The main difference among the free software and open source software concepts are the motivation of the people identifying with each (that is why I tend to use the term FLOSS when I do not want to be specific about either group. From time to time the question of whether some software is open source or rather free software appears. For example, Linus said that the Linux kernel:
… has never been an FSF project, and in fact has never even been a “Free Software” project.
Whether the kernel is or is not a Free Software project is arguable, because it depends on how the developers feel about it or what their intentions are. But what can we say about the set of software grouped under the label of “Free Software” and the set of software gropued under the label of “Open Source Software”? This is far more objective, although not absolutely objective.
We can certainly say that Free Software is software available under a license that fits the Free Software Definition, and that Open Source software is likewise the software available under a license that fits the Open Source Definition. By looking at both definitions, it seems hard to find a license that would fit one definition and not the other, making both sets roughly equivalent.
But let us also compare the list of software licenses that the FSF blesses as Free Software Licenses and compare that list with the software licenses blessed by the OSI as Open Source licenses. The FSF also publishes a list of licenses that are considered to be non-Free Software licenses, whereas the OSI only publishes accepted licenses.
It turns out that there are around 30 licenses (depending on how you count the different versions of the same license) that are accepted by both groups, 32 licenses accepted by the FSF that are not mentioned by the OSI and 22 licenses accepted by the OSI which are not mentioned by the FSF. Then we have some special cases:
- Python: the FSF separates the python license versions into three groups, of which two are compatible and one incompatible with the GPL, though all classified as Free Software
- Perl: the “Perl License” is not evaluated by the OSI. However, since this “disjunctive license” specifies that the licensee may choose either the Artistic License or the GPL, and both are accepted by the OSI, we can deduct that the Perl license should be considered Open Source without a doubt.
- Eiffel Forum v1: does not appear explicitly accepted in the FSF, the document only states that this license is incompatible with the GPL. The conclusion is that it is considered a GPL-incompatible free software license, while version 2 is a GPL-compatible free software license.
- Academic Free License: the FSF accepts versions 1.1 and 2.1, the OSI only mentions acceptance of version 3.0.
And finally we have three licenses that are accepted by the Open Source Initiative and explicitly rejected by the Free Software Foundation: the “Artistic License”, the “Nasa Open Source Agreement” and the “Reciprocal Public License”. However, in both cases the reasons to reject the licenses are not based on failing to fulfill some requirement in the Free Software Definition.
- Artistic License: the FSF does not accept the original artistic license, because of its vagueness, but accepts a modified veresion. However, according to the FSF, “the problems are matters of wording, not substance”.
- Nasa Open Source Agreement: the FSF objects to the requirement that the changes made to the software be the contributor’s “original creation”. However, the Free Software Definition does not require the right to include any third party code. There is no substantial difference between right 3 of the FSD: requiring “the freedom to improve the program, and release your improvements to the public” and the third condition of the OSD stating “the license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software”.
- Reciprocal Public License: there are two criticisms on behalf of the FSF:
1. the licensee has to notify the licensor when she publishes a modified version of the code. This is not in contradiction with the Free Software Definition, so there is no reason to deny this license. Other much stronger restrictions (such as copyleft or the prohibition of modifying files in the LaTeX license) are accepted by the FSF.
2. there is a limit on how much anyone can charge for the source code. This does however not limit the amount of money you may charge for the distribution of both the binary and the source code, or for the binary on its own. And it also does not contradict any of the freedoms in the Free Software Definition.
As a conclusion, we can say that the set of software that fits the Free Software Definition and the set of software that fits the Open Source Definition is essentially equivalent. Some exception may exist depending on the interpretation, but there would be no point in assuming that one is the subset of the other.