mjhjmtu

For all intents and purposes, a typical modern Linux distribution (Ubuntu, Debian, Red Hat, Fedora, Slackware, etc) is a Unix, but strictly speaking, no system can claim to be Unix without being certified, so instead people say they are Unix-like. They are inspired by Unix, and carry on its culture.

This also applies to BSD systems.

Mac OS X is certified Unix, so it's Unix both in name and indeed. (and it's actually based on BSD).

It should be noted that since Linux itself is just a kernel, it can be used to build non-unix-like systems (such as Android).

Linux is more 'Unix-like' so yes simplistically you could call it a clone, the same is true for BSDs (although admittedly BSDs could be considered closer to Unix than Linux).

The main thing that gives Linux the Unix-like title is the fact that it is nearly fully compliant w/ POSIX (Portable Operating System Interface [for Unix]) standards that have built up over time.

The other key thing, is the inheritance of code etc, as demonstrated on Wikipedia, Linux does not actually originate from Unix sources, instead it is pretty much built from scratch (this is disputed however).

So essentially, the main thing that separates Unix from Linux is the ancestry and the standards that it meets.

The choosen answer explains it quite exhaustively, but you would have to watch a whole documentary to know all the details between the UNIX initial developement, and how Linux went out.

First you have to consider that Linux is the kernel, which was made by linus and other programmers. He chose to release it with the GPL, which by the time was a license made by the FSF, (RMS and other folks), who were also developping their own kernel and GNU.

What we usually call Linux is, at first, the kernel, plus all other tools originally coming from the GNU project. Those two project are historically different, because at the time, the kernel from the FSF folks was abandonned to favor the Linux kernel, which was much better.

I remember there is a documentary, you should really watch it, I find it important for your programming culture.

Another thing you have to consider when thinking about "*nix", is also everying involving POSIX and other architecture-standardised stuff. It's subject of OS design/research, but it defines precisely how the involved OSes works, and is crucial when you have a kernel working well with its tools.

There are lots of great answers already but I thought the views of the great Dennis Ritchie (co-creator of Unix) would be an interesting complement to the other more technical answers.

It seems that Dennis Ritchie considered Linux to be a legitimate Unix derivative. In a 1999 interview for LinuxFocus.org, he was asked,

what is your opinion about all the Linux momentum, and the decision of many
companies to start developing software for it?

His response to the question was

I think the Linux phenomenon is quite delightful, because it draws so
strongly on the basis that Unix provided. Linux seems to be the among the
healthiest of the direct Unix derivatives, though there are also the various
BSD systems as well as the more official offerings from the workstation and
mainframe manufacturers. I can't help observing, of course, the "free
source" Unix-derived world seems to be suffering from exactly the same kind
of fragmentation and strife that occurred and is still occurring in the
commercial world.

Historically Linus Torvalds has created an improved clone of Minix OS.
The development of Linux is not focused on a specific platform and customer base, and Linux developers have a variety of experiences and perspectives.
Such OS as HP-UX, AIX, Solaris and others are mostly sharpened for vendors.

In the Linux community there is no strict standard set of tools or environments. This lack of standardization leads to significant inconsistencies within Linux. For some developers, the ability to use the best achievements of other operating systems is a plus, but it's not always convenient to copy UNIX elements on Linux, for example, when the device names inside Linux can be taken from AIX, while the tools for working with the file system are focused on HP-UX. Incompatibilities of this kind are also found between different Linux distributions.

Unix, originally UNICS (UNiplexed Infomation and Computing Service).

Unix (trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix. Development started in 1969 and announced outside Bell abs in October 1973.

Variety of both academic and commercial variants of Unix from vendors such as the University of California, Berkeley (BSD), Microsoft (Xenix), IBM (AIX) and Sun Microsystems (Solaris).

• 
  BSD(Berkeley Software Distribution) releases provided a basis for several open source development projects that are ongoing, e.g., FreeBSD, OpenBSD, NetBSD, Darwin, and TrueOS.


• Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft.


• AIX(Advanced Interactive eXecutive) developed and sold by IBM for several of its computer platforms.

Linux is a family of free and open-source software operating systems built around the Linux kernel.The defining component of Linux is the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds.

Linux is packaged in a form known as a Linux distribution (or distro for short) for both desktop and server use. Some of the most popular and mainstream Linux distributions are Arch Linux, CentOS, Debian, Fedora, Gentoo Linux, Linux Mint, Mageia, openSUSE and Ubuntu, together with commercial distributions such as Red Hat Enterprise Linux

The user interface, also known as the shell, is either a command-line interface (CLI), a graphical user interface (GUI), or through controls attached to the associated hardware, which is common for embedded systems. For desktop systems, the default mode is usually a graphical user interface, although the CLI is commonly available through terminal emulator windows or on a separate virtual console.

• 
  CLI shells are text-based user interfaces, which use text for both input and output. The dominant shell used in Linux is the Bourne-Again Shell (bash), originally developed for the GNU project.


• 
  GUI shells are K Desktop Environment (KDE), GNOME, MATE, Cinnamon, Unity, LXDE, Pantheon and Xfce, though a variety of additional user interfaces exist. Most popular user interfaces are based on the X Window System, often simply called "X".

GNU is an operating system and an extensive collection of computer software. GNU is composed wholly of free software most of which is licensed under the GNU Project's own GPL. GNU is a recursive acronym for "GNU's Not Unix!", chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code. Development of the GNU operating system was initiated by Richard Stallman while he worked at the Massachusetts Institute of Technology (MIT) Artificial Intelligence Laboratory in 1983.

Basic components include the GNU Compiler Collection (GCC), the GNU C library (glibc), and GNU Core Utilities (coreutils), but also the GNU Debugger (GDB), GNU Binary Utilities (binutils), the GNU Bash shell and the GNOME desktop environment.