I've heard or read bits and pieces of the history of Unix at many different places and from different people or books. After a friend of mine reminded me about it, I just felt like jotting it down as I remember it. I used Wikipedia to my advantage.
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Bell Labs
The oldest Unix ever, was developed at (AT&T) Bell Labs. Bell Labs was a Phone company, and had lot of smart people like Dennis Ritchie (The guy who made language C), Ken Thompson (he invented a language caled B which was a predecessor to C).
All these smart people had lot of free time, and in their free time they wrote the code for Unix. On paper, for work stuff, they showed that they are working on Project for Digital Address Book for the phone and blah blah like that. So this entire OS was written by technical people and for the technical people, no mumbo jumbo GUI, where all extremely powerful tools were write at the tip of your fingers from command line.
Since this Unix was developed in Bell labs, Bell Labs had full access and copyright ownership of all its source code, which is why it became commercial (System III, IV and the most popular System V Unix). Normal people couldn't get their hands on this unix, it was exclusive.
This Unix was a revolutionary development, and universities and interested companies wanted a piece of it. AT&T couldnot sell software at that time (for reaosn I dont know). So, they licensed copies of Unix and gave it to universities, companies and individuals, providing no support or bug fixes.
System V is the predecessor to a majority of Commercial Unixes today like HP's HP-UX, IBM's AIX, Novell's NetWare/Unixware etc. Each of these companies took a copy of the Original Unix developped at AT&T and made it into their own flavour of the OS.
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BSD
Meanwhile, Berkley University, California, which was a major university for Electrical, Electronics and Computer Science and Heavy research in these areas was done there. Berkley was one of the few universities who had a computer lab. The computers were Huge back then, the one Berkley had was a PDP-11. The copy of Unix that Berkley recieved from AT&T was installed and executed on the PDP-11 adn they performed research and development on it. After few years, other universities wanted a piece of that code which was being developed at Berkley.
Bill Joy, a student at Berkley, formed the company BSD (Berkley Software Distribution). BSD Distributed the codebase for the unix which was being developed at Berkley University.
BSD Operating System now exists in the form of OpenBSD, FreeBSD and NetBSD each of which are forks from the original BSD unix.
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SUN
Sun Microsystems was formed by 3 students at Stanford University. SUN stood for Stanford University Network.
Bill Joy, the founder and developer of BSD, also joined SUN. Sun Microsystems has always been a pioneer and a technology oriented company. They took BSD unix and formed their Solaris OS. A free open source fork of Solaris exists as OpenSolaris. Sun is also the creator of Java software and the Sparc hardware platform.
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NextStep
NextStep was an OS built using BSD unix a Mach Kernel and some other software technologies like Object Oriented Application Layer etc. At some point NextStep teamed up with Sun and formed the OpenStep standard API. Eventually, NextStep was taken over my Apple which used NextStep to create their Mac OS. Further down the line, Apple came up with Darwin OS which is the command line part of the current Mac OS X, and which gives Macs all the unix tools. Darwin was formed from NextStep, OpenStep and BSD. Darwin exists in open souce form under the variant OpenDarwin.
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Xenix & SCO
Xenix is a version of Unix which was developped at Microsoft, using AT&T's unix. Microsoft lost interest in Xenix when they went into partnership with IBM to work on OS/2. OS/2 was suppossed to be a more powerful command line OS, than DOS, but was eventually a failure due to various issues between IBM and Microsoft. Microsoft eventually sold Xenix to SCO. Further down the line SCO branched Xenix into SCO Unix.
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Minix
Andrew S. Tanenbaum, who was a researcher and known among Computer Science Students for his book "Operating Systems Design and Implementation" wrote the minix operating system for educational purposes, using c. It used some of AT&T's unix code and was inspired from it.
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GNU
Richard Stallman, who was a student at MIT, started GNU after he left MIT. GNU was started as a non-profit organization whose main aim was to create Free software for everyone. It wanted to bring all the unix tools and the OS to the public. It recieved contribution from Voluntary programmers and today pretty much anyone can contribute to it. GNU follows a License Agreement called GPL (General Public License). Today, almost every popular tool that exists under Unix is alternatively available as a GNU utility, each of which is written from scratch by voluntary programmers, and although they mimic Unix tools functionality from a user perspective, the underlying code for these utilities is different.
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Linux
Linux Torwalds, finnish student and researcher used the GNU development tools and wrote the linux kernel. The Linux Kernel was inspired and based on Tanenbaum's Minix microkernel, but was written from scratch and didn't share its codebase with minix. From then on all the GNU software were ported to run on that kernel, and today what we know as Linux is actually a combination if GNU Software and the Linux Kernel that was written by Linus Torwalds. Linux as you know exists in the form of many different variants like Red Hat, Debian, Gentoo.
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Linux Variants
Traditionally the only way to install packages in linux was to compile them from source code. The benefit of compiling code is that you dont have to go around searching for softwares binaries for a specific architecture. The same source code can be compiled on any architecture. So you can get nearly any software to work on your architecture once you have it successfully compiled on your architecture. So if you have a rare processor/architecture, you can compile the softwares on your architecture.
The downside of this is that first of all, compiling softwares from sources, is very time consuming, secondly, you need all the required compiler softwares required to do the build, plus it needs any other library files required for compiling. So, from a practcial standpoint, compiling packages from sources, becomes a big mess, with lots of dependenices amongst softwares, which ultimately is a nightmare.
Because of this, many linux vendors came up with their own mechanism to install and distribute software binaries. These are architecture dependent, so the user has to make sure to get binaries for their specific architecture.
Based on the way packages and softwares are managed in the linux operating system, there became 3 major variants of linux, each being a monster of its own. They are
Red Hat - Uses Red Hat Package Manager (rpm or yum commands). Installer package file is .rpm file
Debian - Uses Aptitute (or apt-get command). Installer package file is .deb
Gentoo - Portage (Emerge). No binary installer, packages are installed from Source.
All other "distros" that exist today are variants of one of the above Linuxes. With recent advances, the distinctions of package management are getting blurred because for example, you can install the rpm package and libraries on a debian system, with which you can then install .rpm softwares on debian. Although not recommended, this is doable, but at the end you may have a broken system, you may have multiple compies of softwares installed on the machine; some using rpm and some using debian, and although at the heart these packages are same (GNU), but how these packages get installed like directory structure, default config files etc might vary from debian to red hat.
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POSIX
With all the different versions and variations of unix coming up all the time, in the 80's, there was a need for standardization. For example, simple commands like grep has many options (arguments) so, if I use grep in a shell script, or just normally from command line, then I should be able to use a standard set of options, irrespective of which OS I'm on and which version of Grep I'm using, whether its GNU, Sun, BSD or any other.
Thats where POSIX comes into the picture. POSIX or Portable Operating System Interface is a comprehensive set of standards defined by IEEE, which determines how various aspects of the unix operating system should behave. This includes all normal commands like ls, grep, awk, sed, ps, cd, cp, mv, etc and compilers like gcc, etc. Korn shell is 100% POSIX compliant and so are Operating systems AIX, HP-UX, Solaris etc. Linux however is mostly POSIX compliant, but not fully. This is why, when writing shell scripts, it is important to write scripts which are based on POSIX, so that the scripts will remain fully portable.
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