What Are Bazel Labels?

Defining Key Terms

  • Workspace: Bazel builds software from source code organized in a directory tree called a workspace.

  • Repositories: The root of the workspace containing the WORKSPACE file is the main repo and is identified with @.

  • Package: Source files in the workspace are organized in a nested hierarchy of packages. Each package is a directory that contains a set of related source files and one BUILD file.

  • BUILD File: The BUILD file specifies what software outputs can be built from the source.

  • Target: A package is a container of targets, which are defined in the package’s BUILD file. Most targets are one of two principal kinds, files and rules. target-name is the name of the target within the package. The name of a rule is the value of the name attribute in the rule’s declaration in a BUILD file; the name of a file is its pathname relative to the directory containing the BUILD file.

Labels

A label is the name that references a target for a specific package:

@myrepo//packages/myapp:targetname

A label is constructed of three parts:

  • The repository name identified with @myrepo.
  • The root path to the package (where the BUILD file of the package is located) //packages/myapp.
  • Lastly, the target name (a name or a path depending on the target if is a rule or file) :targetname.

If a target is a file, the name of the target is the path to the file from where the BUILD file is located:

@myrepo//packages/myapp:testdata/input.txt

For example, here, the name of the target is testdata/input.txt.

To Be Continued