Output Directory Layout


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:

<workspace-name>/                         <== The workspace directory
  bazel-my-project => <...my-project>     <== Symlink to execRoot
  bazel-out => <...bin>                   <== Convenience symlink to outputPath
  bazel-bin => <...bin>                   <== Convenience symlink to most recent written bin dir $(BINDIR)
  bazel-genfiles => <...genfiles>         <== Convenience symlink to most recent written genfiles dir $(GENDIR)

/home/user/.cache/bazel/                  <== Root for all Bazel output on a machine: outputRoot
  _bazel_$USER/                           <== Top level directory for a given user depends on the user name:
      fba9a2c87ee9589d72889caf082f1029/   <== Hash of the Bazel install manifest: installBase
        _embedded_binaries/               <== Contains binaries and scripts unpacked from the data section of
                                              the bazel executable on first run (e.g. helper scripts and the
                                              main Java file BazelServer_deploy.jar)
    7ffd56a6e4cb724ea575aba15733d113/     <== Hash of the client's workspace directory (e.g.
                                              /home/some-user/src/my-project): outputBase
      action_cache/                       <== Action cache directory hierarchy
                                              This contains the persistent record of the file
                                              metadata (timestamps, and perhaps eventually also MD5
                                              sums) used by the FilesystemValueChecker.
      action_outs/                        <== Action output directory. This contains a file with the
                                              stdout/stderr for every action from the most recent
                                              bazel run that produced output.
      command.log                         <== A copy of the stdout/stderr output from the most
                                              recent bazel command.
      external/                           <== The directory that remote repositories are
                                              downloaded/symlinked into.
      server/                             <== The Bazel server puts all server-related files (such
                                              as socket file, logs, etc) here.
        jvm.out                           <== The debugging output for the server.
      execroot/                           <== The working directory for all actions. For special
                                              cases such as sandboxing and remote execution, the
                                              actions run in a directory that mimics execroot.
                                              Implementation details, such as where the directories
                                              are created, are intentionally hidden from the action.
                                              All actions can access its inputs and outputs relative
                                              to the execroot directory.
        <workspace-name>/                 <== Working tree for the Bazel build & root of symlink forest: execRoot
          _bin/                           <== Helper tools are linked from or copied to here.

          bazel-out/                      <== All actual output of the build is under here: outputPath
            local_linux-fastbuild/        <== one subdirectory per unique target BuildConfiguration instance;
                                              this is currently encoded
              bin/                        <== Bazel outputs binaries for target configuration here: $(BINDIR)
                foo/bar/_objs/baz/        <== Object files for a cc_* rule named //foo/bar:baz
                  foo/bar/baz1.o          <== Object files from source //foo/bar:baz1.cc
                  other_package/other.o   <== Object files from source //other_package:other.cc
                foo/bar/baz               <== foo/bar/baz might be the artifact generated by a cc_binary named
                foo/bar/baz.runfiles/     <== The runfiles symlink farm for the //foo/bar:baz executable.
              genfiles/                   <== Bazel puts generated source for the target configuration here:
                foo/bar.h                     e.g. foo/bar.h might be a headerfile generated by //foo:bargen
              testlogs/                   <== Bazel internal test runner puts test log files here
                foo/bartest.log               e.g. foo/bar.log might be an output of the //foo:bartest test with
                foo/bartest.status            foo/bartest.status containing exit status of the test (e.g.
                                              PASSED or FAILED (Exit 1), etc)
              include/                    <== a tree with include symlinks, generated as needed.  The
                                              bazel-include symlinks point to here. This is used for
                                              linkstamp stuff, etc.
            host/                         <== BuildConfiguration for build host (user's workstation), for
                                              building prerequisite tools, that will be used in later stages
                                              of the build (ex: Protocol Compiler)
        <packages>/                       <== Packages referenced in the build appear as if under a regular workspace

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.