Build programs with Bazel

This page covers how to build a program with Bazel, build command syntax, and target pattern syntax.

Quickstart

To run Bazel, go to your base workspace directory or any of its subdirectories and type bazel. See build if you need to make a new workspace.

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 help

• bazel help command: Prints help and options for command.
• bazel helpstartup_options: Options for the JVM hosting Bazel.
• bazel helptarget-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 one target

Before you can start a build, you need a workspace. A workspace is 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.

To build a program with Bazel, type bazel build followed by the target you want to build.

bazel build //foo

After issuing the command to build //foo, you'll see output similar to this:

INFO: Analyzed target //foo:foo (14 packages loaded, 48 targets configured).
INFO: Found 1 target...
Target //foo:foo up-to-date:
  bazel-bin/foo/foo
INFO: Elapsed time: 9.905s, Critical Path: 3.25s
INFO: Build completed successfully, 6 total actions

First, Bazel loads all packages in your target's dependency graph. This includes declared dependencies, files listed directly in the target's BUILD file, and transitive dependencies, files listed in the BUILD files of your target's dependencies. After identifying all dependencies, Bazel analyzes them for correctness and creates the build actions. Last, Bazel executes the compilers and other tools of the build.

During the build's execution phase, Bazel prints progress messages. The progress messages include the current build step (such as, compiler or linker) as it starts, and the number completed over the total number of build actions. As the build starts, the number of total actions often increases as Bazel discovers the entire action graph, but the number stabilizes 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.

If you type the same command again, the build finishes much faster.

bazel build //foo
INFO: Analyzed target //foo:foo (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //foo:foo up-to-date:
  bazel-bin/foo/foo
INFO: Elapsed time: 0.144s, Critical Path: 0.00s
INFO: Build completed successfully, 1 total action

This is a null build. Because nothing changed, there are no packages to reload and no build steps to execute. If something changed in 'foo' or its dependencies, Bazel would re-execute some build actions, or complete an incremental build.

Building multiple targets

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, such as for declaring dependencies in BUILD files, Bazel's target patterns specify multiple targets. Target patterns 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.

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

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"] are not included in wildcard target patterns (..., :*, :all, etc.) when specified in commands like bazel build and bazel test (but they are included in negative wildcard target patterns, that is they will be subtracted). You should specify such test targets with explicit target patterns on the command line if you want Bazel to build/test them. In contrast, bazel query doesn't perform any such filtering automatically (that would defeat the purpose of bazel query).

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 dependencies 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:

• Before you build for the first time.
• 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

The distribution directory is another Bazel mechanism to avoid unnecessary downloads. Bazel searches distribution directories before the repository cache. The primary difference is that the distribution directory requires manual preparation.

Using the --distdir=/path/to-directory 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. This only works if the file hash is specified in the WORKSPACE declaration.

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.

Running Bazel in an airgapped environment

To keep Bazel's binary size small, Bazel's implicit dependencies are fetched over the network while running for the first time. These implicit dependencies contain toolchains and rules that may not be necessary for everyone. For example, Android tools are unbundled and fetched only when building Android projects.

However, these implicit dependencies may cause problems when running Bazel in an airgapped environment, even if you have vendored all of your WORKSPACE dependencies. To solve that, you can prepare a distribution directory containing these dependencies on a machine with network access, and then transfer them to the airgapped environment with an offline approach.

To prepare the distribution directory, use the --distdir flag. You will need to do this once for every new Bazel binary version, since the implicit dependencies can be different for every release.

To build these dependencies outside of your airgapped environment, first checkout the Bazel source tree at the right version:

git clone https://github.com/bazelbuild/bazel "$BAZEL_DIR"
cd "$BAZEL_DIR"
git checkout "$BAZEL_VERSION"

Then, build the tarball containing the implicit runtime dependencies for that specific Bazel version:

bazel build @additional_distfiles//:archives.tar

Export this tarball to a directory that can be copied into your airgapped environment. Note the --strip-components flag, because --distdir can be quite finicky with the directory nesting level:

tar xvf bazel-bin/external/additional_distfiles/archives.tar \
  -C "$NEW_DIRECTORY" --strip-components=3

Finally, when you use Bazel in your airgapped environment, pass the --distdir flag pointing to the directory. For convenience, you can add it as an .bazelrc entry:

build --distdir=path/to/directory

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 exec 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 exec tools, for example some base libraries. Such libraries must be built twice, once for the exec 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.

The exec 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, and 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 exec 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 exec configuration from the request configuration. Most importantly:

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 exec 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.

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 hermetically 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.

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.