"Unix isn't one thing, it's a name for a large family of related operating systems, which share to differing degrees, history and architecture. Solaris, DEC Unix, IRIX, HP-UX are Unix variants. They are to some degree compatible to applications, since they implement POSIX standards to differing degrees, which means they expose similar commands and APIs.
Their kernels are not the same, though if you look up 'unix family tree' you will see a fascinating history of how these variants have evolved from one another, like organisms. That is, a finch and a swallow aren't the same animal but they have much in common.
Linux is a re-write, from scratch, of a Unix-like operating system. Whether programs written for one Unix/Linux versus another is a complex question, but in some cases yes."
"When most people talk about unix they mean a ""Unix-like operating system that is POSIX compliant"", which Linux is.
Do they share a same kernel? No, Linux IS a kernel, not an operating system. Technically, Linux distros use a Linux kernel and a GNU (see gnudotorg) ""userland"". The userland is the basic programs and libraries etc. that bridge big applications and user tasks to the kernel's low-level API.
Is Linux built over Unix? No, it's a clone. It's built to provide a free, usable version of Unix on a lowly desktop PC.
Can programs written for Linux work over Unix and vice versa? Sometimes. For example, on FreeBSD, there is a Linux ABI emulator, which essentially detects that programs were made for Linux, and makes a different set of kernel calls available. Something similar is available for Linux to run standard Unix System V stuff (or some popular commercial version of unix anyway), but it's rarely used, as most programs come as POSIX-compliant source code, and compile on Linux without difficulty. Also, most binary programs for unix are directly available for Linux. FreeBSD is a smaller concern, and doesn't have this luxury. In Linux, the system to load binaries from other systems is most often used to run java programs, windows programs (although this is ill-advised), etc.
Unix is a trademark, and so Linux can't use the name. Unix is the official, old version (inasmuchas there is an official version), which isn't so user friendly. Most other versions of unix were designed a little better, but don't necessarily work as well in practical terms. That's about it really. Many of the companies that used to run Unix now run Linux.
If you want to see the differences in various unix-like operating systems, get yourself a copies of FreeBSD, OpenSolaris, and Debian. I say these distros specifically because they'll be easier to compare, although some others like pc-bsd, nexenta, and ubuntu are arguably nicer, respectively."
UNIX is copyrighted name Only big companies are allowed to use the UNIX copyright and name, so IBM AIX and Sun Solaris and HP-UX all are UNIX. The Open Group holds the UNIX trademark in trust for the industry, and manages the UNIX trademark licensing program. Linux is UNIX clone
But if you consider Portable Operating System Interface (POSIX) standards then Linux can be considered as UNIX. To quote from Official Linux kernel README file: Linux is a Unix clone written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX compliance. Linux is just kernel
Linux is just kernel. Linux distribution includes GUI system, installation and management tools, compilers, editors etc. Linux distribution makes it
Most UNIX oses considered as compete operating system.
As I said earlier Linux is just kernel with Linux distribution makes it complete usable os. Most UNIX oses comes with A-Z programs such as editor, compilers etc. For example FreeBSD comes with A-Z programs. License and cost
Linux is Free (as in beer [freedom]). You will see best community support around. Many UNIX oses are not free (but this is changing fast, for example OpenSolaris/Solaris UNIX). User-friendly
Linux is considered as most user friendly UNIX like os. It makes easy to install sound card, flash players, and other desktop goodies. End user perspective
The differences are not that big for the average end user."
