Unix is an old operating system developed since 1960s as a research project of Bell Labs, which has become one of the most influential pieces of software in history and whose principles (e.g. the Unix philosophy) live on in many so called Unix-like operating systems such as Linux and BSD (at least to some degree). The original system itself is no longer in use, the name UNIX is nowadays a trademark and a certification.
Unix has reached the highest level a software can reach: it has transcended its implementation and became a de facto standard. This means it has become a set of interface conventions, cultural and philosophical ideas rather than being a single system, it lives on as a concept that has many implementations. This is extremely important as we don't depend on any single Unix implementation but we have a great variety of choice between which we can switch without greater issues. This is very important for freedom -- it prevents monopolization -- and its one of the important reasons to use unix-like systems.
In the 1960s, Bell Labs along with other groups were developing Multics, a kind of operating system -- however the project failed and was abandoned for its complexity and expensiveness of development. In 1969 two Multics developers, Ken Thompson and Dennis Ritchie, then started to create their own system, this time with a different philosophy; that of simplicity (see Unix philosophy). They weren't alone in developing the system, a number of other hackers helped program such things as a file system, shell and simple utility programs. At VCF East 2019 Thompson said that they developed Unix as a working system in three weeks. At this point Unix was written in assembly.
In the early 1970s the system got funding as well as its name Unix (a pun on Multix). By now Thompson and Richie were developing a new language for Unix which would eventually become the C language. In version 4 (1973) Unix was rewritten in C.
Unix then started being sold commercially. This led to its fragmentation into different versions such as the BSD or Solaris. In 1983 a version called System V was released which would become one of the most successful. The fragmentation and a lack of a unified standard led to so called Unix Wars in the late 1980s, which led to a few Unix standards such as POSIX and Single Unix Specification.
For zoomers and other noobs: Unix wasn't like Windows, it was more like DOS, things were done in text interface -- if you use the command line in "Linux" nowadays, you'll get an idea of what it was like, except it was all even more primitive. Things we take for granted such as a mouse, copy-pastes, interactive text editors, having multiple user accounts or running multiple programs at once were either non-existent or advanced features in the early days. Anything these guys did you have to see as done with stone tools.
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