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.

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/0.10.1/rules_go-0.10.1.tar.gz",
    sha256 = "4b14d8dd31c6dbaf3ff871adcd03f28c3274e42abc855cb8fb4d01233c0154dc",
)
http_archive(
    name = "bazel_gazelle",
    url = "https://github.com/bazelbuild/bazel-gazelle/releases/download/0.10.0/bazel-gazelle-0.10.0.tar.gz",
    sha256 = "6228d9618ab9536892aa69082c063207c91e777e51bd3c5544c9c060cafe1bd8",
)
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.

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. Currently, this can only be used to import repositories from a vendoring tool's lock file. More functionality will be added in the future.

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 supports several directives, written as top-level comments in build files. These are of the form # gazelle:key value. Most directories apply in both the current directory and in subdirectories.

• # gazelle:build_file_name BUILD.bazel,BUILD: a comma-separated list of file names that Gazelle will recognize as Bazel build files. When Gazelle creates a new file, it will use the first name in this list.
• # gazelle:build_tags foo,bar,baz: a comma-separated list of build tags that Gazelle will consider as always true.
• # gazelle:exclude path: prevents Gazelle from processing a file or directory. If the path refers to a source file, Gazelle won't include it in any rules. If the path refers to a directory, Gazelle won't recurse into it. The path may refer to something in a subdirectory, for example, a testdata directory somewhere in a vendor tree. This directive may be repeated to exclude multiple paths, one per line.
• # gazelle:ignore: prevents Gazelle from updating the build file. Gazelle will still read rules in the build file and may modify build files in subdirectories. Use # gazelle:exclude to ignore a subdirectory.
• # gazelle:importmap_prefix path: a prefix for importmap attributes on library rules. The full importmap for a package is determined by concatenating this prefix with the relative path from the directory where the prefix is set to the package directory. This is set automatically in vendor directories.
• # gazelle:prefix example.com/repo: a prefix for importpath attributes on library rules. The full importpath for a package is determined by concatenating this prefix with the relative path from the directory where the prefix is set to the package directory. This may also be set with the -go_prefix parameter.
• # gazelle:proto <mode>: Tells Gazelle how to generate rules for .proto files. Applies to the current directory and subdirectories. Valid values for mode are:
  • default: proto_library, go_proto_library, go_grpc_library, and go_library rules are generated using @io_bazel_rules_go//proto:def.bzl. This is the default mode.
  • legacy: filegroup rules are generated for use by @io_bazel_rules_go//proto:go_proto_library.bzl. go_proto_library rules must be written by hand. Gazelle will run in this mode automatically if go_proto_library.bzl is loaded to avoid disrupting existing projects, but this can be overridden with a directive.
  • disable: .proto files are ignored. Gazelle will run in this mode automatically if go_proto_library is loaded from any other source, but this can be overridden with a directive.

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
    ],
)

When Gazelle is invoked with the fix command, in addition to updating source files and dependencies of existing rules, Gazelle will remove deprecated usage of the Go rules, analogous to go fix. The following transformations are performed.

Squash cgo libraries: 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.

Remove legacy protos: 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.