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.

Some ideas how to use it:

Install it as the bazel binary in your PATH (e.g. /usr/local/bin). Never worry about upgrading Bazel to the latest version again.
Check it into your repository and recommend users to build your software via ./bazelisk build //my:software. That way, even someone who has never used Bazel or doesn't have it installed can build your software.
As a company using Bazel or as a project owner, add a .bazelversion file to your repository. This will tell Bazelisk to use the exact version specified in the file when running in your workspace. The fact that it's versioned inside your repository will then allow for atomic upgrades of Bazel including all necessary changes. If you install Bazelisk as bazel on your CI machines, too, you can even test Bazel upgrades via a normal presubmit / pull request. It will also ensure that users will not try to build your project with an incompatible version of Bazel, which is often a cause for frustration and failing builds.

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 and where to get it from?

It uses a simple algorithm:

If the environment variable USE_BAZEL_VERSION is set, it will use the version specified in the value.
Otherwise, if a .bazelversion file exists in the current directory or recursively any parent directory, it will read the file and use the version specified in it.
Otherwise it will use the official latest Bazel release.

A version can optionally be prefixed with a fork name. The fork and version should be separated by slash: <FORK>/<VERSION>. If you want to create a fork with your own releases, you have to follow the naming conventions that we use in bazelbuild/bazel for the binary file names. The URL format looks like https://github.com/<FORK>/bazel/releases/download/<VERSION>/<FILENAME>.

Bazelisk currently understands the following formats for version labels:

latest means the latest stable version of Bazel as released on GitHub. Previous releases can be specified via latest-1, latest-2 etc.
A version number like 0.17.2 means that exact version of Bazel. It can also be a release candidate version like 0.20.0rc3.

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

last_green refers to the Bazel binary that was built at the most recent commit that passed Bazel CI. Ideally this binary should be very close to Bazel-at-head.
last_downstream_green points to the most recent Bazel binary that builds and tests all downstream projects successfully.
last_rc points to the most recent release candidate. If there is no active release candidate, Bazelisk uses the latest Bazel release instead.

Other features

The Go version of Bazelisk offers two new flags.

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

bazelisk --strict build //...

--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.

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 if you suspect this affects your results.

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

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.

Releases

Binary and source releases are provided on our Releases page.

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

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

Add support for checked-in Bazel binaries.
When the version label is set to a commit hash, first download a matching binary version of Bazel, then build Bazel automatically at that commit and use the resulting binary.
Add support to automatically bisect a build failure to a culprit commit in Bazel. If you notice that you could successfully build your project using version X, but not using version X+1, then Bazelisk should be able to figure out the commit that caused the breakage and the Bazel team can easily fix the problem.

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.

FaBrand/bazelisk