# Preface

Thank you for taking the time to read about &SCons;. &SCons; is a modern software construction too - a software utility for building software (or other files) and keeping built software up-to-date whenever the underlying input files change.

The most distinctive thing about &SCons; is that its configuration files are actually scripts, written in the &Python; programming language. This is in contrast to most alternative build tools, which typically invent a new language to configure the build. &SCons; still has a learning curve, of course, because you have to know what functions to call to set up your build properly, but the underlying syntax used should be familiar to anyone who has ever looked at a Python script.

Paradoxically, using Python as the configuration file format makes &SCons; easier for non-programmers to learn than the cryptic languages of other build tools, which are usually invented by programmers for other programmers. This is in no small part due to the consistency and readability that are hallmarks of Python. It just so happens that making a real, live scripting language the basis for the configuration files makes it a snap for more accomplished programmers to do more complicated things with builds, as necessary.

## &SCons; Principles

There are a few overriding principles the &SCons; team tries to follow in the design and implementation.

Correctness

  First and foremost, by default, &SCons; guarantees a correct build even if it means sacrificing performance a little. We strive to guarantee the build is correct regardless of how the software being built is structured, how it may have been written, or how unusual the tools are that build it.

Performance

  Given that the build is correct, we try to make &SCons; build software as quickly as possible. In particular, wherever we may have needed to slow down the default &SCons; behavior to guarantee a correct build, we also try to make it easy to speed up &SCons; through optimization options that let you trade off guaranteed correctness in all end cases for a speedier build in the usual cases.

Convenience

  &SCons; tries to do as much for you out of the box as reasonable, including detecting the right tools on your system and using them correctly to build the software.

In a nutshell, we try hard to make &SCons; just "do the right thing" and build software correctly, with a minimum of hassles.

## How to Use this Guide

This guide intends to coach you how to use &SCons; effectively and efficiently, by providing a range of examples and usage scenarios. As such it is not exactly a tutorial (as usually those build a single example topic from start to finish), but if you are just starting with &SCons; it is recommended you step through the first 10 chapters in sequence as this will give a solid grounding in the principles of working with &SCons;. If you follow that trail, you can feel free to initially skip sections on extending &SCons;, such as Writing your own Decider Function, and come back to those if the need arises.

The remaining chapters cover more advanced topics that not all build systems will need, and can be used in more of a single-topic way, to read if you find you need that particular information.

It is often useful to keep &SCons; man page open in a separate browser tab or window to refer to as a complement to this Guide, as the User Guide does not attempt to provide every detail. While this Guide's Appendices A-D do duplicate information that appears in the man page (this is to allow intra-document links to definitions of &consvars;, builders, tools and environment methods to work), the rest of the man page is unique content.

## A Caveat About This Guide's Completeness

&SCons; is a volunteer-run open source project. As such, the &SCons; documentation isn't always completely up-to-date with all the available features - somehow it's almost harder to write high quality, easy to use documentation than it is to implement a feature in software. In other words, there may be a lot that &SCons; can do that isn't yet covered in this User's Guide.

Although this User's Guide may not be as complete as it could be, the development process does emphasize making sure that the &SCons; man page is kept up-to-date with new features. So if you're trying to figure out how to do something that &SCons; supports but can't find enough (or any) information here, it would be worth your while to look at the man page to see if the information is covered there. And if you do, maybe you'd even consider contributing a section to the User's Guide so the next person looking for that information won't have to go through the same thing...?

## Acknowledgements

&SCons; would not exist without a lot of help from a lot of people, many of whom may not even be aware that they helped or served as inspiration. So in no particular order, and at the risk of leaving out someone:

First and foremost, &SCons; owes a tremendous debt to Bob Sidebotham, the original author of the classic Perl-based &Cons; tool which Bob first released to the world back around 1996. Bob's work on Cons classic provided the underlying architecture and model of specifying a build configuration using a real scripting language. My real-world experience working on Cons informed many of the design decisions in SCons, including the improved parallel build support, making Builder objects easily definable by users, and separating the build engine from the wrapping interface.

Greg Wilson was instrumental in getting &SCons; started as a real project when he initiated the Software Carpentry design competition in February 2000. Without that nudge, marrying the advantages of the Cons classic architecture with the readability of Python might have just stayed no more than a nice idea.

The entire &SCons; team have been absolutely wonderful to work with, and &SCons; would be nowhere near as useful a tool without the energy, enthusiasm and time people have contributed over the past few years. The "core team" of Chad Austin, Anthony Roach, Bill Deegan, Charles Crain, Steve Leblanc, Greg Noel, Gary Oberbrunner, Greg Spencer and Christoph Wiedemann have been great about reviewing my (and other) changes and catching problems before they get in the code base. Of particular technical note: Anthony's outstanding and innovative work on the tasking engine has given &SCons; a vastly superior parallel build model; Charles has been the master of the crucial Node infrastructure; Christoph's work on the Configure infrastructure has added crucial Autoconf-like functionality; and Greg has provided excellent support for Microsoft Visual Studio.

Special thanks to David Snopek for contributing his underlying "Autoscons" code that formed the basis of Christoph's work with the Configure functionality. David was extremely generous in making this code available to &SCons;, given that he initially released it under the GPL and &SCons; is released under a less-restrictive MIT-style license.

Thanks to Peter Miller for his splendid change management system, &Aegis;, which has provided the &SCons; project with a robust development methodology from day one, and which showed me how you could integrate incremental regression tests into a practical development cycle (years before eXtreme Programming arrived on the scene).

And last, thanks to Guido van Rossum for his elegant scripting language, which is the basis not only for the &SCons; implementation, but for the interface itself.

## Contact

The best way to contact people involved with SCons, is through the SCons mailing lists.

If you want to ask general questions about how to use &SCons; send email to &scons-users;.

If you want to contact the &SCons; development community directly, send email to &scons-devel;.

For quicker, informal questions, discussion, etc. the project operated a Discord server at and a Libera.chat IRC channel at (the former channel at irc.freenode.net is now unused). Certain discussions may also be moved by administrators from mailing list or chat to GitHub Discussions for greater permanence and easier finding.