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bazelbuild / bazelisk

A user-friendly launcher for Bazel.
Apache License 2.0
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Bazelisk

A user-friendly launcher for Bazel.

About Bazelisk

Bazelisk is a wrapper for Bazel written in Go. It automatically picks a good version of Bazel given your current working directory, downloads it from the official server (if required) and then transparently passes through all command-line arguments to the real Bazel binary. You can call it just like you would call Bazel.

Installation

On macOS: brew install bazelisk.

On Windows: choco install bazelisk.

Each adds bazelisk to the PATH as both bazelisk and bazel.

On Linux: You can download Bazelisk binary on our Releases page and add it to your PATH manually, which also works on macOS and Windows.

Bazelisk is also published to npm. Frontend developers may want to install it with npm install -g @bazel/bazelisk.

You will notice that it serves an analogous function for Bazel as the nvm utility which manages your version of Node.js.

Some ideas how to use it:

Before Bazelisk was rewritten in Go, it was a Python script. This still works and has the advantage that you can run it on any platform that has a Python interpreter, but is currently unmaintained and it doesn't support as many features. The documentation below describes the newer Go version only.

How does Bazelisk know which Bazel version to run?

It uses a simple algorithm:

A version can optionally be prefixed with a fork name. The fork and version should be separated by slash: <FORK>/<VERSION>. Please see the next section for how to work with forks.

Bazelisk currently understands the following formats for version labels:

Additionally, a few special version names are supported for our official releases only (these formats do not work when using a fork):

Where does Bazelisk get Bazel from?

By default Bazelisk retrieves Bazel releases, release candidates and binaries built at green commits from Google Cloud Storage. The downloaded artifacts are validated against the SHA256 value recorded in BAZELISK_VERIFY_SHA256 if this variable is set in the configuration file.

As mentioned in the previous section, the <FORK>/<VERSION> version format allows you to use your own Bazel fork hosted on GitHub:

If you want to create a fork with your own releases, you should follow the naming conventions that we use in bazelbuild/bazel for the binary file names as this results in predictable URLs that are similar to the official ones. The URL format looks like https://github.com/<FORK>/bazel/releases/download/<VERSION>/<FILENAME>.

You can also override the URL by setting the environment variable $BAZELISK_BASE_URL. Bazelisk will then append /<VERSION>/<FILENAME> to the base URL instead of using the official release server. Bazelisk will read file ~/.netrc for credentials for Basic authentication.

If for any reason none of this works, you can also override the URL format altogether by setting the environment variable $BAZELISK_FORMAT_URL. This variable takes a format-like string with placeholders and performs the following replacements to compute the download URL:

Ensuring that your developers use Bazelisk rather than Bazel

Bazel installers typically provide Bazel's shell wrapper script as the bazel on the PATH.

When installed this way, Bazel checks the .bazelversion file itself, but the failure when it mismatches with the actual version of Bazel can be quite confusing to developers. You may find yourself having to explain the difference between Bazel and Bazelisk (especially when you upgrade the pinned version). To avoid this, you can add a check in your tools/bazel wrapper. Since Bazelisk is careful to avoid calling itself in a loop, it always calls the wrapper with the environment variable BAZELISK_SKIP_WRAPPER set to `true'. You can check for the presence of that variable, and when not found, report a useful error to your users about how to install Bazelisk.

Note that if users directly downloaded a Bazel binary and put it in their PATH, rather than running an installer, then tools/bazel and .bazelversion are not checked. You could call the versions.check starlark module from the beginning of your WORKSPACE to require users update their bazel.

Other features

The Go version of Bazelisk offers three new flags.

--strict

--strict expands to the set of incompatible flags which may be enabled for the given version of Bazel.

bazelisk --strict build //...

--migrate

--migrate will run Bazel multiple times to help you identify compatibility issues. If the code fails with --strict, the flag --migrate will run Bazel with each one of the flag separately, and print a report at the end. This will show you which flags can safely enabled, and which flags require a migration.

--bisect

--bisect flag allows you to bisect Bazel versions to find which version introduced a build failure. You can specify the range of versions to bisect with --bisect=<GOOD>..<BAD>, where GOOD is the last known working Bazel version and BAD is the first known non-working Bazel version. Bazelisk uses GitHub's compare API to get the list of commits to bisect. When GOOD is not an ancestor of BAD, GOOD is reset to their merge base commit.

Examples:

# Bisect between 6.0.0 and Bazel at HEAD
bazelisk --bisect=6.0.0..HEAD test //foo:bar_test

# Bisect between 6.1.0 and the second release candidate of Bazel 6.2.0
bazelisk --bisect=6.1.0..release-6.2.0rc2 test //foo:bar_test

# Bisect between two commits on the main branch (or branches with `release-` prefix) of the Bazel GitHub repository.
bazelisk --bisect=<good commit hash>..<bad commit hash> test //foo:bar_test

Note that, Bazelisk uses prebuilt Bazel binaries at commits on the main and release branches, therefore you cannot bisect your local commits.

Useful environment variables for --migrate and --bisect

You can set BAZELISK_INCOMPATIBLE_FLAGS to set a list of incompatible flags (separated by ,) to be tested, otherwise Bazelisk tests all flags starting with --incompatible_.

You can set BAZELISK_GITHUB_TOKEN to set a GitHub access token to use for API requests to avoid rate limiting when on shared networks.

You can set BAZELISK_SHUTDOWN to run shutdown between builds when migrating or bisecting if you suspect this affects your results.

You can set BAZELISK_CLEAN to run clean --expunge between builds when migrating or bisecting if you suspect this affects your results.

tools/bazel

If tools/bazel exists in your workspace root and is executable, Bazelisk will run this file, instead of the Bazel version it downloaded. It will set the environment variable BAZEL_REAL to the path of the downloaded Bazel binary. This can be useful, if you have a wrapper script that e.g. ensures that environment variables are set to known good values. This behavior can be disabled by setting the environment variable BAZELISK_SKIP_WRAPPER to any value (except the empty string) before launching Bazelisk.

You can control the user agent that Bazelisk sends in all HTTP requests by setting BAZELISK_USER_AGENT to the desired value.

.bazeliskrc configuration file

The Go version supports a .bazeliskrc file in the root directory of a workspace and the user home directory. This file allows users to set environment variables persistently.

Example file content:

USE_BAZEL_VERSION=0.19.0
BAZELISK_GITHUB_TOKEN=abc

The following variables can be set:

Configuration variables are evaluated with precedence order. The preferred values are derived in order from highest to lowest precedence as follows:

Requirements

For ease of use, the Python version of Bazelisk is written to work with Python 2.7 and 3.x and only uses modules provided by the standard library.

The Go version can be compiled to run natively on Linux, macOS and Windows. You need at least Go 1.11 to build Bazelisk, otherwise you'll run into errors like undefined: os.UserCacheDir.

To install the Go version, type:

go get github.com/bazelbuild/bazelisk

With Go 1.17 or later, the recommended way to install it is:

go install github.com/bazelbuild/bazelisk@latest

To add it to your PATH:

export PATH=$PATH:$(go env GOPATH)/bin

For more information, you may read about the GOPATH environment variable.

Ideas for the future

FAQ

Where does Bazelisk store the downloaded versions of Bazel?

It creates a directory called "bazelisk" inside your user cache directory and will store them there. Feel free to delete this directory at any time, as it can be regenerated automatically when required.