From Wikipedia “Unix is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, development starting in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.”
But what does this all mean, well in the late 1960’s development was underway on Multics (Multiplexed Information and Computing Service) an operating system with many new and novel ideas. One of these ideas was time-sharing, the ability to share computer resources amongst many users at the same time. Bell Labs, MIT, and General Electric were all involved in the project but Bell Labs dissatisfied with the progress of the project started withdrawing from it. This left just a few researchers from Bell Labs working on Multics one of them being Ken Thompson.
With Bell Labs withdrawing from Multics he decided to write his own operating system (OS) with some of the ideas from the Multics project. While he still had access to Multics he wrote various things for his OS including a game called Space Travel. But writing Space Travel made him realize that he needed a better computer for his purposes and he found a little-used Digital Equipment Corporation PDP-7 at Bell Labs.
At this point there was a team of researchers working on Unix led by Ken Thompson and Dennis Ritchie and they implemented various features on Unix based on the improvements from Multics but simplified. It was around this time that Thompson created the B programming language and the first version of the name was thought up, Uniplexed Information and Computing Service (Unics). A few different departments of Bell Labs were interested in the OS but they wanted to run it on a larger computer and another department wanted to do text processing on it so the team added text processing features to the OS and received funding to get a PDP-11/20. It was also around then that the OS was officially named Unix. Most programs in Unix were at the time written in a programming language called Assembly, the problem with Assembly was that it was very low level meaning that the code was complex and hard for a human to process, but this problem gets solved later.
The system was getting more and more complex and the need for some sort of guide or manual to the OS grew, so one was made and dubbed the ‘UNIX Programmer’s Manual’. Each command or program in the system was documented in a ‘man page’ a format that to this day is widely used on Linux to document commands/programs. Each man page offered a terse reference of information about the usage of the program as well as listing known bugs and giving contact details for the maintainer/maker of the command/program so further questions could be asked and the answers could be delivered and added to the man page.
Cut to 1973, Unix is getting very popular at Bell Labs but the aforementioned problem of the whole OS being written in Assembly is still there, it was the widely accepted opinion at the time that due to an OS being so complex it had to be written in a low-level language like Assembly. The aforementioned B programming language isn’t the ideal contender as it is slow and not very powerful, but in the meantime Ritchie had been working on and improving B which resulted in the successor C which was faster and more powerful. The decision was made to port Unix to C.
Up until this point the use and distribution of Unix had been entirely internal to Bell Labs but this was to change. At the 1973 Symposium on Operating System Principles Unix was presented formally to the outside world. This led to requests for the system and from then on the fate of the system was sealed.
Where is it now?
In the modern day Unix is split into a few different versions, Unix has been mostly taken over in popularity by Linux, more on that later and there are the BSDs: FreeBSD, NetBSD, and OpenBSD. Each of these BSDs have their own strengths and weaknesses but by far the most popular Unix derivative is an operating system you might’ve heard of, MacOS. Apart from MacOS Unix derivative systems have a negligible market share.