Open Source Software Directory
Open source software (OSS) is computer software whose source code is made publicly available, allowing anyone to
use, inspect, modify, and distribute it, typically under a license that ensures these freedoms.
The source code is the underlying set of instructions that programmers use to control how the software operates. By making this code accessible, open source software encourages
collaboration, transparency, and community-driven development.
Key characteristics of open source software include:
- Freedom to use: Anyone can use the software for any purpose.
- Freedom to study: The source code can be examined to understand how the software works.
- Freedom to modify: Users can alter the code to suit their needs or fix issues.
- Freedom to distribute: Both the original and modified versions can be shared with others.
Open source contrasts with proprietary (closed source) software, where the source code is kept secret and only the original creators have the right to modify or distribute the software.
Examples of open source software include the Linux operating system, the Mozilla Firefox browser, and the Python programming language.
The open source model has become a cornerstone of modern software development, fostering rapid innovation and widespread adoption across industries.
Types of Open Source Licenses
Open source licenses define how software can be used, modified, and distributed. They fall into two main categories: permissive and copyleft. Each type has distinct rules and implications for developers and users.
1. Permissive Licenses
Permissive licenses place minimal restrictions on how software can be used. They allow integration into proprietary (closed-source) projects, provided that basic requirements—such as attribution—are met.
Key Features:
- Minimal obligations (often just attribution)
- Allow proprietary use and closed-source derivatives
- Broad compatibility with other licenses
Popular Permissive Licenses:
- MIT License: Extremely simple and permissive; requires only inclusion of the license and copyright notice.
- Apache License 2.0: Similar to MIT but includes an explicit patent grant and protection against patent litigation.
- BSD Licenses (2-Clause and 3-Clause): Allow redistribution and use with minimal restrictions; the 3-Clause version adds a non-endorsement clause.
- ISC License: Very short and permissive, similar to MIT.
- The Unlicense: Effectively dedicates the work to the public domain, with no conditions.
2. Copyleft Licenses
Copyleft licenses require that any derivative works or redistributed versions of the software remain under the same license, ensuring continued openness.
Key Features:
- Derivative works must remain open source under the same (or compatible) license
- Encourage community collaboration and software freedom
- Can be more restrictive for commercial or proprietary use
Types of Copyleft:
- Strong Copyleft: Requires all derivative and combined works to be released under the same license.
- Weak Copyleft: Only requires modifications to the original code (not the entire combined project) to be open source.
Popular Copyleft Licenses:
- GNU General Public License (GPL): The strongest copyleft license; any software using GPL code must also be GPL-licensed.
- GNU Lesser General Public License (LGPL): Weaker than GPL; mainly used for libraries, allowing linking with proprietary code under certain conditions.
- Affero General Public License (AGPL): Extends GPL to cover software accessed over a network (e.g., web services).
- Mozilla Public License (MPL): File-level copyleft; only modifications to MPL-licensed files must remain open source.
- Eclipse Public License (EPL): Module-based copyleft; designed for business and enterprise use.
3. Public Domain and Other Licenses
Some licenses, like Creative Commons Zero (CC0) or the Unlicense, aim to place software in the public domain, waiving all rights and restrictions.
Comparison Table
| License Type |
Examples |
Key Characteristics |
| Permissive |
MIT, Apache 2.0, BSD, ISC |
Minimal restrictions, allows proprietary use |
| Strong Copyleft |
GPL, AGPL |
Derivatives must be open source, strict |
| Weak Copyleft |
LGPL, MPL, EPL |
Only certain parts must remain open |
| Public Domain |
Unlicense, CC0 |
No conditions, effectively public domain |
Summary
- Permissive licenses (MIT, Apache, BSD) maximize flexibility and are business-friendly.
- Copyleft licenses (GPL, LGPL, MPL, EPL) ensure software and derivatives remain open, with varying degrees of strictness.
- Public domain licenses relinquish all rights, allowing unrestricted use.
Choosing the right license depends on your goals for openness, collaboration, and commercial use.