Unix is not a single operating system. It is in fact a general name given to dozens of o.s. by different companies, organizations or groups of individuals. These variants of unix are referred to as flavors. Although based on the same core set of unix commands, different flavors can have their own unique commands and features, and are designed to work with different types of h/w. Linux is often considered a unix flavor.

Among the ways in which the various flavors of UNIX differ are (1) fundamental design, (2) commands and features, (3) the hardware platform(s) (i.e., processors) for which they are intended and (4) whether they are proprietary software (i.e., commercial software) or free software (i.e., software that anyone can obtain at no cost and use for any desired
purpose).

Linux :
The most popular and fastest growing of all the Unix-like operating systems. It is developed by Linus Torvalds, Linux is a product that mimics the form and function of a UNIX system, but is not derived from licensed source code. Rather, it was developed independently; by a group of developers in an informal alliance on the net. A major benefit is that the source code is freely available (under the GNU copyleft), enabling the technically astute to alter and amend the system; it also means that there are many, freely available, utilities and specialist drivers available on the net. Linux is a registered trademark of Linus Torvalds. Recent versions of Glibc include much functionality from the Single UNIX Specification, Version 2 (for UNIX 98) and later.

FreeBSD :
The most popular of the BSD systems (all of which are direct descendants of BSD UNIX, which was developed at the University of California at Berkeley). BSDI is an independent company that markets products derived from the Berkeley Systems Distribution (BSD), developed at the University of California at Berkeley in the 60’s and 70’s. It is the operating
system of choice for many Internet service providers. It is, as with Linux, not a registered. UNIX system, though in this case there is a common code heritage if one looks far enough back in history.

IBM :
IBM has been quietly working on its mainframe operating system (formerly MVS) to add open interfaces for some years. In September 1996, The Open Group announced that OS/390 had been awarded the X/Open UNIX brand, enabling IBM to identify its premier operating system to be marked UNIX 95. This is a significant event as OS/390 is the first product to guarantee conformance to the Single UNIX Specification, and therefore to carry the label UNIX 95, that is not derived from the AT&T/ SCO source code.

NetBSD :
NetBSD is a free, fast, secure, and highly portable Unix-like Open Source operating system. It is available for a wide range of platforms, from large-scale servers and powerful desktop systems to handheld and embedded devices. Features the ability to run on more than 50 platforms, ranging from acorn26 to x68k

OpenBSD :
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system. Our efforts emphasize portability, standardization, correctness, proactive security and integrated cryptography. As an example of the effect
OpenBSD has, the popular OpenSSH software comes from OpenBSD. It May have already attained its goal of becoming the most secure of all computer operating systems.

Darwin :
Darwin is an open-source Unix-like operating system first released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, Mach, and other free software projects The new version of BSD that serves as the core for the Mac OS X

Many of the proprietary flavors have been designed to run only (or mainly) on proprietary hardware sold by the same company that has developed them. Examples include:

• AIX – developed by IBM for use on its mainframe computers
• BSD/OS – a commercial version of BSD developed by Wind River for Intel processors
• HP-UX – developed by Hewlett-Packard for its HP 9000 series of business servers
• IRIX – developed by SGI for applications that use 3-D visualization and virtual reality
• QNX – a real time operating system developed by QNX Software Systems primarily for use in embedded systems
• Solaris – developed by Sun Microsystems for the SPARC platform and the most widely used proprietary flavor for web servers
• Tru64 – developed by Compaq for the Alpha processor