layout: documentation title: Output Directory Layout

Output Directory Layout

Requirements

Requirements for an output directory layout:

  • Don't collide if multiple users are building on the same box.
  • Support building in multiple workspaces at the same time.
  • Support building for multiple target configurations in the same workspace.
  • Don't collide with any other tools.
  • Be easy to access.
  • Be easy to clean, even selectively.
  • Is unambiguous, even if the user relies on symbolic links when changing into their client directory.
  • All the build state per user should be underneath one directory (“I'd like to clean all the .o files from all my clients.”)

Documentation of the current Bazel output directory layout

The solution that's currently implemented:

  • Bazel must be invoked from a directory containing a WORKSPACE file. It reports an error if it is not. We call this the workspace directory.
  • The outputRoot directory is ~/.cache/bazel. (Unless $TEST_TMPDIR is set, as in a test of bazel itself, in which case this directory is used instead.)
  • We stick the Bazel user's build state beneath outputRoot/_bazel_$USER. This is called the outputUserRoot directory.
  • Beneath the outputUserRoot directory, we create an installBase directory whose name is “install” plus the MD5 hash of the Bazel installation manifest.
  • Beneath the outputUserRoot directory, we also create an outputBase directory whose name is the MD5 hash of the path name of the workspace directory. So, for example, if Bazel is running in the workspace directory /home/user/src/my-project (or in a directory symlinked to that one), then we create an output base directory called: /home/user/.cache/bazel/_bazel_user/7ffd56a6e4cb724ea575aba15733d113.
  • Users can use Bazel's --output_base startup option to override the default output base directory. For example, bazel --output_base=/tmp/bazel/output build x/y:z.
  • Users can also use Bazel's --output_user_root startup option to override the default install base and output base directories. For example: bazel --output_user_root=/tmp/bazel build x/y:z.

We put symlinks “bazel-<workspace-name>” and “bazel-out”, as well as “bazel-bin”, “bazel-genfiles”, and “bazel-includes” in the workspace directory; these symlinks points to some directories inside a target-specific directory inside the output directory. These symlinks are only for the user's convenience, as Bazel itself does not use them. Also, we only do this if the workspace directory is writable. The names of the “bazel-bin”, “bazel-genfiles”, and “bazel-include” symlinks are affected by the --symlink_prefix option to bazel, but “bazel-<workspace-name>” and “bazel-out” are not.

Bazel internals: Directory layout

The directories are laid out as follows:

The layout of the *.runfiles directories is documented in more detail in the places pointed to by RunfilesSupport.

bazel clean

bazel clean does an rm -rf on the outputPath and the action_cache directory. It also removes the workspace symlinks. The --expunge option will clean the entire outputBase.