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[yangmingming translating]20160812 What is copyleft.md
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What is copyleft?
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If you've spent much time in open source projects, you have probably seen the term "copyleft" used. While the term is quite commonly used, many people don't understand it. Software licensing is the subject of at least as much heated debate as text editors or packaging formats. An expert understanding of copyleft would fill many books, but this article can be a starting point on your road to copyleft enlightenment.
## What is copyright?
Before we can understand copyleft, we must first introduce the concept of copyright. Copyleft is not a separate legal framework from copyright; copyleft exists within the rules of copyright. So what is copyright?
The exact definition varies based on jurisdiction, but the essence is this: the author of a work has a limited monopoly on the copying (hence the term "copyright"), performance, etc. of the work. In the United States, the Constitution explicitly tasks Congress for creating copyright laws in order to "promote the Progress of Science and useful Arts."
Unlike in the past, copyright attaches to a work immediately -- no registration is required. By default, all rights are reserved. That means no one can republish, perform, or modify a work without permission from the author. This permission is a "license" and may come with certain conditions attached.
For a more thorough introduction to copyright, Coursera's [Copyright for Educators & Librarians](https://www.coursera.org/learn/copyright-for-education) is an excellent resource.
## What is copyleft?
Bear with me, but there's one more step to take before we discuss what copyleft is. First, let's examine what open source means. All open source licenses, by the[Open Source Inititative's definition](https://opensource.org/osd) must, among other things, allow distribution in source form. Anyone who receives open source software has the right to inspect and modify the code.
Where copyleft licenses differ from so-called "permissive" licenses is that copyleft licenses require these same rights to be included in any derivative works. I prefer to think of the distinction in this way: permissive licenses provide the maximum freedom to the immediate downstream developers (including the ability to use the open source code in a closed source project), whereas copyleft licenses provide the maximum freedom through to the end users.
The GNU Project gives this [simple definition](https://www.gnu.org/philosophy/free-sw.en.html) of copyleft: "the rule that when redistributing the program, you cannot add restrictions to deny other people the central freedoms [of free software]." This can be considered the canonical definition, since the [GNU General Public License](https://www.gnu.org/licenses/gpl.html) (GPL) in its various versions remains the most widely-used copyleft license.
## Copyleft in software
While the GPL family are the most popular copyleft licenses, they are by no means the only ones. The [Mozilla Public License](https://www.mozilla.org/en-US/MPL/) and the [Eclipse Public License](https://www.eclipse.org/legal/epl-v10.html) are also very popular. Many [other copyleft licenses](https://tldrlegal.com/licenses/tags/Copyleft) exist with smaller adoption footprints.
As explained in the previous section, a copyleft license means downstream projects cannot add additional restrictions on the use of the software. This is best illustrated with an example. If I wrote MyCoolProgram and distributed it under a copyleft license, you would have the freedom to use and modify it. You could distribute versions with your changes, but you'd have to give your users the same freedoms I gave you. If I had licensed it under a permissive license, you'd be free to incorporate it into a closed software project that you do not provide the source to.
But just as important as what you must do with MyCoolProgram is what you don't have to do. You don't have to use the exact same license I did, so long as the terms are compatible (generally downstream projects use the same license for simplicity's sake). You don't have to contribute your changes back to me, but it's generally considered good form, especially when the changes are bug fixes.
## Copyleft in non-software
Although the notion of copyleft began in the software world, it exists outside as well. The notion of "do what you want, so long as you preserve the right for others to do the same" is the distinguishing characteristic of the [Creative Commons Attribution-ShareAlike](http://creativecommons.org/licenses/by-sa/4.0/) license used for written work, visual art, etc. (CC BY-SA 4.0 is the default license for contributions to Opensource.com.) The [GNU Free Documentation License ](https://www.gnu.org/licenses/fdl.html)is another example of a copyleft non-software license. The use of software licenses for non-software work is generally discouraged.
## Should I choose a copyleft license?
Pages and pages could be (and have been!) written about what type of license should be used for a project. My advice is to first narrow the list of licenses to ones that match your philosophy and your goals for the project. GitHub's[choosealicense.com](http://choosealicense.com/) is a good way to find a license that fits your needs. [tl;drLegal](https://tldrlegal.com/)has plain-language explanations of many common and uncommon software licenses. Also consider the ecosystem that your project lives in. Projects around a specific language or technology will often use the same or similar licenses. If you want your project to be able to play nicely, you may need to make sure the license you choose is compatible.
For more information about copyleft licensing, check out the [Copyleft Guide](https://copyleft.org/)project.
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copyleft 是什么?
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如果你在开源项目中花费很多时间的话,你可能会看到使用的术语 “copyleft”。虽然这个术语使用比较普遍但是很多人却不理解它。软件许可是一个至少不亚于文件编辑器和打包格式的激烈辩论的学科。专家的理解可能会充斥在好多书中但是这篇文章可以作为你理解 copyleft 道路的起点。
## 什么是 copyright
在我们可以理解 copyleft 之前,我们必须先介绍一下 copyright 的概念。copyleft 并不是一个脱离于 copyright 的独立的法律框架copyleft 存在于 copyright 规则中。那么,什么是 copyright
准确的定义随着地域的不同而不同,但是其本质就是:作品的作者对于作品的仿制【注:英文为 copying】因此成为 “copyright”、性能等有有限的垄断权。在美国宪法明确地阐述了美国国会的任务就是制定版权法律来“促进科学和实用艺术的进步”。
不同于以往,版权会立马附加到作品上——而且不需要注册。默认的,所有的权力都是保留的。也就是说,没有经过作者的允许,没有人可以出版、执行或者修改作品。这种允许就是一种许可,可能还会附加一定的条件。
如果希望得到对于 copyright 更彻底的介绍Coursera 上的[教育工作者和图书管理员的著作权](https://www.coursera.org/learn/copyright-for-education)是一个非常优秀的课程。
## 什么是 copyleft
先不要着急,在我们讨论 copyleft 是什么之前,还有一步。首先,让我们解释一下开源意味着什么。所有的开源许可协议,通过[开源 Inititative ](https://opensource.org/osd)定义,除其他形式外,必须以源码的形式发放。获得源码的任何人都有权利查看并修改源码。
copyleft 许可和所谓的 “permissive” 许可不同的地方在于,其衍生的作品中,也需要相同的 copyleft 许可。我倾向于通过这种方式来讨论两者不同: permissive 许可向直接下游的开发者提供了最大的自由(包括闭源项目中使用开源代码的权力),而 copyleft 许可则向最终用户提供最大的自由。
GNU 项目为 copyleft 提供了这个[简单的定义](https://www.gnu.org/philosophy/free-sw.en.html):规则就是当重新编辑程序时,你不可以添加限制来拒绝其他人对于[免费软件]的自由。这可以被认为典型的定义,自从不同版本的 GNU 通用许可证GPL依然是最广泛使用的 copyleft 许可。
## 软件中的 Copyleft
GPL 家族是最出名的 copyleft 许可,但是它们并不是唯一的。[Mozilla 公共许可协议]和[Eclipse 公共许可协议]也很出名。很多[其他的 copyleft 许可](https://tldrlegal.com/licenses/tags/Copyleft) 则存在于很小的脚注那儿。
就像之前章节介绍的那样,一个 copyleft 许可意味着下游的项目不可以在软件的使用上添加额外的限制。这最好用一个例子来说明。如果我写了一个很酷的程序,并且使用 copyleft 许可来发布,你将有使用和修改它的自由。你可以发布你修改后的版本,但是你必须让你的用户有我给你同样的自由。如果我使用 “permissive” 许可,你将可以自由的合并到一个不提供源码的软件中。
对于我的很酷的程序和你必须要做什么同样重要的是你必须不能做什么。你不必用和我完全一样的许可协议只要它们相互兼容就行一般的为了简单起见下游的项目使用相同的许可。你不必向我共享出你的修改但是你这么做的话通常被认为一个很好的形式尤其是一些修改是为了bug的修复。
## 非软件中的 Copyleft
虽然copyleft 起始于软件世界但是它也存在之外的世界。“做你想做的只要你保留其他人也有做同样的事的权力”的概念是应用于文字创作、视觉艺术等的知识共享署名许可的一个显著的特点CC BY-SA 4.0 是贡献于 Opensource.com 默认的许可)。[GNU 自由文档许可证](https://www.gnu.org/licenses/fdl.html)是另一个非软件协议中 copyleft 的例子。在非软件中使用软件协议通常不被建议。
## 我是否需要选择一种 copyleft 许可?
文章可以并且已经写一个项目应该使用哪一种许可。我的建议是首先缩小满足你项目的哲学和目标的许可列表。GitHub 的[choosealicense.com](http://choosealicense.com/) 是一种查找满足你的需求的许可协议的好方法。[tldrLegal](https://tldrlegal.com/)使用纯语言说明许多相同和不同的软件许可。而且研究了你的项目所在的生态系统。围绕一种特定语言和技术的项目经常使用相同或者相似的许可。如果你希望你的项目可以运行的更出色,你可能需要确保你选择的许可是兼容的。
关于更多 copyleft 的信息,请查看 [copyleft 指南](https://copyleft.org/)。
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作者:[Ben Cotton][a]
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[a]: https://opensource.com/users/bcotton