As described in the architecture documentation Sailfish OS consists of a variety of components, some of which are proprietary and closed source (such as some hardware-specific kernel modules, or other software licensed under commercial terms), and most of which are open source.
The various open source components have different licenses depending on the licensing decision by the upstream maintainers. Sailfish OS respects these license requirements, which can also be read from the device. Some of these licenses are highly permissive open source licenses, while others entail specific legal requirements which are described in further detail below.
Most common licenses used in the Sailfish OS are:
In Sailfish OS we follow Fedora’s shortnames for the license names.
There are a number of licenses which permit and encourage the sharing of source code associated with software and you’ll find a mix of these licenses in the source that makes up Sailfish OS. Some of these licenses are highly permissive, while others more strictly enforce the “copyleft” requirements, such as GPL.
The essence of the GPL is that you can share and modify the software but that you must share the source of any modifications you’ve made if you redistribute the software - ie it’s about playing fair.
You can read more about the GPL here.