The classic licenses cover a huge swath of open source software. Each has its own personality based on how and why it came into being.
Considered the progenitor of all open source licenses, and still the overwhelming favorite among programmers (85% of all projects, by some estimates), Richard Stallman’s GPL underpins Linux and the constellation of software around it—everything from email clients to MySQL to more C++ compilers than you could ever possibly need.
The GPL stems from Stallman’s ideological stance that creating “free,” as opposed to closed and proprietary, software is the ethical responsibility of programmers and end users. Whether you subscribe to Stallman’s belief in the “Free World” does not prevent you from using GPL-protected code or profiting from it, as MySQL and commercial distributors of Linux can attest. Nevertheless, understanding where Stallman is starting from is key to grasping the full implications of his central innovation: Copyleft.
All open source licenses share the principle that anyone should be able to use, copy, and distribute—perhaps with modifications and under the right circumstances—source code and the executable software compiled from it. As you will see in this chapter, proprietary software makers dabbling in open source projects tend to have the most restrictive licenses, and the GPL does the most to ensure that software stays open.
The GPL has one notable restriction, however, ...