docs/versions/9.3.0/reference/be/make-variables.mdx

---
title: 'Make Variables'
---

* [Use](#use)
* [Predefined variables](#predefined_variables)
* [Predefined genrule variables](#predefined_genrule_variables)
* [Predefined source/output path variables](#predefined_label_variables)
* [Custom variables](#custom_variables)

"Make" variables are a special class of expandable string variables available
to attributes marked as *"Subject to 'Make variable' substitution"*.

These can be used, for example, to inject specific toolchain paths into
user-constructed build actions.

Bazel provides both *predefined* variables, which are available to all
targets, and *custom* variables, which are defined in dependency targets
and only available to targets that depend on them.

The reason for the term "Make" is historical: the syntax and semantics of
these variables were originally intended to match [GNU
Make](https://www.gnu.org/software/make/manual/html_node/Using-Variables.html).

## Use {#use}

Attributes marked as *"Subject to 'Make variable' substitution"* can
reference the "Make" variable `FOO` as follows:

`my_attr = "prefix $(FOO) suffix"`

In other words, any substring matching `$(FOO)` gets expanded
to `FOO`'s value. If that value is `"bar"`, the final
string becomes:

`my_attr = "prefix bar suffix"`

If `FOO` doesn't correspond to a variable known to the consuming
target, Bazel fails with an error.

"Make" variables whose names are non-letter symbols, such as
`@`, can also be referenced using only a dollar sign, without
the parentheses. For example:

`my_attr = "prefix $@ suffix"`

To write `$` as a string literal (i.e. to prevent variable
expansion), write `$$`.

## Predefined variables {#predefined_variables}

Predefined "Make" variables can be referenced by any attribute marked as
*"Subject to 'Make variable' substitution"* on any target.

To see the list of these variables and their values for a given set of build
options, run

`bazel info --show_make_env [build options]`

and look at the top output lines with capital letters.

[See an example of predefined variables](https://github.com/bazelbuild/examples/tree/main/make-variables#predefined-variables).

**Toolchain option variables**

* `COMPILATION_MODE`:
  `fastbuild`, `dbg`, or `opt`. ([more
  details](https://bazel.build/versions/9.3.0/docs/user-manual#flag--compilation_mode))

**Path variables**

* `BINDIR`: The base of the generated binary tree for the target
  architecture.

  Note that a different tree may be used for programs that run during the
  build on the host architecture, to support cross-compiling.

  If you want to run a tool from within a `genrule`, the
  recommended way to get its path is `$(execpath toolname)`,
  where *toolname* must be listed in the `genrule`'s
  `tools` attribute.
* `GENDIR`:
  The base of the generated code tree for the target architecture.

**Machine architecture variables**

* `TARGET_CPU`:
  The target architecture's CPU, e.g. `k8`.

## Predefined genrule variables {#predefined_genrule_variables}

The following are specially available to `genrule`'s
`cmd` attribute and are
generally important for making that attribute work.

[See an example of predefined genrule variables](https://github.com/bazelbuild/examples/tree/main/make-variables#predefined-genrule-variables).

* `OUTS`: The `genrule`'s `outs` list. If you have
  only one output file, you can also use `$@`.
* `SRCS`: The `genrule`'s `srcs` list (or more
  precisely: the path names of the files corresponding to labels in the
  `srcs` list).
  If you have only one source file, you can also use `$<`.
* `<`: `SRCS`, if it is a single file. Else triggers
  a build error.
* `@`: `OUTS`, if it is a single file. Else triggers a
  build error.
* `RULEDIR`: The output directory of the target, that is, the
  directory corresponding to the name of the package containing the target
  under the `genfiles` or `bin` tree. For
  `//my/pkg:my_genrule` this always ends in `my/pkg`,
  even if `//my/pkg:my_genrule`'s outputs are in subdirectories.
* `@D`: The output directory. If
  [outs](/versions/9.3.0/reference/be/general#genrule.outs) has one entry,
  this expands to the directory containing that file. If it has multiple
  entries, this expands to the package's root directory in the
  `genfiles` tree, *even if all output files are in the same
  subdirectory*!

  **Note:** Use `RULEDIR` over `@D` because
  `RULEDIR` has simpler semantics and behaves the same way
  regardless of the number of output files.

  If the genrule needs to generate temporary intermediate files (perhaps as
  a result of using some other tool like a compiler), it should attempt to
  write them to `@D` (although `/tmp` will also
  be writable) and remove them before finishing.

  Especially avoid writing to directories containing inputs. They may be on
  read-only filesystems. Even if not, doing so would trash the source tree.

**Note:** If the filenames corresponding to the input labels or the output
filenames contain spaces, `'`, or other special characters (or your
genrule is part of a Starlark macro which downstream users may invoke on such
files), then `$(SRCS)` and `$(OUTS)` are not suitable
for interpolation into a command line, as they do not have the semantics that
`"${@}"` would in Bash.

One workaround is to convert to a Bash array, with

```
mapfile SRCS <<< "$$(sed -e 's/ /\\n/g' <<'genrule_srcs_expansion'
$(SRC)
genrule_srcs_expansion
)
```

and then use `"$$\{SRCS[@]}"` in subsequent command lines in place
of `$(SRCS)`. A more robust option is to write a Starlark rule
instead.

## Predefined source/output path variables {#predefined_label_variables}

The predefined variables `execpath`, `execpaths`,
`rootpath`, `rootpaths`, `location`, and
`locations` take label parameters (e.g. `$(execpath //foo:bar)`) and substitute the file paths denoted by that label.

For source files, this is the path relative to your workspace root.
For files that are outputs of rules, this is the file's *output path*
(see the explanation of *output files* below).

[See an example of predefined path variables](https://github.com/bazelbuild/examples/tree/main/make-variables#predefined-path-variables).

* `execpath`: Denotes the path beneath the
  [execroot](/versions/9.3.0/docs/output_directories)
  where Bazel runs build actions.

  In the above example, Bazel runs all build actions in the directory linked
  by the `bazel-myproject` symlink in your workspace root. The
  source file `empty.source` is linked at the path
  `bazel-myproject/testapp/empty.source`. So its exec path (which
  is the subpath below the root) is `testapp/empty.source`. This
  is the path build actions can use to find the file.

  Output files are staged similarly, but are also prefixed with the subpath
  `bazel-out/cpu-compilation_mode/bin` (or for the outputs of
  tools: `bazel-out/cpu-opt-exec-hash/bin`). In the above example,
  `//testapp:app` is a tool because it appears in
  `show_app_output`'s `tools` attribute.
  So its output file `app` is written to
  `bazel-myproject/bazel-out/cpu-opt-exec-hash/bin/testapp/app`.
  The exec path is thus `bazel-out/cpu-opt-exec-hash/bin/testapp/app`. This extra prefix
  makes it possible to build the same target for, say, two different CPUs in
  the same build without the results clobbering each other.

  The label passed to this variable must represent exactly one file. For
  labels representing source files, this is automatically true. For labels
  representing rules, the rule must generate exactly one output. If this is
  false or the label is malformed, the build fails with an error.
* `rootpath`: Denotes the path that a built binary can use to
  find a dependency at runtime relative to the subdirectory of its runfiles
  directory corresponding to the main repository.
  **Note:** This only works if [`--enable_runfiles`](/versions/9.3.0/reference/command-line-reference#flag--enable_runfiles) is enabled, which is not the case on
  Windows by default. Use `rlocationpath` instead for
  cross-platform support.

  This is similar to `execpath` but strips the configuration
  prefixes described above. In the example from above this means both
  `empty.source` and `app` use pure workspace-relative
  paths: `testapp/empty.source` and `testapp/app`.

  The `rootpath` of a file in an external repository
  `repo` will start with `../repo/`, followed by the
  repository-relative path.

  This has the same "one output only" requirements as `execpath`.
* `rlocationpath`: The path a built binary can pass to the `Rlocation` function of a runfiles library to find a dependency at
  runtime, either in the runfiles directory (if available) or using the
  runfiles manifest.

  This is similar to `rootpath` in that it does not contain
  configuration prefixes, but differs in that it always starts with the
  name of the repository. In the example from above this means that `empty.source` and `app` result in the following
  paths: `myproject/testapp/empty.source` and `myproject/testapp/app`.

  The `rlocationpath` of a file in an external repository
  `repo` will start with `repo/`, followed by the
  repository-relative path.

  Passing this path to a binary and resolving it to a file system path using
  the runfiles libraries is the preferred approach to find dependencies at
  runtime. Compared to `rootpath`, it has the advantage that it
  works on all platforms and even if the runfiles directory is not
  available.

  This has the same "one output only" requirements as `execpath`.
* `location`: A synonym for either `execpath` or
  `rootpath`, depending on the attribute being expanded. This is
  legacy pre-Starlark behavior and not recommended unless you really know what
  it does for a particular rule. See [#2475](https://github.com/bazelbuild/bazel/issues/2475#issuecomment-339318016)
  for details.

`execpaths`, `rootpaths`, `rlocationpaths`,
and `locations` are the plural variations of `execpath`,
`rootpath`, `rlocationpath`, and`location`,
respectively. They support labels producing multiple outputs, in which case
each output is listed separated by a space. Zero-output rules and malformed
labels produce build errors.

All referenced labels must appear in the consuming target's `srcs`,
output files, or `deps`. Otherwise the build fails. C++ targets can
also reference labels in `data`.

Labels don't have to be in canonical form: `foo`, `:foo`
and `//somepkg:foo` are all fine.

## Custom variables {#custom_variables}

Custom "Make" variables can be referenced by any attribute marked as
*"Subject to 'Make variable' substitution"*, but only on targets that
depend on other targets that *define* these variables.

As best practice all variables should be custom unless there's a really good
reason to bake them into core Bazel. This saves Bazel from having to load
potentially expensive dependencies to supply variables consuming tarets may
not care about.

**C++ toolchain variables**

The following are defined in C++ toolchain rules and available to any rule
that sets `toolchains = ["@bazel_tools//tools/cpp:toolchain_type"]`
Some rules, like `java_binary`, implicitly
include the C++ toolchain in their rule definition. They inherit these variables
automatically.

The built-in C++ rules are much more sophisticated than "run the compiler on
it". In order to support compilation modes as diverse as \*SAN, ThinLTO,
with/without modules, and carefully optimized binaries at the same time as
fast running tests on multiple platforms, the built-in rules go to great
lengths to ensure the correct inputs, outputs, and command-line flags are set
on each of potentially multiple internally generated actions.

These variables are a fallback mechanism to be used by language experts in
rare cases. If you are tempted to use them, please [contact the Bazel devs](https://bazel.build/versions/9.3.0/help) first.

* `ABI`: The C++ ABI version.
* `AR`: The "ar" command from crosstool.
* `C_COMPILER`:
  The C/C++ compiler identifier, e.g. `llvm`.
* `CC`: The C and C++ compiler command.

  We strongly recommended always using `CC_FLAGS` in
  combination with `CC`. Fail to do so at your own risk.
* `CC_FLAGS`: A minimal set of flags for the C/C++
  compiler to be usable by genrules. In particular, this contains flags to
  select the correct architecture if `CC` supports multiple
  architectures.
* `DUMPBIN`: Microsoft COFF Binary File Dumper (dumpbin.exe) from
  from Microsoft Visual Studio.
* `NM`: The "nm" command from crosstool.
* `OBJCOPY`: The objcopy command from the same suite as the C/C++
  compiler.
* `STRIP`: The strip command from the same suite as the C/C++
  compiler.

**Java toolchain variables**

The following are defined in Java toolchain rules and available to any rule
that sets `toolchains = ["@rules_java//toolchains:current_java_runtime"]` (or
`"@rules_java//toolchains:current_host_java_runtime"`
for the host toolchain equivalent).

Most of the tools in the JDK should not be used directly. The built-in Java
rules use much more sophisticated approaches to Java compilation and packaging
than upstream tools can express, such as interface Jars, header interface
Jars, and highly optimized Jar packaging and merging implementations.

These variables are a fallback mechanism to be used by language experts in
rare cases. If you are tempted to use them, please [contact the Bazel devs](https://bazel.build/versions/9.3.0/help) first.

* `JAVA`: The "java" command (a Java virtual
  machine). Avoid this, and use a `java_binary` rule
  instead where possible. May be a relative path. If you must change
  directories before invoking `java`, you need to capture the
  working directory before changing it.
* `JAVABASE`: The base directory containing the
  Java utilities. May be a relative path. It will have a "bin"
  subdirectory.

**Starlark-defined variables**

Rule and [toolchain](/versions/9.3.0/docs/toolchains) writers can define
completely custom variables by returning a
[TemplateVariableInfo](/versions/9.3.0/rules/lib/TemplateVariableInfo)
provider. Any rules depending on these through the
`toolchains` attribute can then read their values:

[See an example of Starlark-defined variables](https://github.com/bazelbuild/examples/tree/main/make-variables#custom-starlark-defined-variables).