In the starting days of the digital computer (1950-1960) almost all software was developed by academics who mostly shared their software in the public domain as source code. Back then software was rarely seen as a property, in fact software was not even copyrightable until 1974 in the U.S. 1. In the 1960s, however, the cost of developing software increased and hardware companies started to charge for software which was included in their hardware products. With time the cost of software development kept increasing and when software became copyrightable in 1974 a fair amount of software companies started licensing their software including companies like Microsoft and Unix 2. To protect their software, companies started to release their software not in source code but in binary, which restricts people to study or modify licensed software. Many software developers were not pleased with this new trend including Richard Stallman. As a counteract to this trend Stallman founded the GNU project in 1983, a mass-collaboration project with the aim to give computer users the freedom and control over their computers back by developing free software. The software developed in the GNU-project is free in the definition that you as user can freely use it, modify it and distribute it 3. To promote the project Stallman founded the Free Software Foundation (FSF) which created the first free software license called the GNU general public license (GPL), also referred as copyleft, in 1989. This license was used in the release of a free operation system (OS), called Linux 4. In 1997, Eric Raymond published a book called The Cathedral and the Bazaar which gave a reflective analysis on the Linux project 5. The book motivated the Netscape, a software company, to release their web browser software as free software with the aim to fix bugs and improve the web browser in open-development manner. This created a movement where more and more companies released their software free and open, this movement is called the open source movement and was encouraged by the open source initiative (OSI) which was founded by Eric Raymond and Bruce Perens 6. The OSI defines “open source software” (OSS) in a little bit looser but similar way as the FSF defines “free software” which results that free and open software can be called “open source software” or “free software”. Which name you use is a matter of personal opinion as both names stands for different views. The open source movement is a pragmatic movement with the aim to developed better soft- 1 CHAPTER 1. INTRODUCTION ware. The free software movement is a social movement with the aim to have only free software 7. Personally I prefer the term “free software”, but for the sake of clarity I will use the term “open source software”. Over the years the total numbers of open source projects have been growing exponential, see figure 1.1 8. An open source license is a license which allow software to be freely used, modified and shared 9. The OSI have made a list of what they consider open source licenses although the community itself considers a far broader range of licenses as open source licenses. They come in broad range of forms but can be roughly divided in two categories, permissive licenses and copyleft licenses. Copyleft licenses are generally considered more protective and reciprocal than permissive licenses. They tend to protect the copyright of the original developer more in later modified versions of the software. Permissive licenses are generally considered a weaker protection than copyleft licenses. They tend to lose the copyright of the original developer in later modified versions of the software, this makes them more attractive in the commercial world. Although most open source licenses are free to use, in reality a lot of open source 2 CHAPTER 1. INTRODUCTION software is not licensed at all. At GitHub, the biggest software sharing platform, around 20% of all software is licensed, see figure 1.2 10.In principle this no-licensed software is protected by copyright as making software is a creative work 1. In praxis, however, the copyright is very hard to defend without a license and infringement can easily occur. Figure 1.2 shows a big drop in the use of open source licensing over time. There are many speculations about this trend but personally I think young software developers tend to be less protective about their software, this speculation is supported by another trend: a shift from copyleft licenses to permissive licenses. Black Duck’s latest data shows a 31% increase in the use of permissive license and 22% decrease of the use of copyleft licenses in a period of six years 11. Research question As an amateur software developer myself and big fan of open source software, I am interested in this shift from copyleft licenses to permissive licenses. To discover if this shift is a good or bad thing I will compare different open source licenses with each other in this research paper. The main research question will therefore be: Which open source license should you use as a software developer? To answer this question the following subquestions are constructed: 1. What kind of copyleft licenses exist and what are their purposes? 2. What kind of permissive licenses exist and what are their purposes? 3. What kind of other alternatives licenses exist and what are their purposes? In the section Conclusion I will give a comparison of all the former discussed open source licenses and my personal preference. 3 Chapter 2 Copyleft license What is copyleft? Copyleft is a novel form of licensing to ensure a work remains free in further modi- fied versions of the work. Here the word free is defined by the FSF and means the work is free to study, modify and redistribute modified versions 12. The key difference between permissive license and copyleft license is the stipulation that the same rights must be preserved in modified works down the line with a copyleft license, with a permissive license this is not the case. Due this stipulation copyleft licenses are considered to be more protective and reciprocal. Most copyleft licenses meet the criteria of the OSI to be open source licenses 13. Different copyleft licenses The first copyleft license was the GNU GPLv1 writen by the FSF in 1989. The license focus on two issues related to the impendence of free software. The first issue is that distributors may publish in binary. GPLv1 solve this by restricts distributors to distribute GPLv1 licensed software in source code under the same licensing conditions. The second issue is that distributors could change the license in such the software is no longer free by the conditions of FSF or combine the software with other restrictive software. The GPLv1 license solve this by stating that all modified version of the software licensed with GPLv1 should also include the GPLv1 license 14. GPLv2 was released in 1991 and is almost similar to GPLv1. GPLv2 includes some more information with the purpose to prevent future patent infringement claims or litigation which can harm the users freedom under the license 15. The GPLv2 is actively used on BlackDuck and GitHub (18% and 13%, respectively) 1110. Also GPLv2 is used for all the versions of the free OS of Linux 16. With the release of GPLv2 came also the first Library GPL (LGPL) also known in as the lesser GPL. The key difference between the LGPL and the GPL is the allowance of the linkage of non-GPL licensed software with LGPL licensed software 17. The LGPL license was very useful for some software libraries like the C-library (open source computer language). The LGPL is considered to be a weaker protected than the GPLv2. Due this lower protection the LGPL is referred as a weak copyleft license in comparison with the strong copyleft license like GPLv2. The LGPL is actively used on BlackDuck as well on GitHub (6% and 1.3%, respectively) 1110. 4 CHAPTER 2. COPYLEFT LICENSE Another weak copyleft license is the Mozilla Public License (MPL). The MPL is very similar to the LGPL, the only key difference is the linkage of non-GPL licensed software. With the MPL linkage with proprietary software is easier than with the LGPL 18. The MPL is barely used on BlackDuck as well on GitHub (both less than 1%) 1110. The last strong copyleft license created is the GPLv3, written by the FSF in 2005. The GPLv3 includes a lot of changes compared to the GPLv2, the two major changes are mentioned. The GPLv3 is compatible with the Apache license 2.0, one of the most use permissive license at the moment. This means GPLv3 licensed software can include Apache licensed software which makes the GPLv3 more innovative than the GPLv2 which was only compatible with GPLv2, GPLv1 and LGPL (only copyleft licenses). Another major change is the protection against tivoization. Tivoization is the creation of a system which includes open source licensed software, but restricts the access to the open source licensed software by using hardware restrictions. The FSF considers tivoization a tremendous threat to free software and therefore includes a protection against tivoization in GPLv3 19. These two changes makes GPLv3 quiet a different license than the GPLv2, some people argue even that the FSF should have named GPLv3 differently. The GPLv3 is actively used on BlackDuck as well on GitHub (7% and 8.88%, respectively) 1110. 5 Chapter 3 Permissive license What is a permissive license? A permissive license is a open source license with minimal restrictions on the distributions of modified software. Unlike copyleft licenses, a permissive license does not meet the requirement to remain the same rights in later modified work. Thus it is possible that if you license your software under a certain permissive license a company could take your software, modify it and make the modified software proprietary. Due this very permissive character permissive licenses are most of the time compatible with other open source or closed source licenses 20. The difference between a very permissive license and just sharing your work in the public domain is the stipulation of attribution stated in permissive licenses. If the work is shared in the public domain attribution is not legally required in most countries although it is for most people considered ethical to do so 21. Most of the permissive licenses meet the criteria of the OSI to be open source licenses 13. Different permissive licenses The MIT license is a permissive license written by the Massachusetts Institute of Technology (MIT). The MIT license is very short and to the point, the license only states that the code is free to do anything with as long attribution is provided and the developer is not liable in anyway 22. Software licensed with a MIT license can be integrated in software licensed with a GPL but not the other way around, i.e. MIT license is compatible with GPL but GPL is not compatible with a MIT license 23. The MIT license is the most use open source license on BlackDuck as well on GitHub (32% and 44.69%, respectively) 1110. A very similar license as the MIT license is the 3-clause BSD license. The key difference between the 3-clause BSD license and the MIT license is the non-endorsement clause. This clause states that the names of the developers or organization can not be used in advertisement for the software without specific prior written permission. This clause is included to protect your name if you think is needed 24. The 3- clause BSD license is compatible with GPL but again GPL is not compatible with the 3-clause BSD license 23. The BSD license is actively used on BlackDuck as well on GitHub (6% and 4.53%, respectively) 1110. The Apache license 2.0 (ALv2) is a more restrictive permissive license written by the Apache Software Foundation. As ALv2 is permissive license it does not require 6 CHAPTER 3. PERMISSIVE LICENSE to license modified work under the same license (unlike copyleft license). ALv2 does require, however, the same license for all the unmodified parts and in every file original intellectual property rights in the form of patents, copyrights and trademarks must be preserved and every change to the software must have a note to it what is changed 25. Due this requirement the ALv2 has a more restrictive character. The ALv2 is compatible with GPLv3 only, not other copyleft licenses due certain requirements of the ALv2 like certain patent termination and indemnification provisions 23. The ALv2 is actively used on BlackDuck as well on GitHub (14% and 11.19%, respectively) 1110. 7 Chapter 4 Alternative license What are alternative licenses? One might not like to license their open source project under a permissive or copyleft license due various reasons. Fortunately there are some alternatives, they can be roughly be divided in two categories: formal and informal licenses. The formal licenses are still approved by the OSI and FSF, the informal licenses are not approved by the OSI and FSF but some small projects do prefer the informal licenses 13 23. Different alternative licenses The most use alternative license is the Artistic License 2.0 (approved by the OSI and FSF 13 23) which is used around 4% of all projects at BlackDuck 11.