
the workspace at
tools/bazel.rc or system-wide at
/etc/bazel.bazelrc. These files are here to support
installation-wide options or options shared between users. These files do not
override one another; if all of these files exist, all of them will be loaded.
Reading of these files can be disabled using the
.bazelrc syntax and semantics
Like all UNIX "rc" files, the
.bazelrc file is a text file with
a line-based grammar. Lines starting
# are considered comments
and are ignored, as are blank lines. Each line contains a sequence of words,
which are tokenized according to the same rules as the Bourne shell.
Lines that start with
import are special: if Bazel encounters such
a line in a
.bazelrc file, it parses the contents of the file
referenced by the import statement, too. Options specified in an imported file
take precedence over options specified before the import statement. Options
specified after the import statement take precedence over the options in the
imported file. Options in files imported later take precedence over files
imported earlier. To specify a path that is relative to the workspace root,
Most lines of a bazelrc define default option values. The first word on each line specifies when these defaults are applied:
startup: startup options, which go before the command, and are described in
bazel help startup_options.
common: options that apply to all Bazel commands.
command: Bazel command, such as
queryto which the options apply. These options also apply to all commands that inherit from the specified command. (For example,
Each of these lines may be used more than once and the arguments that follow
the first word are combined as if they had appeared on a single line.
(Users of CVS, another tool with a "Swiss army knife" command-line interface,
will find the syntax similar to that of
Options specified in the command line always take precedence over those from
a configuration file. Within the configuration file, precedence is
given by specificity. This means that lines for a more specific command take
precedence over lines for a less specific command, with
getting lowest precedence (for example, the
test command inherits
all the options from the
build command, so the line
test --foo=bar takes precedence over the line
build --foo=baz, regardless of which rc file or what order
these two lines are in). Two lines specifying options for the same command at
equal specificity are parsed in the order in which they appear within the file.
The user-specific configuration file takes precedence over the master file.
Because this precedence rule does not match the file order, we recommend
that the file follows the same order, with
common options at the
top, and most-specific commands near the bottom. This way, the order in which
the options are read is the same as the order in which they are applied,
which is more intuitive.
The arguments specified on a line of an rc file may include arguments that are not options, such as the names of build targets, and so on. These, like the options specified in the same files, have lower precedence than their siblings on the command line, and are always prepended to the explicit list of non- option arguments.
In addition to setting option defaults, the rc file can be used to group
options and provide a shorthand for common groupings. This is done by adding
:name suffix to the command. These options are ignored by
default, but will be included when the option
--config=name is present, either on the command line
or in a
.bazelrc file, recursively, even inside of another
config definition. The options specified by
only be expanded for applicable commands, in the precedence order described
Note that configs can be defined in any
.bazelrc file, and that
all lines of the form
command:name (for applicable commands)
will be expanded, across the different rc files. In order to avoid name
conflicts, we suggest that configs defined in personal rc files start
with an underscore ('_') to avoid unintentional name sharing.
--config=foo expands to the options defined in the rc files
"in-place" so that the options specified for the config have the same
precedence that the
--config=foo option had.
Here's an example
# Bob's Bazel option defaults startup --host_jvm_args=-XX:-UseParallelGC import /home/bobs_project/bazelrc build --show_timestamps --keep_going --jobs 600 build --color=yes query --keep_going # Definition of --config=memcheck build:memcheck --strip=never --test_timeout=3600
Building programs with Bazel
The most important function of Bazel is, of course, building code. Type
bazel build followed by the name of the
target you wish to build. Here's a typical
% bazel build //foo ____Loading package: foo ____Loading package: bar ____Loading package: baz ____Loading complete. Analyzing... ____Building 1 target... ____[0 / 3] Executing Genrule //bar:helper_rule ____[1 / 3] Executing Genrule //baz:another_helper_rule ____[2 / 3] Building foo/foo.bin Target //foo:foo up-to-date: bazel-bin/foo/foo.bin bazel-bin/foo/foo ____Elapsed time: 9.905s
Bazel prints the progress messages as it loads all the packages in the transitive closure of dependencies of the requested target, then analyzes them for correctness and to create the build actions, finally executing the compilers and other tools of the build.
Bazel prints progress messages during the execution phase of the build, showing the current build step (compiler, linker, etc.) that is being started, and the number completed over the total number of build actions. As the build starts the number of total actions will often increase as Bazel discovers the entire action graph, but the number will usually stabilize within a few seconds.
At the end of the build Bazel
prints which targets were requested, whether or not they were
successfully built, and if so, where the output files can be found.
Scripts that run builds can reliably parse this output; see
--show_result for more
Typing the same command again:
% bazel build //foo ____Loading... ____Found 1 target... ____Building complete. Target //foo:foo up-to-date: bazel-bin/foo/foo.bin bazel-bin/foo/foo ____Elapsed time: 0.280s
we see a "null" build: in this case, there are no packages to re-load, since nothing has changed, and no build steps to execute. (If something had changed in "foo" or some of its dependencies, resulting in the reexecution of some build actions, we would call it an "incremental" build, not a "null" build.)
Before you can start a build, you will need a Bazel workspace. This is simply a directory tree that contains all the source files needed to build your application. Bazel allows you to perform a build from a completely read-only volume.
Setting up a
Bazel finds its packages by searching the package path. This is a colon separated ordered list of bazel directories, each being the root of a partial source tree.
To specify a custom package path using the
% bazel build --package_path %workspace%:/some/other/root
Package path elements may be specified in three formats:
If the first character is
/, the path is absolute.
If the path starts with
%workspace%, the path is taken relative to the nearest enclosing bazel directory.
For instance, if your working directory is
/home/bob/clients/bob_client/bazel/foo, then the string
%workspace%in the package-path is expanded to
Anything else is taken relative to the working directory.
This is usually not what you mean to do, and may behave unexpectedly if you use Bazel from directories below the bazel workspace. For instance, if you use the package-path element
., and then cd into the directory
/home/bob/clients/bob_client/bazel/foo, packages will be resolved from the
If you use a non-default package path, we recommend that you specify it in your Bazel configuration file for convenience.
Bazel doesn't require any packages to be in the current directory, so you can do a build from an empty bazel workspace if all the necessary packages can be found somewhere else on the package path.
Example: Building from an empty client
% mkdir -p foo/bazel % cd foo/bazel % touch WORKSPACE % bazel build --package_path /some/other/path //foo
Specifying targets to build
Bazel allows a number of ways to specify the targets to be built. Collectively, these are known as target patterns. The on-line help displays a summary of supported patterns:
% bazel help target-syntax Target pattern syntax ===================== The BUILD file label syntax is used to specify a single target. Target patterns generalize this syntax to sets of targets, and also support working-directory-relative forms, recursion, subtraction and filtering. Examples: Specifying a single target: //foo/bar:wiz The single target '//foo/bar:wiz'. foo/bar/wiz Equivalent to: '//foo/bar/wiz:wiz' if foo/bar/wiz is a package, '//foo/bar:wiz' if foo/bar is a package, '//foo:bar/wiz' otherwise. //foo/bar Equivalent to '//foo/bar:bar'. Specifying all rules in a package: //foo/bar:all Matches all rules in package 'foo/bar'. Specifying all rules recursively beneath a package: //foo/...:all Matches all rules in all packages beneath directory 'foo'. //foo/... (ditto) By default, directory symlinks are followed when performing this recursive traversal, except those that point to under the output base (for example, the convenience symlinks that are created in the root directory of the workspace) But we understand that your workspace may intentionally contain directories with unusual symlink structures that you don't want consumed. As such, if a directory has a file named 'DONT_FOLLOW_SYMLINKS_WHEN_TRAVERSING_THIS_DIRECTORY_VIA_A_RECURSIVE_TARGET_PATTERN' then symlinks in that directory won't be followed when evaluating recursive target patterns. Working-directory relative forms: (assume cwd = 'workspace/foo') Target patterns which do not begin with '//' are taken relative to the working directory. Patterns which begin with '//' are always absolute. ...:all Equivalent to '//foo/...:all'. ... (ditto) bar/...:all Equivalent to '//foo/bar/...:all'. bar/... (ditto) bar:wiz Equivalent to '//foo/bar:wiz'. :foo Equivalent to '//foo:foo'. bar Equivalent to '//foo/bar:bar'. foo/bar Equivalent to '//foo/foo/bar:bar'. bar:all Equivalent to '//foo/bar:all'. :all Equivalent to '//foo:all'. Summary of target wildcards: :all, Match all rules in the specified packages. :*, :all-targets Match all targets (rules and files) in the specified packages, including ones not built by default, such as _deploy.jar files. Subtractive patterns: Target patterns may be preceded by '-', meaning they should be subtracted from the set of targets accumulated by preceding patterns. (Note that this means order matters.) For example: % bazel build -- foo/... -foo/contrib/... builds everything in 'foo', except 'contrib'. In case a target not under 'contrib' depends on something under 'contrib' though, in order to build the former bazel has to build the latter too. As usual, the '--' is required to prevent '-f' from being interpreted as an option. When running the test command, test suite expansion is applied to each target pattern in sequence as the set of targets is evaluated. This means that individual tests from a test suite can be excluded by a later target pattern. It also means that an exclusion target pattern which matches a test suite will exclude all tests which that test suite references. (Targets that would be matched by the list of target patterns without any test suite expansion are also built unless --build_tests_only is set.)
Whereas labels are used to specify individual targets, e.g. for declaring dependencies in BUILD files, Bazel's target patterns are a syntax for specifying multiple targets: they are a generalization of the label syntax for sets of targets, using wildcards. In the simplest case, any valid label is also a valid target pattern, identifying a set of exactly one target.
foo/... is a wildcard over packages,
indicating all packages recursively beneath
foo (for all roots of the package
:all is a wildcard
over targets, matching all rules within a package. These two may be
combined, as in
foo/...:all, and when both wildcards
are used, this may be abbreviated to
:all-targets) is a
wildcard that matches every target in the matched packages,
including files that aren't normally built by any rule, such
_deploy.jar files associated
This implies that
:* denotes a superset
:all; while potentially confusing, this syntax does
allow the familiar
:all wildcard to be used for
typical builds, where building targets like the
is not desired.
In addition, Bazel allows a slash to be used instead of the colon
required by the label syntax; this is often convenient when using
Bash filename expansion. For example,
//foo/bar:wiz (if there is a
foo/bar) or to
there is a package
Many Bazel commands accept a list of target patterns as arguments,
and they all honor the prefix negation operator `
This can be used to subtract a set of targets from the set specified
by the preceding arguments. (Note that this means order matters.)
bazel build foo/... bar/...
means "build all
foo and all targets
bazel build -- foo/... -foo/bar/...
means "build all targets beneath
-- argument is required to prevent the subsequent
arguments starting with
- from being interpreted as
It's important to point out though that subtracting targets this way will not
guarantee that they are not built, since they may be dependencies of targets
that weren't subtracted. For example, if there were a target
//foo:all-apis that among others depended on
//foo/bar:api, then the latter would be built as part of
building the former.
tags=["manual"] will not be included in wildcard target patterns (...,
:*, :all, etc). You should specify such test targets with explicit target patterns on the command
line if you want Bazel to build/test them.
Fetching external dependencies
By default, Bazel will download and symlink external dependencies during the
build. However, this can be undesirable, either because you'd like to know
when new external dependendencies are added or because you'd like to
"prefetch" dependencies (say, before a flight where you'll be offline). If you
would like to prevent new dependencies from being added during builds, you
can specify the
--fetch=false flag. Note that this flag only
applies to repository rules that do not point to a directory in the local
file system. Changes, for example, to
new_local_repository and Android SDK and NDK repository rules
will always take effect regardless of the value
If you disallow fetching during builds and Bazel finds new external dependencies, your build will fail.
You can manually fetch dependencies by running
bazel fetch. If
you disallow during-build fetching, you'll need to run
- Before you build for the first time.
- After you add a new external dependency.
fetch takes a list of targets to fetch dependencies for. For
example, this would fetch dependencies needed to build
$ bazel fetch //foo:bar //bar:baz
To fetch all external dependencies for a workspace, run:
$ bazel fetch //...
You do not need to run bazel fetch at all if you have all of the tools you are
using (from library jars to the JDK itself) under your workspace root.
However, if you're using anything outside of the workspace directory then Bazel
will automatically run
bazel fetch before running
Build configurations and cross-compilation
All the inputs that specify the behavior and result of a given build can be divided into two distinct categories. The first kind is the intrinsic information stored in the BUILD files of your project: the build rule, the values of its attributes, and the complete set of its transitive dependencies. The second kind is the external or environmental data, supplied by the user or by the build tool: the choice of target architecture, compilation and linking options, and other toolchain configuration options. We refer to a complete set of environmental data as a configuration.
In any given build, there may be more than one configuration.
Consider a cross-compile, in which you build
//foo:bin executable for a 64-bit architecture,
but your workstation is a 32-bit machine. Clearly, the build
will require building
//foo:bin using a toolchain
capable of creating 64-bit executables, but the build system must
also build various tools used during the build itself—for example
tools that are built from source, then subsequently used in, say, a
genrule—and these must be built to run on your workstation.
Thus we can identify two configurations: the host
configuration, which is used for building tools that run during
the build, and the target configuration (or request
configuration, but we say "target configuration" more often even
though that word already has many meanings), which is
used for building the binary you ultimately requested.
Typically, there are many libraries that are prerequisites of both
the requested build target (
//foo:bin) and one or more of
the host tools, for example some base libraries. Such libraries must be built
twice, once for the host configuration, and once for the target
Bazel takes care of ensuring that both variants are built, and that the derived files are kept separate to avoid interference; usually such targets can be built concurrently, since they are independent of each other. If you see progress messages indicating that a given target is being built twice, this is most likely the explanation.
Bazel uses one of two ways to select the host configuration, based
--distinct_host_configuration option. This
boolean option is somewhat subtle, and the setting may improve (or
worsen) the speed of your builds.
When this option is false, the host and request configurations are identical: all tools required during the build will be built in exactly the same way as target programs. This setting means that no libraries need to be built twice during a single build, so it keeps builds short. However, it does mean that any change to your request configuration also affects your host configuration, causing all the tools to be rebuilt, and then anything that depends on the tool output to be rebuilt too. Thus, for example, simply changing a linker option between builds might cause all tools to be re-linked, and then all actions using them reexecuted, and so on, resulting in a very large rebuild. Also, please note: if your host architecture is not capable of running your target binaries, your build will not work.
If you frequently make changes to your request configuration, such
as alternating between
-c opt and
builds, or between simple- and cross-compilation, we do not
recommend this option, as you will typically rebuild the majority of
your codebase each time you switch.
If this option is true, then instead of using the same configuration for the host and request, a completely distinct host configuration is used. The host configuration is derived from the target configuration as follows:
- Use the same version of Crosstool
--crosstool_top) as specified in the request configuration, unless
Use the value of
- Use the same values of these options as specified in the request
--host_crosstool_topis used, then the value of
--host_cpuis used to look up a
default_toolchainin the Crosstool (ignoring
--compiler) for the host configuration.
Use the value of
Use the value of
- Use optimized builds for C++ code (
- Generate no debugging information (
- Strip debug information from executables and shared libraries
- Place all derived files in a special location, distinct from that used by any possible request configuration.
- Suppress stamping of binaries with build data
- All other values remain at their defaults.
There are many reasons why it might be preferable to select a distinct host configuration from the request configuration. Some are too esoteric to mention here, but two of them are worth pointing out.
Firstly, by using stripped, optimized binaries, you reduce the time spent linking and executing the tools, the disk space occupied by the tools, and the network I/O time in distributed builds.
Secondly, by decoupling the host and request configurations in all builds, you avoid very expensive rebuilds that would result from minor changes to the request configuration (such as changing a linker options does), as described earlier.
That said, for certain builds, this option may be a hindrance. In particular, builds in which changes of configuration are infrequent (especially certain Java builds), and builds where the amount of code that must be built in both host and target configurations is large, may not benefit.
Correct incremental rebuilds
One of the primary goals of the Bazel project is to ensure correct incremental rebuilds. Previous build tools, especially those based on Make, make several unsound assumptions in their implementation of incremental builds.
Firstly, that timestamps of files increase monotonically. While this is the typical case, it is very easy to fall afoul of this assumption; syncing to an earlier revision of a file causes that file's modification time to decrease; Make-based systems will not rebuild.
More generally, while Make detects changes to files, it does
not detect changes to commands. If you alter the options passed to
the compiler in a given build step, Make will not re-run the
compiler, and it is necessary to manually discard the invalid
outputs of the previous build using
Also, Make is not robust against the unsuccessful termination of one of its subprocesses after that subprocess has started writing to its output file. While the current execution of Make will fail, the subsequent invocation of Make will blindly assume that the truncated output file is valid (because it is newer than its inputs), and it will not be rebuilt. Similarly, if the Make process is killed, a similar situation can occur.
Bazel avoids these assumptions, and others. Bazel maintains a database of all work previously done, and will only omit a build step if it finds that the set of input files (and their timestamps) to that build step, and the compilation command for that build step, exactly match one in the database, and, that the set of output files (and their timestamps) for the database entry exactly match the timestamps of the files on disk. Any change to the input files or output files, or to the command itself, will cause re-execution of the build step.
The benefit to users of correct incremental builds is: less time
wasted due to confusion. (Also, less time spent waiting for
rebuilds caused by use of
make clean, whether necessary
Build consistency and incremental builds
Formally, we define the state of a build as consistent when all the expected output files exist, and their contents are correct, as specified by the steps or rules required to create them. When you edit a source file, the state of the build is said to be inconsistent, and remains inconsistent until you next run the build tool to successful completion. We describe this situation as unstable inconsistency, because it is only temporary, and consistency is restored by running the build tool.
There is another kind of inconsistency that is pernicious: stable
inconsistency. If the build reaches a stable inconsistent
state, then repeated successful invocation of the build tool does
not restore consistency: the build has gotten "stuck", and the
outputs remain incorrect. Stable inconsistent states are the main
reason why users of Make (and other build tools) type
clean. Discovering that the build tool has failed in this
manner (and then recovering from it) can be time consuming and very
Conceptually, the simplest way to achieve a consistent build is to throw away all the previous build outputs and start again: make every build a clean build. This approach is obviously too time-consuming to be practical (except perhaps for release engineers), and therefore to be useful, the build tool must be able to perform incremental builds without compromising consistency.
Correct incremental dependency analysis is hard, and as described above, many other build tools do a poor job of avoiding stable inconsistent states during incremental builds. In contrast, Bazel offers the following guarantee: after a successful invocation of the build tool during which you made no edits, the build will be in a consistent state. (If you edit your source files during a build, Bazel makes no guarantee about the consistency of the result of the current build. But it does guarantee that the results of the next build will restore consistency.)
As with all guarantees, there comes some fine print: there are some known ways of getting into a stable inconsistent state with Bazel. We won't guarantee to investigate such problems arising from deliberate attempts to find bugs in the incremental dependency analysis, but we will investigate and do our best to fix all stable inconsistent states arising from normal or "reasonable" use of the build tool.
If you ever detect a stable inconsistent state with Bazel, please report a bug.
Bazel uses sandboxes to guarantee that actions run hermetically1 and correctly.
Bazel runs Spawns (loosely speaking: actions) in sandboxes that only contain the minimal
set of files the tool requires to do its job. Currently sandboxing works on Linux 3.12 or newer
CONFIG_USER_NS option enabled, and also on macOS 10.11 or newer.
Bazel will print a warning if your system does not support sandboxing to alert you to the fact
that builds are not guaranteed to be hermetic and might affect the host system in unknown ways.
To disable this warning you can pass the
--ignore_unsupported_sandboxing flag to
On some platforms such as Google Kubernetes
Engine cluster nodes or Debian, user namespaces are deactivated by default due to security
concerns. This can be checked by looking at the file
/proc/sys/kernel/unprivileged_userns_clone: if it exists and contains a 0, then
user namespaces can be activated with
sudo sysctl kernel.unprivileged_userns_clone=1.
In some cases, the Bazel sandbox fails to execute rules because of the system setup. The symptom
is generally a failure that output a message similar to
namespace-sandbox.c:633: execvp(argv, argv): No such file or directory. In that
case, try to deactivate the sandbox for genrules with
and for other rules with
--spawn_strategy=standalone. Also please report a bug on our
issue tracker and mention which Linux distribution you're using so that we can investigate and
provide a fix in a subsequent release.
1: Hermeticity means that the action only uses its declared input files and no other files in the filesystem, and it only produces its declared output files.
Deleting the outputs of a build
Bazel has a
clean command, analogous to that of Make.
It deletes the output directories for all build configurations performed
by this Bazel instance, or the entire working tree created by this
Bazel instance, and resets internal caches. If executed without any
command-line options, then the output directory for all configurations
will be cleaned.
Recall that each Bazel instance is associated with a single workspace, thus the
clean command will delete all outputs from all builds you've done
with that Bazel instance in that workspace.
To completely remove the entire working tree created by a Bazel
instance, you can specify the
--expunge option. When
--expunge, the clean command simply
removes the entire output base tree which, in addition to the build
output, contains all temp files created by Bazel. It also
stops the Bazel server after the clean, equivalent to the
shutdown command. For example, to
clean up all disk and memory traces of a Bazel instance, you could
% bazel clean --expunge
Alternatively, you can expunge in the background by using
--expunge_async. It is safe to invoke a Bazel command
in the same client while the asynchronous expunge continues to run.
Note, however, that this may introduce IO contention.
clean command is provided primarily as a means of
reclaiming disk space for workspaces that are no longer needed.
However, we recognize that Bazel's incremental rebuilds might not be
clean may be used to recover a consistent
state when problems arise.
Bazel's design is such that these problems are fixable; we consider
such bugs a high priority, and will do our best fix them. If you
ever find an incorrect incremental build, please file a bug report.
We encourage developers to get out of the habit of
clean and into that of reporting bugs in the
Phases of a build
In Bazel, a build occurs in three distinct phases; as a user, understanding the difference between them provides insight into the options which control a build (see below).
The first is loading during which all the necessary BUILD files for the initial targets, and their transitive closure of dependencies, are loaded, parsed, evaluated and cached.
For the first build after a Bazel server is started, the loading phase typically takes many seconds as many BUILD files are loaded from the file system. In subsequent builds, especially if no BUILD files have changed, loading occurs very quickly.
Errors reported during this phase include: package not found, target not found, lexical and grammatical errors in a BUILD file, and evaluation errors.
The second phase, analysis, involves the semantic analysis and validation of each build rule, the construction of a build dependency graph, and the determination of exactly what work is to be done in each step of the build.
Like loading, analysis also takes several seconds when computed in its entirety. However, Bazel caches the dependency graph from one build to the next and only reanalyzes what it has to, which can make incremental builds extremely fast in the case where the packages haven't changed since the previous build.
Errors reported at this stage include: inappropriate dependencies, invalid inputs to a rule, and all rule-specific error messages.
The loading and analysis phases are fast because Bazel avoids unnecessary file I/O at this stage, reading only BUILD files in order to determine the work to be done. This is by design, and makes Bazel a good foundation for analysis tools, such as Bazel's query command, which is implemented atop the loading phase.
The third and final phase of the build is execution. This phase ensures that the outputs of each step in the build are consistent with its inputs, re-running compilation/linking/etc. tools as necessary. This step is where the build spends the majority of its time, ranging from a few seconds to over an hour for a large build. Errors reported during this phase include: missing source files, errors in a tool executed by some build action, or failure of a tool to produce the expected set of outputs.
The following sections describe the options available during a
--long is used on a help command, the on-line
help messages provide summary information about the meaning, type and
default value for each option.
Most options can only be specified once. When specified multiple times, the last instance wins. Options that can be specified multiple times are identified in the on-line help with the text 'may be used multiple times'.
Options that affect how packages are located
This option specifies the set of directories that are searched to find the BUILD file for a given package.
This option specifies a comma-separated list of packages which Bazel should consider deleted, and not attempt to load from any directory on the package path. This can be used to simulate the deletion of packages without actually deleting them.
Error checking options
These options control Bazel's error-checking and/or warnings.
This option takes an argument that specifies which constraint should be checked.
Bazel performs special checks on each rule that is annotated with the given constraint.
The supported constraints and their checks are as follows:
public: Verify that all java_libraries marked with
constraints = ['public']only depend on java_libraries that are marked as
constraints = ['public']too. If bazel finds a dependency that does not conform to this rule, bazel will issue an error.
If this option is set to false, visibility checks are demoted to warnings. The default value of this option is true, so that by default, visibility checking is done.
--output_filter option will only show build and compilation
warnings for targets that match the regular expression. If a target does not
match the given regular expression and its execution succeeds, its standard
output and standard error are thrown away.
Here are some typical values for this option:
||Show the output for the specified packages.|
||Don't show output for the specified packages.|
These options control which options Bazel will pass to other tools.
This option takes an argument which is to be passed to the compiler. The argument will be passed to the compiler whenever it is invoked for preprocessing, compiling, and/or assembling C, C++, or assembler code. It will not be passed when linking.
This option can be used multiple times. For example:
% bazel build --copt="-g0" --copt="-fpic" //foo
will compile the
foo library without debug tables, generating
Note that changing
--copt settings will force a recompilation
of all affected object files. Also note that copts values listed in specific
cc_library or cc_binary build rules will be placed on the compiler command line
after these options.
Warning: C++-specific options (such as
should be specified in
--cxxopt, not in
--copt. Likewise, C-specific options (such as -Wstrict-prototypes)
should be specified in
--conlyopt, not in
Similarly, compiler options that only have an
effect at link time (such as
-l) should be specified in
--linkopt, not in
This option takes an argument which is to be passed to the compiler for source files
that are compiled in the host configuration. This is analogous to
--copt option, but applies only to the
This option takes an argument which is to be passed to the compiler for C++ source files
that are compiled in the host configuration. This is analogous to
--cxxopt option, but applies only to the
This option takes an argument which is to be passed to the compiler when compiling C source files.
This is similar to
--copt, but only applies to C compilation,
not to C++ compilation or linking. So you can pass C-specific options
Note that copts parameters listed in specific cc_library or cc_binary build rules will be placed on the compiler command line after these options.
This option takes an argument which is to be passed to the compiler when compiling C++ source files.
This is similar to
--copt, but only applies to C++ compilation,
not to C compilation or linking. So you can pass C++-specific options
% bazel build --cxxopt="-fpermissive" --cxxopt="-Wno-error" //foo/cruddy_code
Note that copts parameters listed in specific cc_library or cc_binary build rules will be placed on the compiler command line after these options.
This option takes an argument which is to be passed to the compiler when linking.
This is similar to
--copt, but only applies to linking,
not to compilation. So you can pass compiler options that only make sense
at link time (such as
--linkopt. For example:
% bazel build --copt="-fmudflap" --linkopt="-lmudflap" //foo/buggy_code
Build rules can also specify link options in their attributes. This option's settings always take precedence. Also see cc_library.linkopts.
This option determines whether Bazel will strip debugging information from
all binaries and shared libraries, by invoking the linker with the
--strip=always means always strip debugging information.
--strip=never means never strip debugging information.
The default value of
--strip=sometimes means strip iff the
% bazel build --strip=always //foo:bar
will compile the target while stripping debugging information from all generated binaries.
Note that if you want debugging information, it's not enough to disable stripping; you also need to make
sure that the debugging information was generated by the compiler, which you can do by using either
-c dbg or
Note also that Bazel's
--strip option corresponds with ld's
it only strips debugging information. If for some reason you want to strip all symbols,
not just debug symbols, you would need to use ld's
which you can do by passing
--linkopt=-Wl,--strip-all to Bazel.
An additional option to pass to the
strip command when generating
binary. The default is
-S -p. This option can be used
--stripopt does not apply to the stripping of the main
--fdo_instrument option enables the generation of
FDO (feedback directed optimization) profile output when the
built C/C++ binary is executed. For GCC, the argument provided is used as a
directory prefix for a per-object file directory tree of .gcda files
containing profile information for each .o file.
Once the profile data tree has been generated, the profile tree
should be zipped up, and provided to the
Bazel option to enable the FDO optimized compilation.
For the LLVM compiler the argument is also the directory under which the raw LLVM profile
data file(s) is dumped, e.g.
cannot be used at the same time.
--fdo_optimize option enables the use of the
per-object file profile information to perform FDO (feedback
directed optimization) optimizations when compiling. For GCC, the argument
provided is the zip file containing the previously-generated file tree
of .gcda files containing profile information for each .o file.
Alternatively, the argument provided can point to an auto profile identified by the extension .afdo.
Note that this option also accepts labels that resolve to source files. You
may need to add an
exports_files directive to the corresponding package to
make the file visible to Bazel.
For the LLVM compiler the argument provided should point to the indexed LLVM profile output file prepared by the llvm-profdata tool, and should have a .profdata extension.
--fdo_optimize cannot be used at the same time.
If enabled, each gold-invoked link of a C++ executable binary will output
a symbol counts file (via the
option). For each linker input, the file logs the number of symbols that were
defined and the number of symbols that were used in the binary.
This information can be used to track unnecessary link dependencies.
The symbol counts file is written to the binary's output path with the name
This option is disabled by default.
This option allows option arguments to be passed to the Java VM. It can be used with one big argument, or multiple times with individual arguments. For example:
% bazel build --jvmopt="-server -Xms256m" java/com/example/common/foo:all
will use the server VM for launching all Java binaries and set the startup heap size for the VM to 256 MB.
This option allows option arguments to be passed to javac. It can be used with one big argument, or multiple times with individual arguments. For example:
% bazel build --javacopt="-g:source,lines" //myprojects:prog
will rebuild a java_binary with the javac default debug info (instead of the bazel default).
The option is passed to javac after the Bazel built-in default options for javac and before the per-rule options. The last specification of any option to javac wins. The default options for javac are:
-source 8 -target 8 -encoding UTF-8
Note that changing
--javacopt settings will force a recompilation
of all affected classes. Also note that javacopts parameters listed in
specific java_library or java_binary build rules will be placed on the javac
command line after these options.
This javac option enables extra correctness checks. Any problems found will
be presented as errors.
-extra_checks:on may be used
to force the checks to be turned on.
disables the analysis.
When this option is not specified, the default behavior is used.
This option controls whether javac checks for missing direct dependencies. Java targets must explicitly declare all directly used targets as dependencies. This flag instructs javac to determine the jars actually used for type checking each java file, and warn/error if they are not the output of a direct dependency of the current target.
offmeans checking is disabled.
warnmeans javac will generate standard java warnings of type
[strict]for each missing direct dependency.
errorall mean javac will generate errors instead of warnings, causing the current target to fail to build if any missing direct dependencies are found. This is also the default behavior when the flag is unspecified.
These options affect the build commands and/or the output file contents.
--compilation_mode (fastbuild|opt|dbg) (-c)
This option takes an argument of
opt, and affects various C/C++ code-generation
options, such as the level of optimization and the completeness of
debug tables. Bazel uses a different output directory for each
different compilation mode, so you can switch between modes without
needing to do a full rebuild every time.
fastbuildmeans build as fast as possible: generate minimal debugging information (
-gmlt -Wl,-S), and don't optimize. This is the default. Note:
-DNDEBUGwill not be set.
dbgmeans build with debugging enabled (
-g), so that you can use gdb (or another debugger).
optmeans build with optimization enabled and with
assert()calls disabled (
-O2 -DNDEBUG). Debugging information will not be generated in
optmode unless you also pass
This option specifies the target CPU architecture to be used for the compilation of binaries during the build.
experimental_action_listener option instructs Bazel to use
details from the
action_listener rule specified by label to
extra_actions into the build graph.
If this option is set to true, extra actions specified by the
--experimental_action_listener command line option will only be
scheduled for top level targets.
experimental_extra_action_filter option instructs Bazel to
filter the set of targets to schedule
This flag is only applicable in combination with the
By default all
extra_actions in the transitive closure of the
requested targets-to-build get scheduled for execution.
--experimental_extra_action_filter will restrict scheduling to
extra_actions of which the owner's label matches the specified
The following example will limit scheduling of
to only apply to actions of which the owner's label contains '/bar/':
% bazel build --experimental_action_listener=//test:al //foo/... \ --experimental_extra_action_filter=.*/bar/.*
Build strategy options
Note that a particular combination of crosstool version, compiler version, and target CPU is allowed only if it has been specified in the currently used CROSSTOOL file.
This option specifies the name of the CPU architecture that should be used to build host tools.
The CPUs to build C/C++ libraries for in the transitive
rules. Other C/C++ rules are not affected. For example, if a
appears in the transitive
deps of an
android_binary rule and a
cc_binary rule, the
cc_library will be built at least twice:
once for each CPU specified with
--fat_apk_cpu for the
android_binary rule, and once for the CPU specified with
--cpu for the
The default is
.so file will be created and packaged in the APK for
each CPU specified with
--fat_apk_cpu. The name of the
file will be the name of the
android_binary rule prefixed with "lib", e.g., if the name
android_binary is "foo", then the file will be
Note that an Android-compatible crosstool must be selected.
android_ndk_repository rule is defined in the
WORKSPACE file, an Android-compatible crosstool is automatically selected.
Otherwise, the crostool can be selected using the
When present, any C++ file with a label or an execution path matching one of the inclusion regex
expressions and not matching any of the exclusion expressions will be built
with the given options. The label matching uses the canonical form of the label
The execution path is the relative path to your workspace directory including the base name
(including extension) of the C++ file. It also includes any platform dependent prefixes.
Note, that if only one of the label or the execution path matches the options will be used.
To match the generated files (e.g. genrule outputs)
Bazel can only use the execution path. In this case the regexp shouldn't start with '//'
since that doesn't match any execution paths. Package names can be used like this:
--per_file_copt=base/.*\.pb\.cc@-g0. This will match every
.pb.cc file under a directory called
This option can be used multiple times.
The option is applied regardless of the compilation mode used. I.e. it is possible
to compile with
--compilation_mode=opt and selectively compile some
files with stronger optimization turned on, or with optimization disabled.
Caveat: If some files are selectively compiled with debug symbols the symbols
might be stripped during linking. This can be prevented by setting
regex stands for a regular expression that can be prefixed with
+ to identify include patterns and with
- to identify
option stands for an arbitrary option that is passed
to the C++ compiler. If an option contains a
, it has to be quoted like so
\,. Options can also contain
@, since only the first
@ is used to separate regular expressions from options.
-O0 and the
-fprofile-arcs options to the command
line of the C++ compiler for all
.cc files in
Determines whether C++ binaries will be linked dynamically, interacting with the linkstatic attribute on build rules.
auto: Translates to a platform-dependent mode;
defaultfor linux and
default: Allows bazel to choose whether to link dynamically. See linkstatic for more information.
fully: Links all targets dynamically. This will speed up linking time, and reduce the size of the resulting binaries.
off: Links all targets in mostly static mode. If
-staticis set in linkopts, targets will change to fully static.
Enables Fission, which writes C++ debug information to dedicated .dwo files instead of .o files, where it would otherwise go. This substantially reduces the input size to links and can reduce link times.
When set to
--fission=dbg,fastbuild), Fission is enabled
only for the specified set of compilation modes. This is useful for bazelrc
settings. When set to
yes, Fission is enabled
universally. When set to
no, Fission is disabled
universally. Default is
If this flag is set, any
-static options in linkopts of
cc_* rules BUILD files are ignored. This is only intended as a
workaround for C++ hardening builds.
If enabled, all C++ compilations produce position-independent code ("-fPIC"), links prefer PIC pre-built libraries over non-PIC libraries, and links produce position-independent executables ("-pie"). Default is disabled.
Note that dynamically linked binaries (i.e.
generate PIC code regardless of this flag's setting. So this flag is for cases
where users want PIC code explicitly generated for static links.
Selects whether to perform resource shrinking for android_binary rules. Sets the default for the shrink_resources attribute on android_binary rules; see the documentation for that rule for further details. Defaults to off.
When specified, always use the given malloc implementation, overriding all
malloc="target" attributes, including in those targets that use the
default (by not specifying any
This option specifies the location of the crosstool compiler suite
to be used for all C++ compilation during a build. Bazel will look in that
location for a CROSSTOOL file and uses that to automatically determine
If not specified, bazel uses the value of
--crosstool_top to compile
code in the host configuration, i.e., tools run during the build. The main purpose of this flag
is to enable cross-compilation.
The crosstool to use for compiling C/C++ rules in the transitive
objc_*, ios__*, and apple_* rules. For those targets, this flag overwrites
The crosstool to use for compiling C/C++ rules in the transitive
android_binary rules. This is useful if other targets in the
build require a different crosstool. The default is to use the crosstool
generated by the
android_ndk_repository rule in the WORKSPACE file.
This option specifies the C/C++ compiler version (e.g.
to be used for the compilation of binaries during the build. If you want to
build with a custom crosstool, you should use a CROSSTOOL file instead of
specifying this flag.
Note that only certain combinations of crosstool version, compiler version, and target CPU are allowed.
This option specifies the Android SDK/platform toolchain
and Android runtime library that will be used to build any Android-related
The Android SDK will be automatically selected if an
rule is defined in the WORKSPACE file.
This option specifies the label of the java_toolchain used to compile Java source files.
If not specified, bazel uses the value of
--java_toolchain to compile
code in the host configuration, i.e., tools run during the build. The main purpose of this flag
is to enable cross-compilation.
This option sets the label of the base Java installation to use for bazel run,
bazel test, and for Java binaries built by
java_test rules. The
"Make" variables are derived from this option.
This option sets the label of the base Java installation to use in the host configuration, for example for host build tools including JavaBuilder and Singlejar.
This does not select the Java compiler that is used to compile Java
source files. The compiler can be selected by settings the
Build strategy options
These options affect how Bazel will execute the build. They should not have any significant effect on the output files generated by the build. Typically their main effect is on the speed on the build.
This option controls where and how commands are executed.
standalonecauses commands to be executed as local subprocesses.
sandboxedcauses commands to be executed inside a sandbox on the local machine. This requires that all input files, data dependencies and tools are listed as direct dependencies in the
toolsattributes. This is the default on systems that support sandboxed execution.
This option controls where and how genrules are executed.
standalonecauses genrules to run as local subprocesses.
sandboxedcauses genrules to run inside a sandbox on the local machine. This requires that all input files are listed as direct dependencies in the
srcsattribute, and the program(s) executed are listed in the
toolsattribute. This is the default for Bazel on systems that support sandboxed execution.
--jobs n (-j)
This option, which takes an integer argument, specifies a limit on the number of jobs that should be executed concurrently during the execution phase of the build.
Note that the number of concurrent jobs that Bazel will run
is determined not only by the
--jobs setting, but also
by Bazel's scheduler, which tries to avoid running concurrent jobs
that will use up more resources (RAM or CPU) than are available,
based on some (very crude) estimates of the resource consumption
of each job. The behavior of the scheduler can be controlled by
Bazel periodically prints a progress report on jobs that are not
finished yet (e.g. long running tests). This option sets the
reporting frequency, progress will be printed every
The default is 0, that means an incremental algorithm: the first report will be printed after 10 seconds, then 30 seconds and after that progress is reported once every minute.
This option, which takes an integer argument, specifies what percentage of the system's RAM Bazel should try to use for its subprocesses. This option affects how many processes Bazel will try to run in parallel. The default value is 67. If you run several Bazel builds in parallel, using a lower value for this option may avoid thrashing and thus improve overall throughput. Using a value higher than the default is NOT recommended. Note that Bazel's estimates are very coarse, so the actual RAM usage may be much higher or much lower than specified. Note also that this option does not affect the amount of memory that the Bazel server itself will use.
This option, which takes three comma-separated floating point arguments, specifies the amount of local resources that Bazel can take into consideration when scheduling build and test activities. Option expects amount of available RAM (in MB), number of CPU cores (with 1.0 representing single full core) and workstation I/O capability (with 1.0 representing average workstation). By default Bazel will estimate amount of RAM and number of CPU cores directly from system configuration and will assume 1.0 I/O resource.
If this option is used, Bazel will ignore --ram_utilization_factor.
This option, which is enabled by default, specifies whether the runfiles
symlinks for tests and binaries should be built in the output directory.
--nobuild_runfile_links can be useful
to validate if all targets compile without incurring the overhead
for building the runfiles trees.
When tests (or applications) are executed, their run-time data
dependencies are gathered together in one place. Within Bazel's
output tree, this "runfiles" tree is typically rooted as a sibling of
the corresponding binary or test.
During test execution, runfiles may be accessed using paths of the form
The runfiles tree ensures that tests have access to all the files
upon which they have a declared dependence, and nothing more. By
default, the runfiles tree is implemented by constructing a set of
symbolic links to the required files. As the set of links grows, so
does the cost of this operation, and for some large builds it can
contribute significantly to overall build time, particularly because
each individual test (or application) requires its own runfiles tree.
This option, which is enabled by default, specifies whether runfiles manifests
should be written to the output tree.
Disabling it implies
It can be disabled when executing tests on Forge, as runfiles trees will
be created remotely from in-memory manifests.
When this option is enabled, Bazel will discard the analysis cache right before execution starts, thus freeing up additional memory (around 10%) for the execution phase. The drawback is that further incremental builds will be slower.
As in GNU Make, the execution phase of a build stops when the first error is encountered. Sometimes it is useful to try to build as much as possible even in the face of errors. This option enables that behavior, and when it is specified, the build will attempt to build every target whose prerequisites were successfully built, but will ignore errors.
While this option is usually associated with the execution phase of
a build, it also effects the analysis phase: if several targets are
specified in a build command, but only some of them can be
successfully analyzed, the build will stop with an error
--keep_going is specified, in which case the
build will proceed to the execution phase, but only for the targets
that were successfully analyzed.
This option changes the way
java_library targets are
compiled by Bazel. Instead of using the output of a
java_library for compiling dependent
java_library targets, Bazel will create interface jars
that contain only the signatures of non-private members (public,
protected, and default (package) access methods and fields) and use
the interface jars to compile the dependent targets. This makes it
possible to avoid recompilation when changes are only made to
method bodies or private members of a class.
Note that using
--use_ijars might give you a different
error message when you are accidentally referring to a non visible
member of another class: Instead of getting an error that the member
is not visible you will get an error that the member does not exist.
Note that changing the
--use_ijars setting will force
a recompilation of all affected classes.
This option enables interface shared objects, which makes binaries and other shared libraries depend on the interface of a shared object, rather than its implementation. When only the implementation changes, Bazel can avoid rebuilding targets that depend on the changed shared library unnecessarily.
Output selection options
These options determine what to build or test.
This option causes the execution phase of the build to occur; it is on by default. When it is switched off, the execution phase is skipped, and only the first two phases, loading and analysis, occur.
This option can be useful for validating BUILD files and detecting errors in the inputs, without actually building anything.
If specified, Bazel will build only what is necessary to run the *_test
and test_suite rules that were not filtered due to their
If specified, Bazel will ignore other targets specified on the command line.
By default, this option is disabled and Bazel will build everything
requested, including *_test and test_suite rules that are filtered out from
testing. This is useful because running
bazel test --build_tests_only foo/... may not detect all build
breakages in the
This option causes Bazel not to perform a build, but merely check whether all specified targets are up-to-date. If so, the build completes successfully, as usual. However, if any files are out of date, instead of being built, an error is reported and the build fails. This option may be useful to determine whether a build has been performed more recently than a source edit (e.g. for pre-submit checks) without incurring the cost of a build.
Compile a single dependency of the argument files. This is useful for syntax checking source files in IDEs, for example, by rebuilding a single target that depends on the source file to detect errors as early as possible in the edit/build/test cycle. This argument affects the way all non-flag arguments are interpreted: for each source filename, one rule that depends on it will be built. For C++ and Java sources, rules in the same language space are preferentially chosen. For multiple rules with the same preference, the one that appears first in the BUILD file is chosen. An explicitly named target pattern which does not reference a source file results in an error.
--save_temps option causes temporary outputs from the compiler to be
saved. These include .s files (assembler code), .i (preprocessed C) and .ii
(preprocessed C++) files. These outputs are often useful for debugging. Temps will only be
generated for the set of targets specified on the command line.
Note that our implementation of
--save_temps does not use the compiler's
-save-temps flag. Instead, we do two passes, one with
and one with
-E. A consequence of this is that if your build fails,
Bazel may not yet have produced the ".i" or ".ii" and ".s" files.
If you're trying to use
--save_temps to debug a failed compilation,
you may need to also use
--keep_going so that Bazel will still try to
produce the preprocessed files after the compilation fails.
--save_temps flag currently works only for cc_* rules.
To ensure that Bazel prints the location of the additional output files, check that
setting is high enough.
If specified, Bazel will build only targets that have at least one required tag (if any of them are specified) and does not have any excluded tags. Build tag filter is specified as comma delimited list of tag keywords, optionally preceded with '-' sign used to denote excluded tags. Required tags may also have a preceding '+' sign.
If specified, Bazel will test (or build if
is also specified) only test targets with the given size. Test size filter
is specified as comma delimited list of allowed test size values (small,
medium, large or enormous), optionally preceded with '-' sign used to denote
excluded test sizes. For example,
% bazel test --test_size_filters=small,medium //foo:alland
% bazel test --test_size_filters=-large,-enormous //foo:all
will test only small and medium tests inside //foo.
By default, test size filtering is not applied.
If specified, Bazel will test (or build if
is also specified) only test targets with the given timeout. Test timeout filter
is specified as comma delimited list of allowed test timeout values (short,
moderate, long or eternal), optionally preceded with '-' sign used to denote
excluded test timeouts. See --test_size_filters
for example syntax.
By default, test timeout filtering is not applied.
If specified, Bazel will test (or build if
is also specified) only test targets that have at least one required tag
(if any of them are specified) and does not have any excluded tags. Test tag
filter is specified as comma delimited list of tag keywords, optionally
preceded with '-' sign used to denote excluded tags. Required tags may also
have a preceding '+' sign.
% bazel test --test_tag_filters=performance,stress,-flaky //myproject:all
will test targets that are tagged with either
stress tag but are not tagged with the
By default, test tag filtering is not applied. Note that you can also filter
local tags in
Specifies a comma-separated list of test languages for languages with an official
*_test rule the
(see build encyclopedia for a full list of these). Each
language can be optionally preceded with '-' to specify excluded
languages. The name used for each language should be the same as
the language prefix in the
*_test rule, for example,
If specified, Bazel will test (or build if
is also specified) only test targets of the specified language(s).
% bazel test --test_lang_filters=cc,java foo/...
will test only the C/C++ and Java tests (defined using
java_test rules, respectively)
% bazel test --test_lang_filters=-sh,-java foo/...
will run all of the tests in
foo/... except for the
By default, test language filtering is not applied.
Specifies a filter that the test runner may use to pick a subset of tests for running. All targets specified in the invocation are built, but depending on the expression only some of them may be executed; in some cases, only certain test methods are run.
The particular interpretation of filter-expression is up to
the test framework responsible for running the test. It may be a glob,
substring, or regexp.
--test_filter is a convenience
over passing different
--test_arg filter arguments,
but not all frameworks support it.
Verbosity options: options that control what Bazel printsThese options control the verbosity of Bazel's output, either to the terminal, or to additional log files.
This option, which requires a filename argument, causes the
dependency checker in
bazel build's execution phase to
explain, for each build step, either why it is being executed, or
that it is up-to-date. The explanation is written
If you are encountering unexpected rebuilds, this option can help to
understand the reason. Add it to your
.bazelrc so that
logging occurs for all subsequent builds, and then inspect the log
when you see an execution step executed unexpectedly. This option
may carry a small performance penalty, so you might want to remove
it when it is no longer needed.
This option increases the verbosity of the explanations generated when the --explain option is enabled.
In particular, if verbose explanations are enabled, and an output file is rebuilt because the command used to build it has changed, then the output in the explanation file will include the full details of the new command (at least for most commands).
Using this option may significantly increase the length of the
generated explanation file and the performance penalty of using
--explain is not enabled, then
--verbose_explanations has no effect.
This option, which takes a filename argument, causes Bazel to write
profiling data into a file. The data then can be analyzed or parsed using the
bazel analyze-profile command. The Build profile can be useful in
understanding where Bazel's
build command is spending its time.
This option causes Bazel to output package-loading progress messages. If it is disabled, the messages won't be shown.
This option causes progress messages to be displayed; it is on by default. When disabled, progress messages are suppressed.
This option causes bazel to display only
one progress message per
n seconds, where n is a real number.
n is -1, all progress messages will be displayed. The default value for
this option is 0.03, meaning bazel will limit the progress messages to one per every
This option controls the printing of result information at the end
bazel build command. By default, if a single
build target was specified, Bazel prints a message stating whether
or not the target was successfully brought up-to-date, and if so,
the list of output files that the target created. If multiple
targets were specified, result information is not displayed.
While the result information may be useful for builds of a single
target or a few targets, for large builds (e.g. an entire top-level
project tree), this information can be overwhelming and distracting;
this option allows it to be controlled.
takes an integer argument, which is the maximum number of targets
for which full result information should be printed. By default,
the value is 1. Above this threshold, no result information is
shown for individual targets. Thus zero causes the result
information to be suppressed always, and a very large value causes
the result to be printed always.
Users may wish to choose a value in-between if they regularly
alternate between building a small group of targets (for example,
during the compile-edit-test cycle) and a large group of targets
(for example, when establishing a new workspace or running
regression tests). In the former case, the result information is
very useful whereas in the latter case it is less so. As with all
options, this can be specified implicitly via
The files are printed so as to make it easy to copy and paste the filename to the shell, to run built executables. The "up-to-date" or "failed" messages for each target can be easily parsed by scripts which drive a build.
This option causes Bazel's execution phase to print the full command line for each command prior to executing it.
>>>>> # //examples/cpp:hello-world [action 'Linking examples/cpp/hello-world'] (cd /home/johndoe/.cache/bazel/_bazel_johndoe/4c084335afceb392cfbe7c31afee3a9f/bazel && \ exec env - \ /usr/bin/gcc -o bazel-out/local-fastbuild/bin/examples/cpp/hello-world -B/usr/bin/ -Wl,-z,relro,-z,now -no-canonical-prefixes -pass-exit-codes -Wl,-S -Wl,@bazel-out/local_linux-fastbuild/bin/examples/cpp/hello-world-2.params)
Where possible, commands are printed in a Bourne shell compatible syntax,
so that they can be easily copied and pasted to a shell command prompt.
(The surrounding parentheses are provided to protect your shell from the
exec calls; be sure to copy them!)
However some commands are implemented internally within Bazel, such as
creating symlink trees. For these there's no command line to display.
--subcommands=pretty_print may be passed to print
the arguments of the command as a list rather than as a single line. This may
help make long command lines more readable.
See also --verbose_failures, below.
This option causes Bazel's execution phase to print the full command line for commands that failed. This can be invaluable for debugging a failing build.
Failing commands are printed in a Bourne shell compatible syntax, suitable for copying and pasting to a shell prompt.
Options that control how Bazel embeds workspace status information into binaries ("stamping")
Use these options to "stamp" Bazel-built binaries: to embed additional information into the
binaries, such as the source control revision or other workspace-related information. You can use
this mechanism with rules that support the
stamp attribute, such as
cc_binary, and more.
This flag lets you specify a binary that Bazel runs before each build. The program can report information about the status of the workspace, such as the current source control revision.
The flag's value must be a path to a native program. On Linux/macOS this may be any executable. On Windows this must be a native binary, typically an ".exe", ".bat", or a ".cmd" file.
The program should print zero or more key/value pairs to standard output, one entry on each line, then exit with zero (otherwise the build fails). The key names can be anything but they may only use upper case letters and underscores. The first space after the key name separates it from the value. The value is the rest of the line (including additional whitespaces).
Bazel partitions the keys into two buckets: "stable" and "volatile". (The names "stable" and "volatile" are a bit counter-intuitive, so don't think much about them.)
Bazel then writes the key-value pairs into two files:
bazel-out/stable-status.txtcontains all keys and values where the key's name starts with
bazel-out/volatile-status.txtcontains the rest of the keys and their values
The contract is:
"stable" keys' values should change rarely, if possible. If the contents of
stable-status.txtchange, it invalidates the actions that depend on them. In other words, if a stable key's value changes, it'll make Bazel rebuild stamped actions. Therefore the stable status should not contain things like timestamps, because they change all the time, and would make Bazel rebuild the stamped actions with each build.
Bazel always outputs the following stable keys:
BUILD_EMBED_LABEL: value of
BUILD_HOST: the name of the host machine that Bazel is running on
BUILD_USER: the name of the user that Bazel is running as
"volatile" keys' values may change often. Bazel expects them to change all the time, like timestamps do, and duly updates the
volatile-status.txtfile. In order to avoid rebuilding stamped actions all the time though, Bazel pretends that the volatile file never changes. In other words, if the volatile status file is the only one whose contents changed, that will not invalidate actions that depend on it. If other inputs of the actions have changed, then Bazel rebuilds that action, and the action will use the updated volatile status, but just the volatile status changing alone will not invalidate the action.
Bazel always outputs the following volatile keys:
BUILD_TIMESTAMP: time of the build in seconds since the Unix Epoch (the value of
System.currentTimeMillis()divided by a thousand)
On Linux/macOS you can pass
disable retrieving workspace status, because
true does nothing successfully (exits
with zero) and prints no output. On Windows you can pass the path of MSYS's
for the same effect.
If the workspace status command fails (exits non-zero) for any reason, the build will fail.
Example program on Linux using Git:
#!/bin/bash echo "CURRENT_TIME $(date +%s)" echo "RANDOM_HASH $(cat /dev/urandom | head -c16 | md5sum 2>/dev/null | cut -f1 -d' ')" echo "STABLE_GIT_COMMIT $(git rev-parse HEAD)" echo "STABLE_USER_NAME $USER"
Pass this program's path with
--workspace_status_command, and the stable status file
will include the STABLE lines and the volatile status file will include the rest of the lines.
This option controls whether stamping is enabled for
rule types that support it. For most of the supported rule types stamping is
enabled by default (e.g.
By default, stamping is disabled for all tests. Specifying
--stamp does not force affected targets to be rebuilt,
if their dependencies have not changed.
Stamping can be enabled or disabled explicitly in BUILD using
stamp attribute of certain rule types, please refer to
the build encyclopedia for details. For
rules that are neither explicitly or implicitly configured as
stamp = 1, the
selects whether stamping is enabled. Bazel never stamps binaries that are
built for the host configuration, regardless of the stamp attribute.
Use these options to control the host and target platforms that configure how builds work, and to control what execution platforms and toolchains are available to Bazel rules.
Please see background information on Platforms and Toolchains.
The labels of the platform rules describing the target platforms for the current command.
The label of a platform rule that describes the host system.
The platforms that are available as execution platforms to run actions. Platforms can be specified by exact target, or as a target pattern. These platforms will be considered before those declared in the WORKSPACE file by register_execution_platforms().
The toolchain rules to be considered during toolchain resolution. Toolchains can be specified by exact target, or as a target pattern. These toolchains will be considered before those declared in the WORKSPACE file by register_toolchains().
Print debug information while finding toolchains for a rule. This might help developers of Bazel or Skylark rules with debugging failures due to missing toolchains.
Enable toolchain resolution for the given toolchain type, if the rules used support that. This does not directly change the core Bazel machinery, but is a signal to participating rule implementations that toolchain resolution should be used.
Changes the prefix of the generated convenience symlinks. The
default value for the symlink prefix is
will create the symlinks
If the symbolic links cannot be created for any reason, a warning is issued but the build is still considered a success. In particular, this allows you to build in a read-only directory or one that you have no permission to write into. Any paths printed in informational messages at the conclusion of a build will only use the symlink-relative short form if the symlinks point to the expected location; in other words, you can rely on the correctness of those paths, even if you cannot rely on the symlinks being created.
Some common values of this option:
Suppress symlink creation:
--symlink_prefix=/will cause Bazel to not create or update any symlinks, including the
bazel-<workspace>symlinks. Use this option to suppress symlink creation entirely.
--symlink_prefix=.bazel/will cause Bazel to create symlinks called
bin(etc) inside a hidden directory
Adds a suffix to the configuration short name, which is used to determine the output directory. Setting this option to different values puts the files into different directories, for example to improve cache hit rates for builds that otherwise clobber each others output files, or to keep the output files around for comparisons.
Temporary flag for testing bazel default visibility changes. Not intended for general use but documented for completeness' sake.
This option is enabled by default. If disabled, Bazel will not use its local action cache. Disabling the local action cache saves memory and disk space for clean builds, but will make incremental builds slower.
Using Bazel for releases
Bazel is used both by software engineers during the development cycle, and by release engineers when preparing binaries for deployment to production. This section provides a list of tips for release engineers using Bazel.
When using Bazel for release builds, the same issues arise as for other scripts that perform a build, so you should read the scripting section of this manual. In particular, the following options are strongly recommended:
These options (q.v.) are also important:
--symlink_prefix: for managing builds for multiple configurations, it may be convenient to distinguish each build with a distinct identifier, e.g. "64bit" vs. "32bit". This option differentiates the
Running tests with Bazel
To build and run tests with bazel, type
bazel test followed by
the name of the test targets.
By default, this command performs simultaneous build and test
activity, building all specified targets (including any non-test
targets specified on the command line) and testing
test_suite targets as soon as
their prerequisites are built, meaning that test execution is
interleaved with building. Doing so usually results in significant
If this option is set to 'auto' (the default) then Bazel will only rerun a test if any of the following conditions applies:
- Bazel detects changes in the test or its dependencies
- the test is marked as
- multiple test runs were requested with
- the test failed.
If 'no', all tests will be executed unconditionally.
If 'yes', the caching behavior will be the same as auto
except that it may cache test failures and test runs with
Note that test results are always saved in Bazel's output tree,
regardless of whether this option is enabled, so
you needn't have used
--cache_test_results on the
prior run(s) of
bazel test in order to get cache hits.
The option only affects whether Bazel will use previously
saved results, not whether it will save results of the current run.
Users who have enabled this option by default in
.bazelrc file may find the
-t (on) or
convenient for overriding the default on a particular run.
This option tells Bazel not to run the tests, but to merely check and report the cached test results. If there are any tests which have not been previously built and run, or whose tests results are out-of-date (e.g. because the source code or the build options have changed), then Bazel will report an error message ("test result is not up-to-date"), will record the test's status as "NO STATUS" (in red, if color output is enabled), and will return a non-zero exit code.
This option also implies
This option may be useful for pre-submit checks.
This option tells Bazel to explicitly warn the user if a test's timeout is significantly longer then the test's actual execution time. While a test's timeout should be set such that it is not flaky, a test that has a highly over-generous timeout can hide real problems that crop up unexpectedly.
For instance, a test that normally executes in a minute or two should not have a timeout of ETERNAL or LONG as these are much, much too generous. This option is useful to help users decide on a good timeout value or sanity check existing timeout values.
Note that each test shard is allotted the timeout of the entire
XX_test target. Using this option does not affect a test's timeout
value, merely warns if Bazel thinks the timeout could be restricted further.
By default, all tests are run to completion. If this flag is disabled,
however, the build is aborted on any non-passing test. Subsequent build steps
and test invocations are not run, and in-flight invocations are canceled.
Do not specify both
This option specifies the maximum number of times a test should be attempted
if it fails for any reason. A test that initially fails but eventually
succeeds is reported as
FLAKY on the test summary. It is,
however, considered to be passed when it comes to identifying Bazel exit code
or total number of passed tests. Tests that fail all allowed attempts are
considered to be failed.
By default (when this option is not specified, or when it is set to
"default"), only a single attempt is allowed for regular tests, and
3 for test rules with the
flaky attribute set. You can specify
an integer value to override the maximum limit of test attempts. Bazel allows
a maximum of 10 test attempts in order to prevent abuse of the system.
This option specifies the number of times each test should be executed. All test executions are treated as separate tests (e.g. fallback functionality will apply to each of them independently).
The status of a target with failing runs depends on the value of the
- If absent, any failing run causes the entire test to fail.
- If present and two runs from the same shard return PASS and FAIL, the test will receive a status of flaky (unless other failing runs cause it to fail).
If a single number is specified, all tests will run that many times. Alternatively, a regular expression may be specified using the syntax regex@number. This constrains the effect of --runs_per_test to targets which match the regex (e.g. "--runs_per_test=^//pizza:.*@4" runs all tests under //pizza/ 4 times). This form of --runs_per_test may be specified more than once.
If this option is specified (by default it is not), Bazel will detect flaky test shards through --runs_per_test. If one or more runs for a single shard fail and one or more runs for the same shard pass, the target will be considered flaky with the flag. If unspecified, the target will report a failing status.
Specifies how the test result summary should be displayed.
shortprints the results of each test along with the name of the file containing the test output if the test failed. This is the default value.
short, but even shorter: only print information about tests which did not pass.
detailedprints each individual test case that failed, not only each test. The names of test output files are omitted.
nonedoes not print test summary.
Specifies how test output should be displayed:
summaryshows a summary of whether each test passed or failed. Also shows the output log file name for failed tests. The summary will be printed at the end of the build (during the build, one would see just simple progress messages when tests start, pass or fail). This is the default behavior.
errorssends combined stdout/stderr output from failed tests only into the stdout immediately after test is completed, ensuring that test output from simultaneous tests is not interleaved with each other. Prints a summary at the build as per summary output above.
allis similar to
errorsbut prints output for all tests, including those which passed.
streamedstreams stdout/stderr output from each test in real-time.
This option causes the Java virtual machine of a java test to wait for a connection from a JDWP-compliant debugger before starting the test. This option implies --test_output=streamed.
By default this option is enabled, causing test times and other additional
information (such as test attempts) to be printed to the test summary. If
--noverbose_test_summary is specified, test summary will
include only test name, test status and cached test indicator and will
be formatted to stay within 80 characters when possible.
Specifies temporary directory for tests executed locally. Each test will be
executed in a separate subdirectory inside this directory. The directory will
be cleaned at the beginning of the each
bazel test command.
By default, bazel will place this directory under Bazel output base directory.
Note that this is a directory for running tests, not storing test results
(those are always stored under the
Overrides the timeout value for all tests by using specified number of seconds as a new timeout value. If only one value is provided, then it will be used for all test timeout categories.
Alternatively, four comma-separated values may be provided, specifying individual timeouts for short, moderate, long and eternal tests (in that order). In either form, zero or a negative value for any of the test sizes will be substituted by the default timeout for the given timeout categories as defined by the page Writing Tests. By default, Bazel will use these timeouts for all tests by inferring the timeout limit from the test's size whether the size is implicitly or explicitly set.
Tests which explicitly state their timeout category as distinct from their size will receive the same value as if that timeout had been implicitly set by the size tag. So a test of size 'small' which declares a 'long' timeout will have the same effective timeout that a 'large' tests has with no explicit timeout.
Passes command-line options/flags/arguments to each test process. This
option can be used multiple times to pass several arguments, e.g.
Specifies additional variables that must be injected into the test
environment for each test. If value is not specified it will be
inherited from the shell environment used to start the
The environment can be accessed from within a test by using
getenv("var") (C or C++),
This specifies a prefix that the test runner will insert in front of the test command before running it. The command-prefix is split into words using Bourne shell tokenization rules, and then the list of words is prepended to the command that will be executed.
If the first word is a fully qualified label (i.e. starts with
//) it is built. Then the label is substituted by the
corresponding executable location that is prepended to the command
that will be executed along with the other words.
Some caveats apply:
The PATH used for running tests may be different than the PATH in your environment,
so you may need to use an absolute path for the
--run_undercommand (the first word in command-prefix).
stdinis not connected, so
--run_undercan't be used for interactive commands.
--run_under=/usr/bin/valgrind --run_under=/usr/bin/strace --run_under='/usr/bin/strace -c' --run_under='/usr/bin/valgrind --quiet --num-callers=20'
As documented under Output selection options, you can filter tests by size, timeout, tag, or language. A convenience general name filter can forward particular filter args to the test runner.
Other options for
The syntax and the remaining options are exactly like bazel build.
Running executables with Bazel
bazel run command is similar to
bazel build, except
it is used to build and run a single target. Here is a typical session:
% bazel run -- java/myapp:myapp --arg1 --arg2 Welcome to Bazel INFO: Loading package: java/myapp INFO: Loading package: foo/bar INFO: Loading complete. Analyzing... INFO: Found 1 target... ... Target //java/myapp:myapp up-to-date: bazel-bin/java/myapp:myapp INFO: Elapsed time: 0.638s, Critical Path: 0.34s INFO: Running command line: bazel-bin/java/myapp:myapp --arg1 --arg2 Hello there $EXEC_ROOT/java/myapp/myapp --arg1 --arg2
Note the use of the
--. This is needed so that Bazel
does not interpret
Bazel options, but rather as part of the command line for running the binary.
(The program being run simply says hello and prints out its args.)
This has the same effect as the
--run_under option for
bazel test (see above),
except that it applies to the command being run by
run rather than to the tests being run by
and cannot run under label.
bazel run can also execute test binaries, which has the effect of
running the test in a close approximation of the environment described at
Writing Tests. Note that none of the
--test_*code> arguments have an effect when running a test in this manner except
Querying the dependency graph with Bazel
Bazel includes a query language for asking questions about the dependency graph used during the build. The query language is used by two commands: query and cquery. The major difference between the two commands is that query runs after the loading phase and cquery runs after the analysis phase. These tools are an invaluable aid to many software engineering tasks.
The query language is based on the idea of algebraic operations over graphs; it is documented in detail in Bazel Query Reference. Please refer to that document for reference, for examples, and for query-specific command-line options.
The query tool accepts several command-line
--output selects the output format.
--[no]keep_going (disabled by default) causes the query
tool to continue to make progress upon errors; this behavior may be
disabled if an incomplete result is not acceptable in case of errors.
enabled by default, causes dependencies on "host
configuration" targets to be included in the dependency graph over
which the query operates.
--[no]implicit_deps option, enabled by default, causes
implicit dependencies to be included in the dependency graph over which the query operates. An
implicit dependency is one that is not explicitly specified in the BUILD file
but added by bazel.
Example: "Show the locations of the definitions (in BUILD files) of all genrules required to build all the tests in the PEBL tree."
bazel query --output location 'kind(genrule, deps(kind(".*_test rule", foo/bar/pebl/...)))'
Querying the action graph with BazelCaution: The aquery command is still experimental and its API will change.
aquery command allows you to query for actions in your build graph.
It operates on the post-analysis configured target graph and exposes
information about actions, artifacts and their relationships.
The tool accepts several command-line options.
--output selects the output format
proto is the default, use
text for human readable
Notably, the aquery command runs on top of a regular Bazel build and inherits
the set of options available during a build.
It supports the same set of functions that is also available to traditional
Miscellaneous Bazel commands and options
help command provides on-line help. By default, it
shows a summary of available commands and help topics, as shown in
the Bazel overview section above.
Specifying an argument displays detailed help for a particular
topic. Most topics are Bazel commands, e.g.
query, but there are some additional help topics
that do not correspond to commands.
bazel help [topic] prints only a
summary of the relevant options for a topic. If
--long option is specified, the type, default value
and full description of each option is also printed.
Bazel server processes (see Client/server
implementation) may be stopped by using the
command. This command causes the Bazel server to exit as soon as it
becomes idle (i.e. after the completion of any builds or other
commands that are currently in progress).
Bazel servers stop themselves after an idle timeout, so this command
is rarely necessary; however, it can be useful in scripts when it is
known that no further builds will occur in a given workspace.
shutdown accepts one
--iff_heap_size_greater_than n, which
requires an integer argument (in MB). If specified, this makes the shutdown
conditional on the amount of memory already consumed. This is
useful for scripts that initiate a lot of builds, as any memory
leaks in the Bazel server could cause it to crash spuriously on
occasion; performing a conditional restart preempts this condition.
info command prints various values associated with
the Bazel server instance, or with a specific build configuration.
(These may be used by scripts that drive a build.)
info command also permits a single (optional)
argument, which is the name of one of the keys in the list below.
In this case,
bazel info key will print only
the value for that one key. (This is especially convenient when
scripting Bazel, as it avoids the need to pipe the result
sed -ne /key:/s/key://p:
release: the release label for this Bazel instance, or "development version" if this is not a released binary.
workspacethe absolute path to the base workspace directory.
install_base: the absolute path to the installation directory used by this Bazel instance for the current user. Bazel installs its internally required executables below this directory.
output_base: the absolute path to the base output directory used by this Bazel instance for the current user and workspace combination. Bazel puts all of its scratch and build output below this directory.
execution_root: the absolute path to the execution root directory under output_base. This directory is the root for all files accessible to commands executed during the build, and is the working directory for those commands. If the workspace directory is writable, a symlink named
bazel-<workspace>is placed there pointing to this directory.
output_path: the absolute path to the output directory beneath the execution root used for all files actually generated as a result of build commands. If the workspace directory is writable, a symlink named
bazel-outis placed there pointing to this directory.
server_pid: the process ID of the Bazel server process.
command_log: the absolute path to the command log file; this contains the interleaved stdout and stderr streams of the most recent Bazel command. Note that running
bazel infowill overwrite the contents of this file, since it then becomes the most recent Bazel command. However, the location of the command log file will not change unless you change the setting of the
max-heap-size: reports various JVM heap size parameters. Respectively: memory currently used, memory currently guaranteed to be available to the JVM from the system, maximum possible allocation.
gc-time: The cumulative count of garbage collections since the start of this Bazel server and the time spent to perform them. Note that these values are not reset at the start of every build.
package_path: A colon-separated list of paths which would be searched for packages by bazel. Has the same format as the
--package_pathbuild command line argument.
Example: the process ID of the Bazel server.
% bazel info server_pid 1285
These data may be affected by the configuration options passed
bazel info, for
info command accepts all
the options that control dependency
analysis, since some of these determine the location of the
output directory of a build, the choice of compiler, etc.
bazel-genfiles: reports the absolute path to the
bazel-*directories in which programs generated by the build are located. This is usually, though not always, the same as the
bazel-*symlinks created in the base workspace directory after a successful build. However, if the workspace directory is read-only, no
bazel-*symlinks can be created. Scripts that use the value reported by
bazel info, instead of assuming the existence of the symlink, will be more robust.
"Make" environment. If the
--show_make_envflag is specified, all variables in the current configuration's "Make" environment are also displayed (e.g.
GLIBC_VERSION, etc). These are the variables accessed using the
varref("CC")syntax inside BUILD files.
Example: the C++ compiler for the current configuration.
This is the
$(CC) variable in the "Make" environment,
--show_make_env flag is needed.
% bazel info --show_make_env -c opt COMPILATION_MODE opt
bazel-bin output directory for the current
configuration. This is guaranteed to be correct even in cases where
bazel-bin symlink cannot be created for some reason
(e.g. you are building from a read-only directory).
The version command prints version details about the built Bazel binary, including the changelist at which it was built and the date. These are particularly useful in determining if you have the latest Bazel, or if you are reporting bugs. Some of the interesting values are:
changelist: the changelist at which this version of Bazel was released.
label: the release label for this Bazel instance, or "development version" if this is not a released binary. Very useful when reporting bugs.
mobile-install command installs apps to mobile devices.
Currently only Android devices running ART are supported.
See bazel mobile-install
for more information.
Note that this command does not install the same thing that
bazel build produces: Bazel tweaks the app so that it can be
built, installed and re-installed quickly. This should, however, be mostly
transparent to the app.
The following options are supported:
If set, Bazel tries to install the app incrementally, that is, only those
parts that have changed since the last build. This cannot update resources
AndroidManifest.xml, native code or Java
resources (i.e. ones referenced by
Class.getResource()). If these
things change, this option must be omitted. Contrary to the spirit of Bazel
and due to limitations of the Android platform, it is the
responsibility of the user to know when this command is good enough and
when a full install is needed.
If you are using a device with Marshmallow or later, consider the
Whether to use split apks to install and update the application on the device.
Works only with devices with Marshmallow or later. Note that the
is not necessary when using
Starts the app in a clean state after installing. Equivalent to
Waits for debugger to be attached before starting the app in a clean state after installing.
How the app should be started after installing it. Supported start_types are:
NODoes not start the app. This is the default.
COLDStarts the app from a clean state after install.
WARMPreserves and restores the application state on incremental installs.
DEBUGWaits for the debugger before starting the app in a clean state after install.
--debug_appis set, the last value will be used.
adb binary to be used.
The default is to use the adb in the Android SDK specified by
Extra arguments to
adb. These come before the subcommand in the
command line and are typically used to specify which device to install to.
For example, to select the Android device or emulator to use:
% bazel mobile-install --adb_arg=-s --adb_arg=deadbeefwill invoke
adb -s deadbeef install ...
The verbosity for incremental install. Set to 1 for debug logging to be printed to the console.
dump command prints to stdout a dump of the
internal state of the Bazel server. This command is intended
primarily for use by Bazel developers, so the output of this command
is not specified, and is subject to change.
By default, command will just print help message outlining possible options to dump specific areas of the Bazel state. In order to dump internal state, at least one of the options must be specified.
Following options are supported:
--action_cachedumps action cache content.
--packagesdumps package cache content.
--skyframedumps state of internal Bazel dependency graph.
--rulesdumps rule summary for each rule and aspect class, including counts and action counts. This includes both native and Skylark rules. If memory tracking is enabled, then the rules' memory consumption is also printed.
pprofcompatible .gz file to the specified path. You must enable memory tracking for this to work.
--action_graph=/path/to/filedumps the state of the internal Bazel action graph in proto format to
/path/to/file. You have to run (at least) the analysis phase for the targets you are interested in (for example,
bazel build --nobuild //foo:bar). Note that this feature is still experimental, subject to change and will probably be integrated into
cqueryin the future.
--action_graph:targets=target1,target2,...filters the actions to the comma-separated list of targets when dumping the action graph.
dump commands require memory tracking. To turn this on, you have to pass
startup flags to Bazel:
The java-agent is checked into bazel at
third_party/allocation_instrumenter/java-allocation-instrumenter-3.0.1.jar, so make
sure you adjust
$BAZEL for where you keep your bazel repository.
Do not forget to keep passing these options to Bazel for every command or the server will
% bazel --host_jvm_args=-javaagent:$BAZEL/third_party/allocation_instrumenter/java-allocation-instrumenter-3.0.1.jar \ --host_jvm_args=-DRULE_MEMORY_TRACKER=1 \ build --nobuild <targets> # Dump rules % bazel --host_jvm_args=-javaagent:$BAZEL/third_party/allocation_instrumenter/java-allocation-instrumenter-3.0.1.jar \ --host_jvm_args=-DRULE_MEMORY_TRACKER=1 \ dump --rules # Dump Skylark heap and analyze it with pprof % bazel --host_jvm_args=-javaagent:$BAZEL/third_party/allocation_instrumenter/java-allocation-instrumenter-3.0.1.jar \ --host_jvm_args=-DRULE_MEMORY_TRACKER=1 \ dump --skylark_memory=$HOME/prof.gz % pprof -flame $HOME/prof.gz
analyze-profile command analyzes data previously gathered
during the build using
--profile option. It provides several
options to either perform analysis of the build execution or export data in
the specified format.
The following options are supported:
--dump=textdisplays all gathered data in a human-readable format
--dump=rawdisplays all gathered data in a script-friendly format
--htmlgenerates an HTML file visualizing the actions and rules executed in the build, as well as summary statistics for the build
--html_detailsadds more fine-grained information on actions and rules to the HTML visualization
--html_histogramsadds histograms for Skylark functions clicked in the statistics table. This will increase file size massively
--nocharthides the task chart from generated HTML
--combinecombines multiple profile data files into a single report. Does not generate HTML task charts
--task_treeprints the tree of tasks matching the given regular expression
--task_tree_thresholdskip tasks with duration less than threshhold, in milliseconds. Default is 50ms
See the section on Troubleshooting performance by profiling for format details and usage help.
canonicalize-flags command, which takes a list of options for
a Bazel command and returns a list of options that has the same effect. The
new list of options is canonical, i.e., two lists of options with the same
effect are canonicalized to the same new list.
--for_command option can be used to select between different
commands. At this time, only
supported. Options that the given command does not support cause an error.
Note that a small number of options cannot be reordered, because Bazel cannot ensure that the effect is identical.
Bazel startup options
The options described in this section affect the startup of the Java virtual machine used by Bazel server process, and they apply to all subsequent commands handled by that server. If there is an already running Bazel server and the startup options do not match, it will be restarted.
All of the options described in this section must be specified using the
syntax. Also, these options must appear before the name of the Bazel
This option requires a path argument, which must specify a writable directory. Bazel will use this location to write all its output. The output base is also the key by which the client locates the Bazel server. By changing the output base, you change the server which will handle the command.
By default, the output base is derived from the user's login name,
and the name of the workspace directory (actually, its MD5 digest),
so a typical value looks like:
Note that the client uses the output base to find the Bazel server
instance, so if you specify a different output base in a Bazel
command, a different server will be found (or started) to handle the
request. It's possible to perform two concurrent builds in the same
workspace directory by varying the output base.
% bazel --output_base /tmp/1 build //foo & bazel --output_base /tmp/2 build //bar
In this command, the two Bazel commands run concurrently (because of
& operator), each using a different Bazel
server instance (because of the different output bases).
In contrast, if the default output base was used in both commands,
then both requests would be sent to the same server, which would
handle them sequentially: building
//foo first, followed
by an incremental build of
We recommend you do not use NFS locations for the output base, as the higher access latency of NFS will cause noticeably slower builds.
By default, the
output_base value is chosen to as to
avoid conflicts between multiple users building in the same workspace directory.
In some situations, though, it is desirable to build from a directory
shared between multiple users; release engineers often do this. In
those cases it may be useful to deliberately override the default so
as to ensure "conflicts" (i.e., sharing) between multiple users.
--output_user_root option to achieve this: the
output base is placed in a subdirectory of the output user root,
with a unique name based on the workspace, so the result of using an
output user root that is not a function of
sharing. Of course, it is important to ensure (via umask and group
membership) that all the cooperating users can read/write each
--output_base option is specified, it overrides
--output_user_root to calculate the output base.
The install base location is also calculated based on
--output_user_root, plus the MD5 identity of the Bazel embedded
You can also use the
--output_user_root option to choose an
alternate base location for all of Bazel's output (install base and output
base) if there is a better location in your filesystem layout.
Specifies the Java virtual machine in which Bazel itself runs. The value must be a path to the directory containing a JDK or JRE. It should not be a label. This option should appear before any bazel command, and not be confused with the build option --server_javabase, for example:
% bazel --server_javabase=/usr/local/buildtools/java/jdk9 build //foo
This flag does not affect the JVMs used by Bazel subprocesses such as applications, tests, tools, and so on. Use build options --javabase or --host_javabase instead.
This flag was previously named
--host_javabase (sometimes referred to as the
--host_javabase), but was renamed to avoid confusion with the
build flag --host_javabase (sometimes referred to as the
Specifies a startup option to be passed to the Java virtual machine in which Bazel itself runs. This can be used to set the stack size, for example:
% bazel --host_jvm_args="-Xss256K" build //foo
This option can be used multiple times with individual arguments. Note that setting this flag should rarely be needed. You can also pass a space-separated list of strings, each of which will be interpreted as a separate JVM argument, but this feature will soon be deprecated.
That this does not affect any JVMs used by
subprocesses of Bazel: applications, tests, tools, and so on. To pass
JVM options to executable Java programs, whether run by
run or on the command-line, you should use
--jvm_flags argument which
support. Alternatively for tests, use
test --test_arg=--jvm_flags=foo ....
This option causes the Java virtual machine to wait for a connection from a JDWP-compliant debugger before calling the main method of Bazel itself. This is primarily intended for use by Bazel developers.
(Please note that this does not affect any JVMs used by subprocesses of Bazel: applications, tests, tools, etc.)
--batch is deprecated. For build isolation, we recommend
using the command option
--nokeep_state_after_build, which guarantees
that no incremental in-memory state is kept between builds. In order to restart the
Bazel server and JVM after a build, please explicitly do so using the “shutdown” command.
Batch mode causes Bazel to not use the standard client/server mode described above, instead running a bazel java process for a single command, which has been used for more predictable semantics with respect to signal handling, job control, and environment variable inheritance, and is necessary for running bazel in a chroot jail.
Batch mode retains proper queueing semantics within the same output_base. That is, simultaneous invocations will be processed in order, without overlap. If a batch mode Bazel is run on a client with a running server, it first kills the server before processing the command.
Bazel will run slower in batch mode, or with the alternatives described above. This is because, among other things, the build file cache is memory-resident, so it is not preserved between sequential batch invocations. Therefore, using batch mode often makes more sense in cases where performance is less critical, such as continuous builds.
This option specifies how long, in seconds, the Bazel server process should wait after the last client request, before it exits. The default value is 10800 (3 hours).
This option may be used by scripts that invoke Bazel to ensure that
they do not leave Bazel server processes on a user's machine when they
would not be running otherwise.
For example, a presubmit script might wish to
bazel query to ensure that a user's pending
change does not introduce unwanted dependencies. However, if the
user has not done a recent build in that workspace, it would be
undesirable for the presubmit script to start a Bazel server just
for it to remain idle for the rest of the day.
By specifying a small value of
--max_idle_secs in the
query request, the script can ensure that if it caused a new
server to start, that server will exit promptly, but if instead
there was already a server running, that server will continue to run
until it has been idle for the usual time. Of course, the existing
server's idle timer will be reset.
If enabled, Bazel will wait for other Bazel commands holding the server lock to complete before progressing. If disabled, Bazel will exit in error if it cannot immediately acquire the lock and proceed. Developers might use this in presubmit checks to avoid long waits caused by another Bazel command in the same client.
Sets a level from 0-7 for best-effort IO scheduling. 0 is highest priority, 7 is lowest. The anticipatory scheduler may only honor up to priority 4. Negative values are ignored.
batch CPU scheduling for Bazel. This policy is useful for
workloads that are non-interactive, but do not want to lower their nice value.
See 'man 2 sched_setscheduler'. This policy may provide for better system
interactivity at the expense of Bazel throughput.
Controls whether Bazel announces command options read from the bazelrc file when starting up. (Startup options are unconditionally announced.)
This option determines whether Bazel will use colors to highlight its output on the screen.
If this option is set to
yes, color output is enabled.
If this option is set to
auto, Bazel will use color output only if
the output is being sent to a terminal and the TERM environment variable
is set to a value other than
If this option is set to
no, color output is disabled,
regardless of whether the output is going to a terminal and regardless
of the setting of the TERM environment variable.
Selects additional config section from the rc files; for the current
command, it also pulls in the options from
command:name if such a section exists. Can be specified multiple
times to add flags from several config sections. Expansions can refer to other
definitions (i.e. expansions can be chained).
This option determines whether Bazel will use cursor controls
in its screen output. This results in less scrolling data, and a more
compact, easy-to-read stream of output from Bazel. This works well with
If this option is set to
yes, use of cursor controls is enabled.
If this option is set to
no, use of cursor controls is disabled.
If this option is set to
auto, use of cursor controls will be
enabled under the same conditions as for
If specified, a timestamp is added to each message generated by Bazel specifying the time at which the message was displayed.
Calling Bazel from scripts
Bazel can be called from scripts in order to perform a build, run tests or query the dependency graph. Bazel has been designed to enable effective scripting, but this section lists some details to bear in mind to make your scripts more robust.
Choosing the output base
--output_base option controls where the Bazel process should
write the outputs of a build to, as well as various working files used
internally by Bazel, one of which is a lock that guards against
concurrent mutation of the output base by multiple Bazel processes.
Choosing the correct output base directory for your script depends
on several factors. If you need to put the build outputs in a
specific location, this will dictate the output base you need to
use. If you are making a "read only" call to Bazel
bazel query), the locking factors will be more important.
In particular, if you need to run multiple instances of your script
concurrently, you will need to give each one a different (or random) output
If you use the default output base value, you will be contending for the same lock used by the user's interactive Bazel commands. If the user issues long-running commands such as builds, your script will have to wait for those commands to complete before it can continue.
Notes about Server Mode
By default, Bazel uses a long-running server process as an optimization. When running Bazel
in a script, don't forget to call
shutdown when you're finished
with the server, or, specify
that idle servers shut themselves down promptly.
What exit code will I get?
Bazel attempts to differentiate failures due to the source code under consideration from external errors that prevent Bazel from executing properly. Bazel execution can result in following exit codes:Exit Codes common to all commands:
2- Command Line Problem, Bad or Illegal flags or command combination, or Bad Environment Variables. Your command line must be modified.
8- Build Interrupted but we terminated with an orderly shutdown.
32- External Environment Failure not on this machine.
33- OOM failure. You need to modify your command line.
34- Reserved for Google-internal use.
35- Reserved for Google-internal use.
36- Local Environmental Issue, suspected permanent.
37- Unhandled Exception / Internal Bazel Error.
38- Reserved for Google-internal use.
40-44- Reserved for errors in Bazel's command line launcher,
bazel.ccthat are not command line related. Typically these are related to bazel server being unable to launch itself.
1- Build failed.
3- Build OK, but some tests failed or timed out.
4- Build successful but no tests were found even though testing was requested.
1- Build failed.
- If the build succeeds but the executed subprocess returns a non-zero exit code it will be the exit code of the command as well.
3- Partial success, but the query encountered 1 or more errors in the input BUILD file set and therefore the results of the operation are not 100% reliable. This is likely due to a
--keep_goingoption on the command line.
7- Command failure.
Future Bazel versions may add additional exit codes, replacing generic failure
1 with a different non-zero value with a particular
meaning. However, all non-zero exit values will always constitute an error.
Reading the .bazelrc file
By default, Bazel will read the
.bazelrc file from the base workspace
directory or the user's home directory. Whether or not this is
desirable is a choice for your script; if your script needs to be
perfectly hermetic (e.g. when doing release builds), you should
disable reading the .bazelrc file by using the option
--bazelrc=/dev/null. If you want to perform a build
using the user's preferred settings, the default behavior is better.
The Bazel output is also available in a command log file which you can find with the following command:
% bazel info command_log
The command log file contains the interleaved stdout and stderr streams
of the most recent Bazel command. Note that running
will overwrite the contents of this file, since it then becomes the most
recent Bazel command. However, the location of the command log file will
not change unless you change the setting of the
The Bazel output is quite easy to parse for many purposes. Two
options that may be helpful for your script are
--noshow_progress which suppresses progress messages,
--show_result n, which controls whether
or not "build up-to-date" messages are printed; these messages may
be parsed to discover which targets were successfully built, and the
location of the output files they created. Be sure to specify a
very large value of n if you rely on these messages.
Troubleshooting performance by profiling
The first step in analyzing the performance of your build is to profile your build with the
The file generated by the
command is a binary file. Once you have generated this binary profile, you can analyze it using
analyze-profile command. By default, it will
print out summary analysis information for each of the specified profile datafiles. This includes
cumulative statistics for different task types for each build phase and an analysis of the
critical execution path.
The first section of the default output describes an overview of the time spent on the different build phases:
=== PHASE SUMMARY INFORMATION === Total launch phase time 6.00 ms 0.01% Total init phase time 864 ms 1.11% Total loading phase time 21.841 s 28.05% Total analysis phase time 5.444 s 6.99% Total preparation phase time 155 ms 0.20% Total execution phase time 49.473 s 63.54% Total finish phase time 83.9 ms 0.11% Total run time 77.866 s 100.00%
The following sections show the execution time of different tasks happening during a particular phase:
=== INIT PHASE INFORMATION === Total init phase time 864 ms Total time (across all threads) spent on: Type Total Count Average VFS_STAT 2.72% 1 23.5 ms VFS_READLINK 32.19% 1 278 ms === LOADING PHASE INFORMATION === Total loading phase time 21.841 s Total time (across all threads) spent on: Type Total Count Average SPAWN 3.26% 154 475 ms VFS_STAT 10.81% 65416 3.71 ms [...] SKYLARK_BUILTIN_FN 13.12% 45138 6.52 ms === ANALYSIS PHASE INFORMATION === Total analysis phase time 5.444 s Total time (across all threads) spent on: Type Total Count Average SKYFRAME_EVAL 9.35% 1 4.782 s SKYFUNCTION 89.36% 43332 1.06 ms === EXECUTION PHASE INFORMATION === Total preparation time 155 ms Total execution phase time 49.473 s Total time finalizing build 83.9 ms Action dependency map creation 0.00 ms Actual execution time 49.473 s Total time (across all threads) spent on: Type Total Count Average ACTION 2.25% 12229 10.2 ms [...] SKYFUNCTION 1.87% 236131 0.44 ms
The last section shows the critical path:
Critical path (32.078 s): Id Time Percentage Description 1109746 5.171 s 16.12% Building [...] 1109745 164 ms 0.51% Extracting interface [...] 1109744 4.615 s 14.39% Building [...] [...] 1109639 2.202 s 6.86% Executing genrule [...] 1109637 2.00 ms 0.01% Symlinking [...] 1109636 163 ms 0.51% Executing genrule [...] 4.00 ms 0.01% [3 middleman actions]
You can use the following options to display more detailed information:
This option prints all recorded tasks in the order they occurred. Nested tasks are indented relative to the parent. For each task, output includes the following information:
[task type] [task description] Thread: [thread id] Id: [task id] Parent: [parent task id or 0 for top-level tasks] Start time: [time elapsed from the profiling session start] Duration: [task duration] [aggregated statistic for nested tasks, including count and total duration for each nested task]
This option is most useful for automated analysis with scripts. It outputs each task record on a single line using '|' delimiter between fields. Fields are printed in the following order:
- thread id - integer positive number, identifies owner thread for the task
- task id - integer positive number, identifies specific task
- parent task id for nested tasks or 0 for root tasks
- task start time in ns, relative to the start of the profiling session
- task duration in ns. Please note that this will include duration of all subtasks.
- aggregated statistic for immediate subtasks per type. This will include type name (lower case), number of subtasks for that type and their cumulative duration. Types are space-delimited and information for single type is comma-delimited.
- task type (upper case)
- task description
1|1|0|0|0||PHASE|Launch Bazel 1|2|0|6000000|0||PHASE|Initialize command 1|3|0|168963053|278111411||VFS_READLINK|/[...] 1|4|0|571055781|23495512||VFS_STAT|/[...] 1|5|0|869955040|0||PHASE|Load packages [...]
This option writes a file called
<profile-file>.htmlin the directory of the profile file. Open it in your browser to see the visualization of the actions in your build. Note that the file can be quite large and may push the capabilities of your browser – please wait for the file to load.
In most cases, the HTML output from
--htmlis easier to read than the
--dumpoutput. It includes a Gantt chart that displays time on the horizontal axis and threads of execution along the vertical axis. If you click on the Statistics link in the top right corner of the page, you will jump to a section that lists summary analysis information from your build.
Additionally passing this option will render a more detailed execution chart and additional tables on the performance of built-in and user-defined Skylark functions. Beware that this increases the file size and the load on the browser considerably.
If Bazel appears to be hung, you can hit ctrl + \ or send
SIGQUIT signal (
kill -3 $(bazel info server_pid)) to get a
thread dump in the file
$(bazel info output_base)/server/jvm.out.
Since you may not be able to run
bazel info if bazel is hung, the
output_base directory is usually the parent of the
symlink in your workspace directory.