Gazelle is a build file generator for Go projects. It can create new BUILD.bazel files for a project that follows "go build" conventions, and it can update existing build files to include new files and options. Gazelle can be invoked directly in a project workspace, or it can be run on an external repository during the build as part of the go_repository rule.

Gazelle is under active development. Its interface and the rules it generates may change. Gazelle is not an official Google product.

See also:

• Architecture of Gazelle
• Repository rules
  • go_repository
  • git_repository
  • http_archive

To use Gazelle in a new project, add the bazel_gazelle repository and its dependencies to your WORKSPACE file before go_rules_dependencies is called. It should look like this:

http_archive(
    name = "io_bazel_rules_go",
    url = "https://github.com/bazelbuild/rules_go/releases/download/0.12.0/rules_go-0.12.0.tar.gz",
    sha256 = "c1f52b8789218bb1542ed362c4f7de7052abcf254d865d96fb7ba6d44bc15ee3",
)
http_archive(
    name = "bazel_gazelle",
    url = "https://github.com/bazelbuild/bazel-gazelle/releases/download/0.12.0/bazel-gazelle-0.12.0.tar.gz",
    sha256 = "ddedc7aaeb61f2654d7d7d4fd7940052ea992ccdb031b8f9797ed143ac7e8d43",
)
load("@io_bazel_rules_go//go:def.bzl", "go_rules_dependencies", "go_register_toolchains")
go_rules_dependencies()
go_register_toolchains()
load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies")
gazelle_dependencies()

Add the code below to the BUILD or BUILD.bazel file in the root directory of your repository. Replace the string in prefix with the portion of your import path that corresponds to your repository.

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    prefix = "github.com/example/project",
)

After adding this code, you can run Gazelle with Bazel.

$ bazel run //:gazelle

This will generate new BUILD.bazel files for your project. You can run the same command in the future to update existing BUILD.bazel files to include new source files or options.

You can pass additional arguments to Gazelle after a -- argument. This can be used to run alternate commands like update-repos that the gazelle rule does not support directly.

$ bazel run //:gazelle -- update-repos -from_file=Gopkg.lock

If you have a Go SDK installed, you can install Gazelle in your GOPATH with the command below:

go get -u github.com/bazelbuild/bazel-gazelle/cmd/gazelle

Make sure to re-run this command to upgrade Gazelle whenever you upgrade rules_go in your repository.

To generate BUILD.bazel files in a new project, run the command below, replacing the prefix with the portion of your import path that corresponds to your repository.

gazelle -go_prefix github.com/my/project

The prefix only needs to be specified the first time you run Gazelle. To update existing BUILD.bazel files, you can just run gazelle without arguments.

Gazelle generates build files that require a minimum version of rules_go to build. Check the table below to ensure that you're using compatible versions.

gazelle <command> [flags...] [package-dirs...]

The first argument to Gazelle may be one of the commands below. If no command is specified, update is assumed. The remaining arguments are specific to each command and are documented below.

update

Scans sources files, then generates and updates build files.

fix

Same as the update command, but it also fixes deprecated usage of rules.

update-repos

Updates repository rules in the WORKSPACE file.

Gazelle may be run via a rule. See Running Gazelle with Bazel for setup instructions. This rule builds Gazelle and generates a wrapper script that executes Gazelle with baked-in set of arguments. You can run this script with bazel run, or you can copy it into your workspace and run it directly.

The following attributes are available on the gazelle rule.

The update command is the most common way of running Gazelle. Gazelle will scan sources in directories throughout the repository, then create and update build files.

The fix command does everything update does, but it also fixes deprecated usage of rules, analogous to go fix. For example, cgo_library will be consolidated with go_library. This command may delete or rename rules, so it's not on by default. See Fix command transformations for details.

Both commands accept a list of directories to process as positional arguments. If no directories are specified, Gazelle will process the current directory. Subdirectories will be processed recursively.

The following flags are accepted:

The update-repos command updates repository rules in the WORKSPACE file. It can be used to add new repository rules or update existing rules to the latest version. It can also import repository rules from a dep Gopkg.lock file.

# Add or update a repository by import path
$ gazelle update-repos example.com/new/repo

# Import repositories from Gopkg.lock
$ gazelle update-repos -from_file=Gopkg.lock

$ bazel run //:gazelle -- update-repos example.com/new/repo

The following flags are accepted:

When Gazelle is run by Bazel, most of the flags above can be encoded in the gazelle rule. For example:

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    command = "fix",
    prefix = "github.com/example/project",
    external = "vendored",
    build_tags = [
        "integration",
        "debug",
    ],
    extra_args = [
        "-build_file_name",
        "BUILD,BUILD.bazel",
    ],
)

Gazelle can be configured with directives, which are written as top-level comments in build files. Most options that can be set on the command line can also be set using directives. Some options can only be set with directives.

Directive comments have the form # gazelle:key value. For example:

load("@io_bazel_rules_go//go:def.bzl", "go_library")

# gazelle:prefix github.com/example/project
# gazelle:build_file_name BUILD,BUILD.bazel

go_library(
    name = "go_default_library",
    srcs = ["example.go"],
    importpath = "github.com/example/project",
    visibility = ["//visibility:public"],
)

Directives apply in the directory where they are set and in subdirectories. This means, for example, if you set # gazelle:prefix in the build file in your project's root directory, it affects your whole project. If you set it in a subdirectory, it only affects rules in that subtree.

The following directives are recognized:

In addition to directives, Gazelle supports # keep comments that protect parts of build files from being modified. # keep may be written before a rule, before an attribute, or after a string within a list.

Suppose you have a library that includes a generated .go file. Gazelle won't know what imports to resolve, so you may need to add dependencies manually with # keep comments.

load("@io_bazel_rules_go//go:def.bzl", "go_library")
load("@com_github_example_gen//:gen.bzl", "gen_go_file")

gen_go_file(
    name = "magic",
    srcs = ["magic.go.in"],
    outs = ["magic.go"],
)

go_library(
    name = "go_default_library",
    srcs = ["magic.go"],
    visibility = ["//visibility:public"],
    deps = [
        "@com_github_example_gen//:go_default_library",  # keep
    ],
)

Gazelle will generate and update build files when invoked with either gazelle update or gazelle fix (update is the default). Both commands perform several transformations to fix deprecated usage of the Go rules. update performs a safe set of tranformations, while fix performs some additional transformations that may delete or rename rules.

The following transformations are performed:

Migrate library to embed (fix and update): Gazelle replaces library attributes with embed attributes.

Migrate gRPC compilers (fix and update): Gazelle converts go_grpc_library rules to go_proto_library rules with compilers = ["@io_bazel_rules_go//proto:go_grpc"].

Flatten srcs (fix and update): Gazelle converts srcs attributes that use OS and architecture-specific select expressions to flat lists. rules_go filters these sources anyway.

Squash cgo libraries (fix only): Gazelle will remove cgo_library rules named cgo_default_library and merge their attributes with a go_library rule in the same package named go_default_library. If no such go_library rule exists, a new one will be created. Other cgo_library rules will not be removed.

Squash external tests (fix only): Gazelle will squash go_test rules named go_default_xtest into go_default_test. Earlier versions of rules_go required internal and external tests to be built separately, but this is no longer needed.

Remove legacy protos (fix only): Gazelle will remove usage of go_proto_library``rules loaded from ``@io_bazel_rules_go//proto:go_proto_library.bzl and filegroup rules named go_default_library_protos. Newly generated proto rules will take their place. Since filegroup isn't needed anymore and go_proto_library has different attributes and was always written by hand, Gazelle will not attempt to merge anything from these rules with the newly generated rules.

This transformation is only applied in the default proto mode. Since Gazelle will run in legacy proto mode if go_proto_library.bzl is loaded, this transformation is not usually applied. You can set the proto mode explicitly using the directive # gazelle:proto default.