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# Using Bazel on Windows

## Installation

See Install Bazel on Windows for installation instructions.

## Known issues

We mark Windows-related Bazel issues on GitHub with the "multi-platform > windows" label. You can see the open issues here.

## Running Bazel: MSYS2 shell vs. Command Prompt vs. PowerShell

It's best to run Bazel from the Command Prompt (cmd.exe) or from PowerShell.

You can also run Bazel from the MSYS2 shell, but you need to disable MSYS2's automatic path conversion. See this StackOverflow answer for details.

## Setting environment variables

Environment variables you set in the Windows Command Prompt (cmd.exe) are only set in that command prompt session. If you start a new cmd.exe, you need to set the variables again. To always set the variables when cmd.exe starts, you can add them to the User variables or System variables in the Control Panel > System Properties > Advanced > Environment Variables... dialog box.

## Using Bazel on Windows

The first time you build any target, Bazel auto-configures the location of Python and the Visual C++ compiler. If you need to auto-configure again, run bazel clean then build a target.

You can also tell Bazel where to find the Python binary and the C++ compiler: - use the --python_path=c:\path\to\python.exe flag for Python - use the BAZEL_VC or BAZEL_VS environment variable. See the Build C++ section below.

### Build C++

To build C++ targets, you need:

• The Visual C++ compiler.

You can install it in one of the following ways:

• Install Visual Studio 2015 or later (Community Edition is enough) with Visual C++.

Make sure to also install the Visual C++ > Common Tools for Visual C++ and Visual C++ > Microsoft Foundation Classes for C++ features. These features are not installed by default.

• Install the Visual C++ Build Tools 2015 or later.

If alwayslink doesn't work with VS 2017, that is due to a known issue, please upgrade your VS 2017 to the latest version.

• The BAZEL_VS or BAZEL_VC environment variable.

Bazel tries to locate the C++ compiler the first time you build any target. To tell Bazel where the compiler is, you can set one of the following environment variables:

• BAZEL_VS storing the Visual Studio installation directory
• BAZEL_VC storing the Visual C++ Build Tools installation directory

Setting one of these variables is enough. For example:

set BAZEL_VS=C:\Program Files (x86)\Microsoft Visual Studio 14.0


or

set BAZEL_VC=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC

• The Windows SDK.

The Windows SDK contains header files and libraries you need when building Windows applications, including Bazel itself.

If everything is set up, you can build a C++ target now!

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/cpp:hello-world

C:\projects\bazel> bazel-bin\examples\cpp\hello-world.exe


### Build Java

There's no setup necessary.

On Windows, Bazel builds two output files for java_binary rules:

• a .jar file
• a .exe file that can set up the environment for the JVM and run the binary

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/java-native/src/main/java/com/example/myproject:hello-world

C:\projects\bazel> bazel-bin\examples\java-native\src\main\java\com\example\myproject\hello-world.exe


### Build Python

To build Python targets, you need:

• Both Python 2 and Python 3 are supported.

To tell Bazel where Python is, you can use --python_path=<path/to/python>. For example:

bazel build --python_path=C:/Python27/python.exe ...


If --python_path is not specified, Bazel uses python.exe as the interpreter and the binary looks for it in $PATH during runtime. If it is not in $PATH(for example, when you use py_binary as an action's executable, Bazel will sanitize \$PATH), then the execution will fail.

On Windows, Bazel builds two output files for py_binary rules:

• a self-extracting zip file
• an executable file that can launch the Python interpreter with the self-extracting zip file as the argument

You can either run the executable file (it has a .exe extension) or you can run Python with the self-extracting zip file as the argument.

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/py_native:bin

C:\projects\bazel> bazel-bin\examples\py_native\bin.exe

C:\projects\bazel> python bazel-bin\examples\py_native\bin.zip


If you are interested in details about how Bazel builds Python targets on Windows, check out this design doc.