Open-source software

A computer program's source code is the human-readable instructions from which executable software is built. Open-source software is made available under terms that permit people to inspect and use that code, make changes, and redistribute the original or modified program. The Open Source Initiative's Open Source Definition sets licensing criteria that include access to source code, permission to create and distribute derived works, and freedom from discrimination against particular users or fields of activity. The criteria concern the licence attached to a program, not just whether its code can be viewed.

A public repository without an open-source licence is not automatically open source. GitHub's documentation explains that, without a licence, ordinary copyright rules generally leave the author with exclusive rights to copy, distribute or create derivative works; a repository's visibility alone does not grant those permissions. Licences differ in their conditions. Permissive licences such as MIT and BSD generally impose fewer obligations on redistribution, while copyleft licences such as the GNU General Public License require certain distributed derivatives to preserve specified freedoms. The precise obligations depend on the licence.

The word “free” in “free software” refers to users' freedom, not necessarily to price: copies may be sold. Open-source software may also be commercial. The Free Software Foundation and Open Source Initiative share substantial practical ground, but the former emphasizes the ethical and social importance of users' freedoms, while the latter uses a defined set of licence criteria to identify open-source software.

Programmers shared and adapted code long before the phrase “open source” became common. The GNU Project, announced by computer scientist Richard Stallman in 1983, sought to develop a Unix-like operating system that users could share and improve; the Free Software Foundation was established in 1985. In the 1990s, networked collaboration made it easier for geographically dispersed developers to work on common projects. The Linux kernel, announced by Linus Torvalds in 1991, became a prominent example of Internet-based collaborative development.

The label “open source” gained prominence in 1998, as developers sought language that would help explain collaborative software development to a wider audience, including businesses. The Open Source Initiative (OSI) was founded that year and adopted the Open Source Definition as a standard for assessing licences. The change in label did not mark the beginning of shared software development; it gave a name and a licensing framework to an established practice.

Open-source development is not a single management method. A project may be coordinated by volunteers, a nonprofit foundation, a university, a company, or a mix of these. Contributors can submit proposed changes for review; maintainers decide whether to accept them, release versions and set project priorities. Public discussion and a visible code history can make participation and inspection possible, but the governance of each project determines how those opportunities work in practice.

Projects also vary in how they are funded and sustained. Some depend on volunteer work, while others are supported by paid developers, donations, institutional sponsorship, or companies that sell services and products built around the software. Open licensing permits reuse under its conditions; it does not dictate who may lead the project, how its contributors are paid, or whether the program itself is sold.

Open-source software is used in operating systems, programming tools, web infrastructure and many other kinds of software. Examples include the Linux kernel, the Apache HTTP Server, Python and the Firefox browser. Shared code and permissive licences can make it easier to adapt, combine and redistribute software. Independent review can help people find defects, but the public availability of code does not ensure that anyone will inspect it or that reported problems will be fixed.

Security and long-term reliability depend on more than a licence. The U.S. National Institute of Standards and Technology notes that open-source projects have diverse operating models, and their provenance, integrity, support and maintenance may be difficult to determine. Organizations using software components therefore need to consider how each project is maintained, obtain components from trustworthy sources, and track known vulnerabilities. These qualifications do not make open source inherently insecure; they show why openness should not be mistaken for a security guarantee.