Output Directory Layout

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Requirements

• Doesn’t collide if multiple users are building on the same box.
• Supports building in multiple workspaces at the same time.
• Supports building for multiple target configurations in the same workspace.
• Doesn’t collide with any other tools.
• Is easy to access.
• Is 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.”)

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Current layout

• Bazel must be invoked from a directory containing a repo boundary file, or a subdirectory thereof. In other words, Bazel must be invoked from inside a repository. Otherwise, an error is reported.
• The outputRoot directory defaults to ${XDG_CACHE_HOME}/bazel (or ~/.cache/bazel, if the XDG_CACHE_HOME environment variable is not set) 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 $TEST_TMPDIR is set, as in a test of Bazel itself, then that value overrides the default.
• The Bazel user’s build state is located beneath outputRoot/_bazel_$USER. This is called the outputUserRoot directory.
• Beneath the outputUserRoot directory there is an install directory, and in it is an installBase directory whose name is the MD5 hash of the Bazel installation manifest.
• Beneath the outputUserRoot directory, an outputBase directory is also created whose name is the MD5 hash of the path name of the workspace root. So, for example, if Bazel is running in the workspace root /home/user/src/my-project (or in a directory symlinked to that one), then an output base directory is created called: /home/user/.cache/bazel/_bazel_user/7ffd56a6e4cb724ea575aba15733d113. You can also run echo -n $(pwd) | md5sum in the workspace root to get the MD5.
• You 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.
• You 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.

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Layout diagram

<workspace-name>/                         <== The workspace root
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 $(BINDIR)
bazel-testlogs => <…testlogs>         <== Convenience symlink to the test logs directory/home/user/.cache/bazel/                  <== Root for all Bazel output on a machine: outputRoot
bazel$USER/                           <== Top level directory for a given user depends on the user name:
outputUserRoot
install/
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 (such as helper scripts and the
main Java file BazelServer_deploy.jar)
7ffd56a6e4cb724ea575aba15733d113/     <== Hash of the client’s workspace root (such as
/home/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.
Every action can access its inputs and outputs relative
to the execroot directory.
_main/                            <== 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/        &lt;== Object files for a cc_* rule named //foo/bar:baz
                    foo/bar/baz1.o          &lt;== Object files from source //foo/bar:baz1.cc
                    other_package/other.o   &lt;== Object files from source //other_package:other.cc
                  foo/bar/baz               &lt;== foo/bar/baz might be the artifact generated by a cc_binary named
                                                //foo/bar:baz
                  foo/bar/baz.runfiles/     &lt;== The runfiles symlink farm for the //foo/bar:baz executable.
                    MANIFEST                   _main/
                      ...
                genfiles/                   &lt;== Bazel puts generated source for the target configuration here:
                                                (GENDIR)
foo/bar.h                     such as foo/bar.h might be a headerfile generated by //foo:bargen
testlogs/                   <== Bazel internal test runner puts test log files here
foo/bartest.log               such as 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 (such as
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

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bazel clean