Zero-dependency scripts to simplify acquiring and running SCons on Windows machines

SCons is a powerful build tool, but using it to build a project on a Windows computer requires several manual steps: first, one must install Python, and then SCons. Only then can one type "scons" to build the project. On UNIX-derived systems, this is not such a problem, because SCons and Python can usually be installed via the system's native package manager (e.g., apt, rpm).

The goal of bootstrap-scons is to make SCons-based projects effortless to build on Windows. By using bootstrap-scons, a SCons-based project may be cloned from source control, and then built by running a single command:

D:\> git clone <my-project-url.git> my-project
D:\> cd my-project
D:\my-project\> build

The "build" command first locates or installs Python, then locates or installs SCons, and finally invokes SCons to build the project.

To use this in your project, do the following:

1. Drop a copy of bootstrap-scons as a subdirectory of your project, or better yet add bootstrap-scons as a submodule of your project. E.g.:

   D:\my-project\> git submodule add git@github.com:jediry/bootstrap-scons.git
   

   By adding bootstrap-scons as a submodule, you can easily pick up updates and bugfixes, just by updating your submodule version. (Alternately, if you want to make significant customizations to your copy of bootstrap-scons, consider forking it on GitHub, and then adding your fork as the submodule.)

2. Copy bootstrap-scons/build.bat.example and modify it to your liking. E.g.:

   D:\my-project\> copy bootstrap-scons\build.bat.example build.bat
   

The main things that you might want to customize are:

• Which versions of Python/SCons are downloaded, if none can be found
• Where the downloaded Python/SCons are installed
• Whether the detected Python/SCons are cached in environment variables

Take a look at build.bat.example included with bootstrap-scons to see how to alter these things, or invoke find_python.bat or find_scons.bat with the /? parameter to see all the things that can be customized.

A primary goal of bootstrap-scons is to be zero dependency: it does what it does without requiring any software that isn't already available on any Windows machine. This is fairly limiting: no wget/curl, no PowerShell...however, once Python has been found, pretty much anything can be done with relative ease.

When you run build.bat, here's what happens:

• build.bat sets default configuration values, as chosen by the project authors (see above). However, the user may override these defaults by explicitly setting the appropriate environment variables.
• build.bat invokes bootstrap-scons/scons.bat. Most of the logic (but none of the configuration) lives inside of scons.bat, since build.bat is intended to be copied and modified by project authors. This split is advantageous because it allows the core of bootstrap-scons to be updated (e.g., for bugfixes or new features) without stomping on your project's customizations.
  • scons.bat checks the %FOUND_SCONS_AT% environment variable to see whether it has already located and/or installed scons. If this variable is unset, it invokes find_scons.bat.
    • But before we can worry about SCons, we need Python. find_scons.bat checks the %FOUND_PYTHON_AT% environment variable to see whether it has already located and/or installed Python. If this variable is unset, it invokes find_python.bat.
      • find_python.bat searches in a number of places for an already-installed copy of Python, including the %PATH%, %PYTHONHOME%, and several places that Python is typically placed by installers, and %FIND_PYTHON_LOCAL_ROOT%, where any auto-downloaded copy of Python will be unpacked.
      • If no copy of Python is discovered, and if configured to download/install Python, find_python.bat constructs the download URL for the python-embed package of the specified version, and then invokes download-and-unzip.vbs to do the actual download and unpacking of the .zip archive (cmd.exe itself cannot do either of these things, but VBScript can, with a litle help from some COM objects).
      • Else, if not configured to download/install Python, find_python.bat aborts with an error message.
      • If Python was found or installed, %FOUND_PYTHON_AT% is set to point to it. This tells find_scons.bat and scons.bat where to find it, and also lets find_python.bat avoid re-doing this search in the future.
    • Now find_scons.bat knows where Python is. find_scons.bat searches in a few places for an already-installed copy of SCons, including the "scripts" directory of the version of Python selected, and %FIND_SCONS_LOCAL_ROOT%, where any auto-downloaded copy of SCons will be unpacked.
    • If no copy of SCons is discovered, and if configured to download/install SCons, find_scons.bat invokes itself as a Python script, and then uses Python to download and unpack SCons under %FIND_SCONS_LOCAL_ROOT%.
    • Else, if not configured to download/install SCons, find_scons.bat aborts with an error message.
    • If SCons was successfully found/installed, %FOUND_SCONS_AT% is set to point to it. This tells scons.bta where to find it, and also lets find_scons.bat avoid re-doing this search in the future.
  • If Python & SCons were successfully located/installed, scons.bat now launches SCons to build the project.
• Finally, build.bat propagates the values of %FOUND_SCONS_AT% and %FOUND_PYTHON_AT% into the calling shell's environment, so that future runs of build.bat can avoid re-doing all this work. (If this behavior is not desired, build.bat can be easily modified to avoid any changes to the shell environment; see comments in build.bat.example.)