Skip to content


Repository files navigation

Aspect's Bazel helpers library

Base Starlark libraries and basic Bazel rules which are useful for constructing rulesets and BUILD files.

This module depends on bazel-skylib. In theory all these utilities could be upstreamed to bazel-skylib, but the declared scope of that project is narrow and it's very difficult to get anyone's attention to review PRs there.

bazel-lib is just a part of what Aspect provides:


Installation instructions are included on each release:

To use a commit rather than a release, you can point at any SHA of the repo.

For example to use commit abc123:

  1. Replace url = "" with a GitHub-provided source archive like url = ""
  2. Replace strip_prefix = "bazel-lib-0.1.0" with strip_prefix = "bazel-lib-abc123"
  3. Update the sha256. The easiest way to do this is to comment out the line, then Bazel will print a message with the correct value.

Note that GitHub source archives don't have a strong guarantee on the sha256 stability, see

Public API

Copying files

  • copy_directory Copies directories to another package.
  • copy_file Copies files to another package.
  • copy_to_bin Copies a source file to output tree at the same workspace-relative path.
  • copy_to_directory Copies and arranges files and directories into a new directory.
  • write_source_files Write to one or more files or folders in the source tree. Stamp out tests that ensure the sources exist and are up to date.

Transforming files

  • jq A toolchain and custom rule for running jq, a tool that is "like sed for json".
  • yq A toolchain and custom rule for running yq, a "YAML, JSON and XML processor".

Manipulating paths

  • directory_path Provide a label to reference some path within a directory, via DirectoryPathInfo.
  • output_files Forwards a subset of the files (via the DefaultInfo provider) from a given target's DefaultInfo or OutputGroupInfo.

Writing rules

  • expand_make_vars Perform make variable and location substitions in strings..
  • expand_template Substitute templates with make variables, location resolves, stamp variables, and arbitrary strings.
  • paths Useful path resolution methods.
  • transitions Transition sources to a provided platform.
  • lists Functional-style helpers for working with list data structures.
  • utils Various utils for labels and globs.
  • params_file Generate encoded params file from a list of arguments.
  • repo_utils Useful methods for repository rule implementations.
  • run_binary Like skylib's run_binary but adds directory output support.
  • stamping Support version stamping in custom rules.
  • base64 Starlark Base64 encoder & decoder.


Generating documentation

  • docs Rules for generating docs and stamping tests to ensure they are up to date.