OSS (Open Source Software) is software whose source code is made public and that anyone may use, modify and redistribute under the terms of its license.
Use
Well-known OSS
OS
Linux, Android, FreeBSD
Web server
Apache HTTP Server
DBMS
MySQL, PostgreSQL
Web browser
Firefox
Email client
Thunderbird
Source code is the text of a program written in a form people can read and edit, and a license is the set of usage terms a copyright holder grants to users. Software counts as OSS only when its license allows not just reading the source code but also changing and redistributing it. It generally comes with no guarantee of how well or how safely it works, so whoever adopts it checks the quality first.
AnalogyIt is like publishing a detailed recipe and letting anyone adapt it and share it with others, as long as they keep to the agreed rules.
Exam points
ExamOSS lets whoever receives it improve the contents and pass the improved version on. It also does not treat people differently by who receives it or what field it is used in.
ExamThe copyright (著作権) in OSS is not given up. The copyright holder keeps it and sets the terms of use through a license (ライセンス), and users check and follow the terms, which differ from license to license.
Mix-upOSS does not mean the same thing as free-of-charge software. Charging a fee for redistribution and offering it with paid support are both allowed.
GPL (GNU General Public License) is a leading OSS license built on the idea that modified versions carry the same conditions.
GPL was written for the software of the GNU Project, and the Linux kernel is among the programs that use it. A GPL program can be used, modified and redistributed freely, but anyone who distributes it, modified or not, must let the recipients obtain its source code and attach the same GPL to it. Someone who only uses a modified version themselves and does not distribute it has no such obligation.
AnalogyIt is like a recipe that says: if you adapt this and hand it on, include your adapted method and attach the same set of rules.
How to rememberThe G stands for GNU, a name that is an acronym of "GNU's Not Unix" and so repeats itself inside its own expansion.
Exam points
ExamWhen you modify a GPL program and distribute it, apply the same GPL to what you distribute and let recipients obtain the matching source code (ソースコード).
Mix-upSome OSS licenses, such as the BSD license (BSDライセンス) and the MIT license (MITライセンス), allow a modified version to be distributed under other terms. GPL differs from them in requiring the same GPL for modified versions too.
Copyleft (コピーレフト) is the idea that when software is modified and passed on, the recipients still receive the same freedom to use, modify and redistribute it as the original gave.
Original OSS: comes with freedoms and conditions
Modify: add features
Distribute: make the source code obtainable and attach the original conditions
Recipient: gets the same freedoms
The next recipient gets the same freedoms and the same conditions, and the same flow repeats when they modify and distribute it in turn.
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The copyright holder uses their copyright to set a license, and the license requires that anyone passing the software on does so under the same terms. This stops a modified version from turning into a form without those freedoms partway along, so the freedoms carry on from one recipient to the next.
AnalogyIt is like an arrangement where someone given a seedling, if they share the plants they grow, must pass on the same promise they received: grow it freely and share it freely.
How to rememberThe name is wordplay that swaps the "right" in copyright for "left".
Exam points
ExamUnder copyleft (コピーレフト), a person who distributes a modified version passes the conditions they received on to the next recipient unchanged.
Make the source code obtainable and attach the GPL
Pass the original conditions on to the next recipient
Key points
OSS is software whose source code is public and whose license grants the freedom to use, modify and redistribute it, while the copyright is not given up and stays with the copyright holder.
People who use OSS check and follow the terms, which differ from license to license.
When you modify a GPL program and distribute it, let recipients obtain the source code and attach the same GPL.
Copyleft is the idea that software modified and passed on keeps the same freedoms and conditions as the original.