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"Make" Variables

This section describes how to use a special class of built-in string variables that are called the "Make" environment. Defining custom "Make" variables is not supported.

(The reason for the term "Make" is historical: the syntax and semantics of these variables are somewhat similar to those of GNU Make.)

To see the list of all common "Make" variables and their values, run bazel info --show_make_env.

Build rules can introduce additional rule specific variables. One example is the cmd attribute of a genrule.

"Make" variable substitution

Variables can be referenced in attributes using $(FOO) where FOO is the variable name. In the attribute documentation of rules, it is mentioned when an attribute is subject to "Make" variable substitution. For those attributes this means that any substrings of the form $(X) within those attributes will be interpreted as references to the "Make" variable X, and will be replaced by the appropriate value of that variable for the applicable build configuration. The parens may be omitted for variables whose name is a single character.

It is an error if such attributes contain embedded strings of the form $(X) where X is not the name of a "Make" variable, or unclosed references such as $( not matched by a corresponding ).

Within such attributes, literal dollar signs must be escaped as $$ to prevent this expansion.

Those attributes that are subject to this substitution are explicitly indicated as such in their definitions in this document.

Predefined "Make" Variables

Bazel defines a set of "Make" variables for you.

The build system also provides a consistent PATH environment variable for genrules and tests which need to execute shell commands. For genrules, you can indirect your commands using the "Make" variables below. For basic Unix utilities, prefer relying on the PATH environment variable to guarantee correct results. For genrules involving compiler and platform invocation, you must use the "Make" variable syntax. The same basic command set is also available during tests. Simply rely on the PATH.

Compiler and Platforms available to genrules

These tools may not be in the PATH, therefore you must use "Make" variable syntax in your genrule's cmd attribute.
  • CC: The C and C++ compiler command. 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, LIPO, 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 environment variables are a fallback mechanism to be used by language experts in rare cases. If you are tempted to use them, please talk to us first.

    It is strongly recommended to always use CC_FLAGS in combination with CC. Fail to do so at your own risk.

  • C_COMPILER: The C/C++ compiler identifier, e.g. "gcc".
  • 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.
  • STRIP: The strip command from the same suite as the C/C++ compiler.
  • AR: The "ar" command from crosstool.
  • NM: The "nm" command from crosstool.
  • OBJCOPY: The objcopy command from the same suite as the C/C++ compiler.

Tool option Variables

  • CC_FLAGS: A minimal set of flags for the C/C++ compiler to be used by genrules. In particular, it contains flags to select the correct architecture if CC supports multiple architectures.
  • COMPILATION_MODE: "fastbuild", "dbg", or "opt".

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 of specifying the path to the tool is to use $(location toolname), where toolname must be listed in the tools attribute for the genrule.
  • GENDIR: The base of the generated code tree for the target architecture.
  • JAVABASE: for Java, most of the tools in the JDK should not be used as-is. The built-in Java rules use much more sophisticated approaches to Java compilation and packaging than the upstream tools can express, such as interface Jars, header interface Jars, and highly optimized Jar packaging and merging implementations.

    The base directory containing the Java utilities. May be a relative path. It will have a "bin" subdirectory.

Architecture Variables

  • ABI: The C++ ABI version; currently "gcc-3.4".
  • TARGET_CPU: The target architecture's cpu, e.g. "piii" or "k8".

Other Variables available to the cmd attribute of a genrule

  • OUTS: The outs list. If you have only one output file, you can also use $@.
  • SRCS: The srcs list (or more precisely, the pathnames 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.
  • @: outs, if it is a single file.
  • @D: The output directory. If there is only one filename in outs, this expands to the directory containing that file. If there are multiple filenames, this variable instead expands to the package's root directory in the genfiles tree, even if all the generated files belong to the same subdirectory! If the genrule needs to generate temporary intermediate files (perhaps as a result of using some other tool like a compiler) then it should attempt to write the temporary files to @D (although /tmp will also be writable), and to remove any such generated temporary files. Especially, avoid writing to directories containing inputs - they may be on read-only filesystems, and even if they aren't, doing so would trash the source tree.

"$(location)" substitution

In attributes that support it, all occurrences of $(location label) are replaced by the path to the file denoted by label. Use location if the label outputs exactly one filename. This allows bazel to perform a check and give an error if no or more than one files are represented by the given label; a label referring to a source file always represents a single file, but a label referring to a rule refers to all output files of that rule. Otherwise use $(locations label); bazel will then raise an error if no files are generated. In both cases, if the label is malformed then an error is raised.

The label needs not be in canonical form: foo, :foo and //somepkg:foo are all fine. It may also be the name of an output file from the outs attribute.

The expanded paths are relative to the runfiles directory of the *_test or *_binary rule.