A user's guide to Bazel

To run Bazel, go to your base workspace directory or any of its subdirectories and type bazel. 

  % 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.

The bazel tool performs many functions, called commands. The most commonly used ones are bazel build and bazel test. You can browse the online help messages using bazel help.

Building programs with Bazel

The build command

Type bazel build followed by the name of the target you wish to build. Here's a typical session: 

  % 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 details.

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 re-execution 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.

Specifying targets to build

Bazel allows a number of ways to specify the targets to be built. Collectively, these are known as target patterns. This syntax is used in commands like build, test, or query.

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.

All target patterns starting with // are resolved relative to the current workspace.

//foo/bar:wiz Just the single target //foo/bar:wiz.
//foo/bar Equivalent to //foo/bar:bar.
//foo/bar:all All rules in the package foo/bar.
//foo/... All rules in all packages beneath the directory foo.
//foo/...:all All rules in all packages beneath the directory foo.
//foo/...:* All targets (rules and files) in all packages beneath the directory foo.
//foo/...:all-targets All targets (rules and files) in all packages beneath the directory foo.

Target patterns which do not begin with // are resolved relative to the current working directory. These examples assume a working directory of foo:

:foo Equivalent to //foo:foo.
bar:wiz Equivalent to //foo/bar:wiz.
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.
bar:all Equivalent to //foo/bar:all.
:all Equivalent to //foo:all.
...:all Equivalent to //foo/...:all.
... Equivalent to //foo/...:all.
bar/...:all Equivalent to //foo/bar/...:all.

By default, directory symlinks are followed for recursive target patterns, except those that point to under the output base, such as the convenience symlinks that are created in the root directory of the workspace.

In addition, Bazel does not follow symlinks when evaluating recursive target patterns in any directory that contains a file named as follows: 

DONT_FOLLOW_SYMLINKS_WHEN_TRAVERSING_THIS_DIRECTORY_VIA_A_RECURSIVE_TARGET_PATTERN

foo/... is a wildcard over packages, indicating all packages recursively beneath directory foo (for all roots of the package path). :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 foo/....

In addition, :* (or :all-targets) is a wildcard that matches every target in the matched packages, including files that aren't normally built by any rule, such as _deploy.jar files associated with java_binary rules.

This implies that :* denotes a superset of :all; while potentially confusing, this syntax does allow the familiar :all wildcard to be used for typical builds, where building targets like the _deploy.jar 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 is equivalent to //foo/bar:wiz (if there is a package foo/bar) or to //foo:bar/wiz (if there is a package foo).

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. For example, 

  bazel build foo/... bar/...

means "build all targets beneath foo and all targets beneath bar", whereas 

  bazel build -- foo/... -foo/bar/...

means "build all targets beneath foo except those beneath foo/bar". (The -- argument is required to prevent the subsequent arguments starting with - from being interpreted as additional options.)

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.

Targets with 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 local_repository, new_local_repository and Android SDK and NDK repository rules will always take effect regardless of the value --fetch .

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 bazel fetch:

  1. Before you build for the first time.
  2. After you add a new external dependency.
Once it has been run, you should not need to run it again until the WORKSPACE file changes.

fetch takes a list of targets to fetch dependencies for. For example, this would fetch dependencies needed to build //foo:bar and //bar:baz: 

$ 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 bazel build.

The repository cache

Bazel tries to avoid fetching the same file several times, even if the same file is needed in different workspaces, or if the definition of an external repository changed but it still needs the same file to download. To do so, bazel caches all files downloaded in the repository cache which, by default, is located at ~/.cache/bazel/_bazel_$USER/cache/repos/v1/. The location can be changed by the --repository_cache option. The cache is shared between all workspaces and installed versions of bazel. An entry is taken from the cache if Bazel knows for sure that it has a copy of the correct file, that is, if the download request has a SHA256 sum of the file specified and a file with that hash is in the cache. So specifying a hash for each external file is not only a good idea from a security perspective; it also helps avoiding unnecessary downloads.

Upon each cache hit, the modification time of the file in the cache is updated. In this way, the last use of a file in the cache directory can easily be determined, for example to manually clean up the cache. The cache is never cleaned up automatically, as it might contain a copy of a file that is no longer available upstream.

Distribution files directories

Besides the repository cache, Bazel has a second mechanism to avoid unnecessary downloads. Using the --distdir option, you can specify additional read-only directories to look for files instead of fetching them. A file is taken from such a directory if the file name is equal to the base name of the URL and additionally the hash of the file is equal to the one specified in the download request (again, no file is taken from a distdir, if no hash is specified). While the condition on the file name is not necessary for correctness, it reduces the number of candidate files to one per specified directory. In this way, specifying distribution files directories remains efficient, even if the number of files in such a directory grows large.

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 a //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 configuration.
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 on the --distinct_host_configuration option. This boolean option is somewhat subtle, and the setting may improve (or worsen) the speed of your builds.

--distinct_host_configuration=false

We do not recommend this option.

  • If you frequently make changes to your request configuration, such as alternating between -c opt and -c dbg builds, or between simple- and cross-compilation, you will typically rebuild the majority of your codebase each time you switch.

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. 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 re-executed, 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.

--distinct_host_configuration=true (default)

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 --host_crosstool_top is specified.
  • Use the value of --host_cpu for --cpu (default: k8).
  • Use the same values of these options as specified in the request configuration: --compiler, --use_ijars, If --host_crosstool_top is used, then the value of --host_cpu is used to look up a default_toolchain in the Crosstool (ignoring --compiler) for the host configuration.
  • Use the value of --host_javabase for --javabase
  • Use the value of --host_java_toolchain for --java_toolchain
  • Use optimized builds for C++ code (-c opt).
  • Generate no debugging information (--copt=-g0).
  • Strip debug information from executables and shared libraries (--strip=always).
  • Place all derived files in a special location, distinct from that used by any possible request configuration.
  • Suppress stamping of binaries with build data (see --embed_* options).
  • 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 make clean.

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 or pre-emptive.)

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 make clean. Discovering that the build tool has failed in this manner (and then recovering from it) can be time consuming and very frustrating.

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.

Sandboxed execution

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 with the 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 Bazel.

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[0], argv): No such file or directory. In that case, try to deactivate the sandbox for genrules with --strategy=Genrule=standalone 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.

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).

Loading phase

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.

Analysis phase

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.

Execution 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.

Client/server implementation

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 build and 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 --max_idle_secs).

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 x or ps -e f as bazel(dirname), where dirname is the basename of the directory enclosing the root of your workspace directory. For example: 

  % 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 to ps, Bazel server processes may be named just java.) Bazel servers can be stopped using the shutdown command.

When running 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, the --bazelrc=file option, and the --config=value option

Bazel accepts many options. Typically, some of these are varied frequently (for example, --subcommands) while others stay the same across several builds (e.g. --package_path). To avoid having to specify these unchanged options for every build (and other commands) Bazel allows you to specify options in a configuration file.

Where are the .bazelrc files?

Bazel looks for optional configuration files in the following locations, in the order shown below. The options are interpreted in this order, so options in later files can override a value from an earlier file if a conflict arises. All options that control which of these files are loaded are startup options, which means they must occur after bazel and before the command (build, test, etc).

  1. The system RC file, unless --nosystem_rc is present.

    Path:

    • On Linux/macOS/Unixes: /etc/bazel.bazelrc
    • On Windows: %ProgramData%\bazel.bazelrc

    It is not an error if this file does not exist.

    If another system-specified location is required, you must build a custom Bazel binary, overriding the BAZEL_SYSTEM_BAZELRC_PATH value in //src/main/cpp:option_processor. The system-specified location may contain environment variable references, such as ${VAR_NAME} on Unix or %VAR_NAME% on Windows.

  2. The workspace RC file, unless --noworkspace_rc is present.

    Path: .bazelrc in your workspace directory (next to the main WORKSPACE file).

    It is not an error if this file does not exist.

  3. The home RC file, unless --nohome_rc is present.

    Path:

    • On Linux/macOS/Unixes: $HOME/.bazelrc
    • On Windows: %USERPROFILE%\.bazelrc if exists, or %HOME%/.bazelrc

    It is not an error if this file does not exist.

  4. The user-specified RC file, if specified with -bazelrc=file

    This flag is optional. However, if the flag is specified, then the file must exist.

.bazelrc syntax and semantics

Like all UNIX "rc" files, the .bazelrc file is a text file with a line-based grammar. Empty lines and lines starting with # (comments) are ignored. Each line contains a sequence of words, which are tokenized according to the same rules as the Bourne shell.

Imports

Lines that start with import or try-import are special: use these to load other "rc" files. To specify a path that is relative to the workspace root, write import %workspace%/path/to/bazelrc.

The difference between import and try-import is that Bazel fails if the import'ed file is missing (or can't be read), but not so for a try-import'ed file.

Import precedence:

  • Options in the 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.
Option defaults

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 build or query to which the options apply. These options also apply to all commands that inherit from the specified command. (For example, test inherits from build.)

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 .cvsrc.) For example, the lines: 

build --test_tmpdir=/tmp/foo --verbose_failures
build --test_tmpdir=/tmp/bar
are combined as: 
build --test_tmpdir=/tmp/foo --verbose_failures --test_tmpdir=/tmp/bar
so the effective flags are --verbose_failures and --test_tmpdir=/tmp/bar.

Option precedence:

  • Options on the command line always take precedence over those in rc files. For example, if a rc file says build -c opt but the command line flag is -c dbg, the command line flag takes precedence.

  • Within the rc file, precedence is governed by specificity: lines for a more specific command take precedence over lines for a less specific command.

    Specificity is defined by inheritance. Some commands inherit options from other commands, making the inheriting command more specific than the base command. For example test inherits from the build command, so all bazel build flags are valid for bazel test, and all build lines apply also to bazel test unless there's a test line for the same option. If the rc file says: 

    test -c dbg --test_env=PATH
build -c opt --verbose_failures
    then bazel build //foo will use -c opt --verbose_failures, and bazel test //foo will use --verbose_failures -c dbg --test_env=PATH.

    The inheritance (specificity) graph is:

    • Every command inherits from common
    • The following commands inherit from (and are more specific than) build: test, run, clean, mobile-install, info, print_action, config, cquery, and aquery
    • coverage inherits from test
  • Two lines specifying options for the same command at equal specificity are parsed in the order in which they appear within the file.
  • Because this precedence rule does not match the file order, it helps readability if you follow the precedence order within rc files: start with common options at the top, and end with the most-specific commands at the bottom of the file. 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.

--config

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 a :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 command:name will only be expanded for applicable commands, in the precedence order described above.

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.

Example

Here's an example ~/.bazelrc file: 

  # 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

Other files governing Bazel's behavior

.bazelignore

You can specify directories within the workspace that you want Bazel to ignore, such as related projects that use other build systems. Place a file called .bazelignore at the root of the workspace and add the directories you want Bazel to ignore, one per line. Entries are relative to the workspace root.

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

The --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 (e.g. 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 base.

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 --max_idle_secs=5 so 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:
  • 0 - Success
  • 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.cc that are not command line related. Typically these are related to bazel server being unable to launch itself.
  • 45 - Error publishing results to the Build Event Service.
Return codes for commands bazel build, bazel test.
  • 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.
For bazel run:
  • 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.
For bazel query:
  • 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_going option on the command line.
  • 7 - Command failure.

Future Bazel versions may add additional exit codes, replacing generic failure exit code 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.

Command log

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 bazel info 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 --output_base or --output_user_root options.

Parsing output

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, and --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 --profile option.

The file generated by the --profile command is a binary file. Once you have generated this binary profile, you can analyze it using Bazel's 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:

  • --dump=text

    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]
  • --dump=raw

    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:

    1. thread id - integer positive number, identifies owner thread for the task
    2. task id - integer positive number, identifies specific task
    3. parent task id for nested tasks or 0 for root tasks
    4. task start time in ns, relative to the start of the profiling session
    5. task duration in ns. Please note that this will include duration of all subtasks.
    6. 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.
    7. task type (upper case)
    8. task description
    Example: 
    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
[...]
  • --html

    This option writes a file called <profile-file>.html in 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 --html is easier to read than the --dump output. 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.

    • --html_details

      Additionally passing this option will render a more detailed execution chart and additional tables on the performance of built-in and user-defined Starlark 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 Bazel a 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 bazel-<workspace> symlink in your workspace directory.