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.”)

Current layout

The solution that's currently implemented:

  • Bazel must be invoked from a directory containing a WORKSPACE file (the “workspace directory”), or a subdirectory thereof. It reports an error if it is not.
  • The outputRoot directory defaults to ~/.cache/bazel on Linux, /private/var/tmp on macOS, and on Windows it defaults to %HOME% if set, else %USERPROFILE% if set, else the result of calling SHGetKnownFolderPath() with the FOLDERID_Profile flag set. If the environment variable $TEST_TMPDIR is set, as in a test of bazel itself, then that value overrides the default.
  • We stick the Bazel user's build state beneath outputRoot/_bazel_$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>”, “bazel-out”, “bazel-testlogs”, and “bazel-bin” in the workspace directory; these symlinks point 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.

Layout diagram

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.