Are UNIX and GNU/Linux the Same Operating Systems?

Often, in technical discussions and debates, a common question arises: Are UNIX and GNU/Linux the same operating systems (OSs)? While this query does not have a straightforward answer, it is crucial to understand the nuances and differences between these two concepts.

The Connection Between UNIX and GNU/Linux

The relationship between UNIX and GNU/Linux is complex and multifaceted. It is often said that Linux is not a clone of UNIX because the name and source code of UNIX are protected.

Shortly after I started working at a networking equipment company, I was introduced to a "BBS" system where I had to learn UNIX to access it. However, I quickly realized that what we were using was more precisely a "UNIX-like" or "Unix-like" system, consisting of the Linux kernel, GNU utilities, and a suite of application programs. This realization highlighted the interconnectedness yet distinct nature of these operating systems.

User Perspective vs. Technical Perspective

From a user's perspective, Linux is indeed Unix or nearly indistinguishable from it. Both offer a robust command-line interface, a versatile file system, and a wide range of utilities that make system administration and development tasks efficient. However, to a kernel developer, the differences are more pronounced. The Linux kernel and its utilities do not share any code with the original UNIX, which means they do not have to pay royalties.

Even so-called "Unix" systems like Ultrix, Xenix, and AIX vary in both kernel code and utilities. These variations have led to the development of different Unix-like systems with unique features and functionalities.

The Origin and Philosophy of UNIX

According to The Bell System Technical Journal, Volume 57, Number 6, Part 2 (1978), UNIX's uniqueness lies not in its specific implementation but in the philosophies and concepts that it embodies. Murray Campbell and Dennis Ritchie, who co-authored a seminal article on UNIX, highlighted the following key features:

A hierarchical file system with demountable volumes Compatible file device and inter-process I/O The ability to initiate asynchronous processes System command language selectable on a per-user basis Over 100 subsystems, including a dozen programming languages

These features and the "Unix style" maxims emphasize simplicity, modularity, and efficiency. Despite these principles, the original UNIX has evolved through various implementations and licenses, leading to the diverse family of Unix-like systems, including GNU/Linux.

The Current Landscape

Looking at today's landscape, Linux has become the dominant player in the Unix-like operating systems. Its popularity is bolstered by its flexibility, community-driven development, and the permissive nature of the GNU General Public License (GPL) that encourages sharing and improving code. In contrast, some UNIX-like systems, such as those based on the BSD license, may not share the same spirit of community collaboration.

I, like many others, "grew up" with ATT Unix V at Bell Labs and have not been involved in any Linux work. However, my personal experience and observations suggest that the term "Linux" or "GNU/Linux" is an implementation of ideas originally implemented as UNIX, whether through Ultrix, Xenix, or AIX. These systems share a common philosophy and set of principles that make them inherently similar, especially from a user's standpoint.

Ultimately, whether we classify these operating systems as "nix" or something else, they share a lineage rooted in the early computing advancements of the 1970s and 1980s. The distinctions between them are mainly technical and legal, while their similarities are rooted in user experience and functionality.