Bazel cheatsheet

This cheatsheet provides quick tips on how to build and test code in our repository using Bazel.

Start here if you're completely new to Bazel.

The reference documentation for our Bazel build can be found at the following Golinks:

Initial setup

This section includes steps every engineer should follow to get a consistent development experience.

Install Bazelisk

Bazelisk is a wrapper for Bazel that downloads and runs the version of Bazel specified in //.bazelversion. It serves a similar purpose as nvm for NodeJS.

Bazelisk is recommended over plain Bazel because the bazel command on our gLinux workstations is automatically updated every time a new version of Bazel is released.

The easiest way to install Bazelisk is via npm, e.g.:

npm install -g @bazel/bazelisk

An alternate method is to install bazelisk to a temporary directory and then copy the correct binary to the PATH. For example:

mkdir /tmp/bazelisk && cd /tmp/bazelisk
npm install @bazel/bazelisk
cp node_modules/@bazel/bazelisk/bazelisk-linux_amd64 ~/bin/bazelisk
ln ~/bin/bazelisk ~/bin/bazel


  • Make a Bash alias with alias bazel="bazelisk" and add it to your ~/.bash_aliases file.
  • Set the full path to bazel to be the full path to bazelisk in your IDE of choice. This is necessary for some extensions to work correctly, such as the Bazel plugin for Visual Studio Code.


We use Gazelle to automatically generate BUILD.bazel files for most of our Go and TypeScript code.

Note that we occasionally edit Gazelle-generated BUILD.bazel files by hand, e.g. to mark tests as flaky.


Run make gazelle from the repository's root directory.

TypeScript support

Currently, Gazelle only generates front-end Bazel targets for the directories explicitly listed in //bazel/gazelle/frontend/allowlist.go. Edit this file to enable generation of front-end targets for your app.

TypeScript support is provided via a custom Gazelle extension which can be found in //bazel/gazelle/frontend.

Tip: see here for details on how this extension decides which rule to generate for a given TypeScript file.


Buildifier is a linter and formatter for BUILD.bazel and other Bazel files (WORKSPACE, *.bzl, etc.).


Run bazel run //:buildifier.

Bazel CI tasks

Our Bazel build is tested on RBE via the following tasks:

  • Infra-PerCommit-Build-Bazel-RBE (roughly equivalent to bazel build //... --config=remote)
  • Infra-PerCommit-Test-Bazel-RBE (roughly equivalent to bazel test //... --config=remote)

We regard the above tasks as the source of truth for build and test correctness.

As an insurance policy against RBE outages, we also have the following tasks:

  • Infra-PerCommit-Build-Bazel-Local (roughly equivalent to bazel build //...)
  • Infra-PerCommit-Test-Bazel-Local (roughly equivalent to bazel test //...)

The non-RBE tasks tend to be a bit more brittle than the RBE ones, which is why they are excluded from the CQ.

Building and testing

Use commands bazel build and bazel test to build and test Bazel targets, respectively. Examples:

# Single target.
$ bazel build //go/util:util
$ bazel test //go/util:util_test

# All targets under a directory and any subdirectoriews.
$ bazel build //go/...
$ bazel test //go/...

# All targets in the repository.
$ bazel build //...
$ bazel test //...

Any build artifacts produced by bazel build or bazel test will be found under //_bazel_bin.

Note that it‘s not necessary to bazel build a test target before bazel test-ing it. bazel test will automatically build the test target if it wasn’t built already (i.e. if it wasn't found in the Bazel cache).

More on bazel build here.

More on bazel test here.

Building and testing on RBE

By default, Bazel will build and test targets on the host system (aka a local build). To build on RBE, add flag --config=remote, e.g.:

$ bazel build //go/util:util --config=remote
$ bazel test //go/util:util_test --config=remote

Running Bazel-built binaries

Use command bazel run to run binary Bazel targets (such as go_binary, sh_binary, etc.), e.g.:

# Without command-line parameters.
$ bazel run //scripts/run_emulators:run_emulators

# With command-line parameters.
$ bazel run //scripts/run_emulators:run_emulators -- start

Alternatively, you can run the Bazel-built artifact directly, e.g.:

$ bazel build //scripts/run_emulators:run_emulators
$ _bazel_bin/scripts/run_emulators/run_emulators_/run_emulators start

The exact path of the binary under //_bazel_bin depends on the Bazel rule (go_binary, py_binary, etc.). As you can see, said path can be non-obvious, so it's generally recommended to use bazel run.

More on bazel run here.

Back-end development in Go

Our Go codebase is built and tested using Bazel rules from the rules_go repository. The go_test rule documentation is a great read to get started.

As mentioned in the Gazelle section, all Bazel targets for Go code are generated with Gazelle.

Read go/skia-infra-bazel-backend for the full details.

Building Go code

Simply use bazel build (and optionally bazel run) as described earlier.

Testing Go code

Tip: Start by reading the General testing tips section.

Our setup differs slightly from typical Go + Bazel projects in that we use a wrapper macro around go_test to handle manual tests. Gazelle is configured to use this macro via a gazelle:map_kind directive in //BUILD.bazel. The macro is defined in //bazel/go/go_test.bzl. Read the macro's docstring for the full details.

Manual Go tests

To mark specific Go test cases as manual, extract them out into a separate file ending with _manual_test.go within the same directory, and call unittest.ManualTest(t) from each test case in said file.

The go_test macro in //bazel/go/go_test.bzl places files ending with _manual_test.go in a separate go_test target, which is tagged as manual.

More on manual tests here.

Passing flags to Go tests

The go test command supports flags such as -v to print verbose outputs, -run to run a specific test case, etc. Under Bazel, these flags can be passed to a go_test test target via --test_arg, but they need to be prefixed with -test., e.g.:

# Equivalent to "go test ./go/util -v".
$ bazel test //go/util:util_test --test_arg=-test.v

# Equivalent to "go test ./go/util -run=TestFoo"
$ bazel test //go/util:util_test

Example bazel test invocation for Go tests

The following example shows what a typical bazel test invocation might look like while debugging a go_test target locally.

# Equivalent to "$ MY_ENV_VAR=foo go test ./go/my_pkg -v -logtostderr"
$ bazel test //go/my_pkg:my_pkg_test \
             --test_output=streamed \
             --nocache_test_results \
             --test_arg=-test.v \
             --test_arg=-logtostderr \

Front-end development in TypeScript

Our front-end code is built and tested using a set of custom Bazel macros built on top of rules provided by the rules_nodejs repository. All such macros are either defined in or re-exported from //infra-sk/index.bzl. This section uses the terms macro and rule interchangeably when referring to the macros exported from said file.

As mentioned in the Gazelle section, most Bazel targets for front-end code are generated with Gazelle.

Read go/skia-infra-bazel-frontend for the full details.

Building TypeScript code

Simply use bazel build (and optionally bazel run) as described earlier.

Working with demo pages

Demo pages are served via a Gazelle-generated sk_demo_page_server rule.

Use bazel run to serve a demo page via its sk_demo_page_server rule, e.g.:

$ bazel run //golden/modules/dots-sk:demo_page_server

Watching for changes

To rebuild the demo page automatically upon changes in the custom element‘s directory, use the script found in the repository’s root directory, e.g.:

$ ./ golden/modules/dots-sk

This script uses entr to watch for file changes and re-execute the bazel run command as needed. The above invocation is equivalent to:

$ ls golden/modules/dots-sk/* | entr -r bazel run //golden/modules/dots-sk:demo_page_server

Install entr on a gLinux workstation with sudo apt-get install entr.

In the future, we might replace this script with ibazel, which requires changes to the sk_demo_page_server rule.

Testing TypeScript code

Tip: Start by reading the General testing tips section.

Front-end code testing is done via three different Bazel rules:

  • karma_test for in-browser tests based on the Karma test runner.
  • sk_element_puppeteer_test for Puppeteer tests that require a running sk_demo_page_server.
  • nodejs_test for any other server-side TypeScript tests (i.e. NodeJS tests).

Gazelle decides which rule to generate for a given *_test.ts file based the following patterns:

  • karma_test is used for files matching //<app>/modules/<element>/<element>_test.ts.
  • sk_element_puppeteer_test is used for files matching //<app>/modules/<element>/<element>_puppeteer_test.ts.
  • nodejs_test is used for files matching *_nodejs_test.ts.

Karma tests (karma_test rule)

Use bazel test to run a Karma test in headless mode:

$ bazel test //golden/modules/dots-sk:dots-sk_test

To run a Karma test in the browser during development, use bazel run instead:

$ bazel run //golden/modules/dots-sk:dots-sk_test
Karma v4.4.1 server started at

Puppeteer tests (sk_element_puppeteer_test rule)

Use bazel test to run a Puppeteer test, e.g.:

$ bazel test //golden/modules/dots-sk:dots-sk_puppeteer_test

To view the screenshots captured by a Puppeteer test, use the //:extract_puppeteer_screenshots target:

$ mkdir /tmp/screenshots
$ bazel run //:extract_puppeteer_screenshots -- --output_dir /tmp/screenshots

To step through a Puppeteer test with a debugger, run your test with bazel run, and append _debug at the end of the target name, e.g.:

# Normal test execution (for reference).
$ bazel test //golden/modules/dots-sk:dots-sk_puppeteer_test

# Test execution in debug mode.
$ bazel run //golden/modules/dots-sk:dots-sk_puppeteer_test_debug

This will print a URL to stdout that you can use to attach a Node.js debugger (such as the VS Code Node.js debugger, or Chrome DevTools). Your test will wait until a debugger is attached before continuing.

Example debug session with Chrome DevTools:

  1. Add one or more debugger statements in your test code to set breakpoints, e.g.:
// //golden/modules/dots-sk/dots-sk_puppeteer_test.ts

describe('dots-sk', () => {
  it('should do something', () => {
  1. Run bazel run //golden/modules/dots-sk:dots-sk_puppeteer_test_debugger.
  2. Launch Chrome in the machine where the test is running, otherwise Chrome won't see the Node.js process associated to your test.
  3. Enter chrome://inspect in the URL bar, then press return.
  4. You should see an “inspect” link under the “Remote Target” heading.
  5. Click that link to launch a Chrome DevTools window attached to your Node.js process.
  6. Click the “Resume script execution” button (looks like a play/pause icon).
  7. Test execution should start, and eventually pause at your debugger statement.

By default, Puppeteer starts a Chromium instance in headless mode. If you would like to run your test in headful mode, invoke your test with bazel run, and append _debug_headful at the end of the target name, e.g.:

$ bazel run //golden/modules/dots-sk:dots-sk_puppeteer_test_debug_headful

Run your test in headful mode to visually inspect how your test interacts with the demo page under test as you step through your test code with the attached debugger.

NodeJS tests (nodejs_test rule)

Use bazel test to run a NodeJS test, e.g.:

$ bazel test //puppeteer-tests:util_nodejs_test

General testing tips

The below tips apply to all Bazel test targets (e.g. go_test, karma_test, etc.).

Test output

By default, Bazel omits the standard output of tests (e.g. fmt.Println("Hello")).

Use flag --test_output=all to see the full output of your tests:

$ bazel test //perf/... --test_output=all

Note that Bazel runs tests in parallel, so it will only print out their output once all tests have finished running.

Flag --test_output=errors can be used to only print out the output of failing tests.

To see the tests' output in real time, use flag --test_output=streamed. Note however that this forces serial execution of tests, so this can be significantly slower.


Bazel caches successful test runs, and reports (cached) PASSED on subsequent bazel test invocations, e.g.:

$ bazel test //go/util:util_test
//go/util:util_test                                                      PASSED in 0.1s

$ bazel test //go/util:util_test
//go/util:util_test                                             (cached) PASSED in 0.1s

To disable caching, use flag --nocache_test_results, e.g.

$ bazel test //go/util:util_test
//go/util:util_test                                             (cached) PASSED in 0.1s

$ bazel test //go/util:util_test --nocache_test_results
//go/util:util_test                                                      PASSED in 0.1s

Flaky tests

Flaky tests can cause the CI to fail (see Bazel CI tasks).

Tests can be marked as flaky via the flaky argument, e.g.:

    name = "some_flaky_test",
    srcs = ["some_flaky_test.go"],
    flaky = True,

Bazel will execute tests marked as flaky up to three times, and report test failure only if the three attempts fail.

Using flaky is generally discouraged, but can be useful until the root cause of the flake is diagnosed (see Debugging flaky tests) and fixed.

As a last resort, consider marking your flaky test as manual (see Manual tests).

More on the flaky attribute here.

Debugging flaky tests

While --nocache_test_results can be useful for debugging flaky tests, flag --runs_per_test was specifically added for this purpose. Example:

$ bazel test //path/to:some_flaky_test --runs_per_test=10
//path/to:some_flaky_test                                             FAILED in 4 out of 10 in 0.1s

Manual tests

Manual tests are excluded from Bazel wildcards such as bazel test //....

To mark a test target as manual, use the manual tag, e.g.:

    name = "some_manual_nodejs_test",
    src = "some_manual_nodejs_test.ts",
    tags = ["manual"],

Note that the instructions to mark go_test targets as manual are different. See Manual Go tests for more.

Note that manual tests are excluded from the Bazel CI tasks.

More on manual tests and Bazel tags here.

Test timeouts

By default, Bazel will report TIMEOUT if the test does not finish within 5 minutes. This can be overridden via the --test_timeout flag, e.g.

$ bazel test //go/util:slow_test --test_timeout=20

This can also be overridden via the timeout and size arguments of the test target, e.g.

    name = "my_test",
    srcs = ["my_test.go"],
    timeout = "long",

More on how to handle timeouts and slow tests here.

Passing command-line flags to test binaries

Use flag --test_arg to pass flags to the binary produced by a test target.

For example, our go_test targets define custom command-line flags such as flag.Bool("logtostderr", ...). This flag can be enabled with --test_arg, e.g.:

$ bazel test //go/util:util_test --test_arg=-logtostderr

As an alternative, command-line flags can be specified via the args argument of the Bazel test target, as follows:

    name = "my_test",
    srcs = ["my_test.go],
    args = ["-logtostderr"],

More on test arguments here.

Overriding environment variables

By default, Bazel isolates test targets from the host system's environment variables, and sets the environment with a number of variables with Bazel-specific information that some *_test rules depend on (documented here).

Use flag --test_env to specify any environment variables, e.g.

$ bazel test //path/to:my_cockroachdb_test --test_env=COCKROACHDB_EMULATOR_STORE_DIR=/tmp/crdb

To pipe through an environment variable from the host's system:

$ bazel test //path/to:my_cockroachdb_test --test_env=COCKROACHDB_EMULATOR_STORE_DIR

More on the --test_env flag here.

Faster Sandboxing

By default, Bazel sandboxes every build step. Effectively, it runs the compile command with only the given source files for a particular rule and the specified dependencies visible, to force all dependencies to be properly listed.

For steps that have a lot of files, this can have a bit of I/O overhead. To speed this up, one can use tempfs (e.g. a RAM disk) for the sandbox by adding --sandbox_base=/dev/shm to the build command. When compiling Skia, for example, this reduces compile time by 2-3x.

Sandboxing can make diagnosing failing rules a bit harder. To see what command got run and to be able to view the sandbox after failure, add --subcommands --sandbox_debug to the command.

BUILD Debugging

Bazel builds fast and correct by making use of cached outputs and reusing them when the input file is identical. This can make it hard to debug a slow or non-deterministic build.

To get a detailed log of all the actions your build is taking:

  1. Add the following to your .bazelrc
# ensure there are no disk cache hits
build --disk_cache=/path/to/debugging/cache
# IMPORTANT Generate execution logs
build --experimental_execution_log_file=yourLogFile.log

  1. Run bazel clean --expunge. We want all actions to get executed, so nothing cached.
  2. Look at the yourLogFile.log, it will contain a record of every action bazel executed, environment variables, command line, input files, and output files of every action.