UNIX, Linux and the Key Differences
What is Unix?

All Unix, Linux and other Unix-like operating systems are families of computer operating systems derived from the original Unix System from Bell Telephone Laboratories. Today, the largest Unix descendent directly certified as "UNIX" is macOS by Apple. The original commercial Unix derivatives included HP-UX, AIX and SunOS, to name a few. The diversity and perceived incompatibility between various Unix systems led to the creation of interoperability standards including the Portable Operating System Interface (POSIX).
Unix is the original and most powerful and popular multi-user and multi-tasking Operating System. The basic concepts of Unix were originated in the Multics project of 1969. The Multics system was intended as a time-sharing system that would allow multiple users to simultaneously access a mainframe computer. Ken Thompson, Dennis Ritchie, and others developed the basic building blocks of Unix including a hierarchical file system and a command line interpreter for the PDP-7. Multiple generations of Unix systems were developed for myriad computers.
As mentioned, historical perceived incompatibility between the various early Unix systems led to the creation of POSIX and the Single Unix Specification. Historically, the rigid "standardization" approach led to the creation of various open source approaches to Unix including the Free Software Foundation (FSF), GNU and Linux. Unix programs were originally created around core design philosophies that included requirements like single purpose, interoperable, and working with a simple standardized text interface. Unix systems are built around a core kernel that manages the system and the other processes. Kernel subsystems may include process management, file management, memory management, network management and others.
Key Features:
  1. Unix is a multi-user system where the resources can be shared by various system users.
  2. Unix provides multi-tasking. Users can execute many processes at the same time (practically speaking).
  3. Unix was the first computer operating system that was written in a high-level language (C Language).
  4. Unix provides a hierarchical file structure which facilitated fast data access.
  5. Unix has built-in networking functions so that computers can easily exchange information.
  6. Unix functionality can be extended through user programs built on a standard programming interface.
  7. UNIX certifies compliance with a full set of interoperability standard, managed and maintained by the Open Group, which includes, not only the kernel, but the entire operating system.
What is Linux?

Linux is a Unix-like operating system created by Linus Torvalds at the University of Helsinki in 1991. The name Linux refers to the Linux kernel, the software on a computer which permits applications and users to access the devices on the computer to perform specific functions. The development of Linux was a landmark example of global, free and open source software collaboration. Many companies and similar numbers of individuals have released their own version of Unix-like operating systems based on the Linux Kernel.
Similar to Unix kernels, the Linux kernel relays instructions from an application from the computer's processor and sends the results back to the application.
Broader in scope than commercial Unix products, Linux has been installed on a variety of platforms including mobile phones, tablets, video game consoles, NAS storage arrays, routers and small embedded controllers.
Currently the world's largest and most powerful data centers and scientific research centers use some "flavor" of Linux.
The development of Linux is a stellar example of the power of free and open source software development and global human collaboration.
A Linux "distribution" is a release of a Unix-like operating system based on the specifications of the Linux kernel.
Key Features:
  1. Linux, like Unix, is a multi-user system where the resources can be shared by various system users.
  2. Linux, like Unix, provides multi-tasking. Users can execute many processes at the same time (practically speaking).
  3. Linux, like Unix, was written in C.
  4. Linux, like Unix, provides a hierarchical file structure which facilitated fast data access.
  5. Linux, like Unix, has built-in networking functions so that computers can easily exchange information.
  6. Linux, unlike UNIX, specifies only the kernel but not the entire operating system.
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An Interest Fact about the History of Unix and Linux
"Unix-like"

A "Unix-like" operating system is an OS that behaves in a manner similar to a Unix system but is generally not certified to conform to the Single UNIX Specification. Then again, a "Unix-like" application is an application that performs like the corresponding Unix command or shell. In practical terms, there is no standard for defining the term "Unix-like" and so you will also see terms like UN*X or *nix.
Historically, there have been myriad opinions as to the degree to which a given operating system or an application is "Unix-like". "Unix-like" could refer to a free or open-source operating system inspired by the Bell Labs original Unix design. "Unix-like" might refer to a design which emulates Unix features, similar commercial and proprietary versions, and Unix versions based on the licensed UNIX source code. "Unix-like", for others, simply means an operating system is sufficiently "Unix-like" to pass certification and bear the "UNIX" trademark. Obviously, people used to find this phrase controversial. However, over time people realized that all the historical in-fighting did not help the Unix brand or the community as a whole; and so the controversy has subsided, for the most part.
"Unix-like" operating systems burst into the computing scene in the late 1970s and early 1980s. Back then, proprietary Unix versions such as Idris (1978), UNOS (1982), Coherent (1983), and UniFlex (1985) marketed the functionality available to academic users of UNIX as "Unix-like". AIX, HP-UX, IRIX, SunOS, Tru64, Ultrix, and Xenix (among others) were developed based on relatively inexpensive AT&T commercial binary sub-licensing of UNIX back in 1979. These licensed Unix variations displaced most proprietary Unix clones back then. Incompatibility issues with all these glorious "Unix" operating systems created a need for robust interoperability standards, including POSIX and the Single UNIX Specification. The Open Group adopted the UNIX trademark to oversee the Single UNIX Specification and the "UNIX" name became a certification mark for Unix system interoperability.
Dennis Ritchie, one of the original Unix legends, has publicly stated that Unix-like systems such as Linux are de facto Unix systems. The historical years of in-fighting and legal battles over "Unix", "Unix-like", and "UNIX" standards arguably contributed to the overall downward trend for Unix over the years and the rise of Linux as a dominate "Unix-like" global operating system. In 2019, macOS has the honor of having the largest commercial UNIX in the Unix user market. Yes, it can be confusing for many people, especially the younger generations.
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