Output Directory Layout

This page covers requirements and layout for output directories.

Requirements {:#requirements}

Requirements for an output directory layout:

• 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.")

Current layout {:#layout}

The solution that's currently implemented:

• 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 ~/.cache/bazel on Linux, ~/Library/Caches/bazel on macOS (when using Bazel 9 and newer), 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 $XDG_CACHE_HOME is set on either Linux or macOS, the value ${XDG_CACHE_HOME}/bazel will override the default. If the environment variable $TEST_TMPDIR is set, as in a test of Bazel itself, then that value overrides any defaults.
  • Note that Bazel 8.x and earlier on macOS used /private/var/tmp as outputRoot, and ignored $XDG_CACHE_HOME.
• The Bazel user's build state is located beneath outputRoot/_bazel_$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.

The symlinks for "bazel-<workspace-name>", "bazel-out", "bazel-testlogs", and "bazel-bin" are put 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, this is done only if the workspace root is writable.

Layout diagram {:#layout-diagram}

The directories are laid out as follows:

<pre> &lt;workspace-name&gt;/ <== The workspace root bazel-my-project => <..._main> <== Symlink to execRoot bazel-out => <...bazel-out> <== 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. 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 _tmp/actions/ <== Action output directory. This contains a file with the stdout/stderr for every action from the most recent bazel run that produced output. 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 foo/bar/baz.runfiles/ <== The runfiles symlink farm for the //foo/bar:baz executable. MANIFEST _main/ ... genfiles/ <== 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) 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) &lt;packages&gt;/ <== Packages referenced in the build appear as if under a regular workspace </pre>

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.