@Desmond David Linux is definitely not unix. It is "unix-like" to the extent that it follows some similar design principles and userland API tends to be very unix-like on most distributions. You will quickly find that to not be the case once you move out of standard GNU/Linux distros or distros that try to copy GNU coreutils completely.
There are some important things to note here though:
1. Linux is merely the kernel. When talking about unix you have to consider the operating system as a whole. So a debian or a fedora or an arch linux may or may not be unix based on their ability to comply with the posix standard and the single unix specification.
2. Linux does not aim to follow the single unix specification, and implementing even the complete POSIX set is optional for distro developers. Only one distribution that I know of complies with POSIX fully, which is a chinese OS derived from Debian. I forget the name.
3. Most unices are proprietary, and even the BSDs do not aim to completely match POSIX.
4. There is a thought that sometimes POSIX goes in contravention of operating system design.
You'll notice I bring up POSIX several times: this is because POSIX is a required standard under the single unix specification, and posix allows different system implementations to talk to each other natively. So even if the implementation changes, the behaviour remains predictable.
The current unices, per the definition of single unix specification are:
1. MacOS (but not Darwin)
2. IBM z/OS
3. IBM AIX
4. HP-UX
Even with this, only AIX meets the latest version of the spec, aka UNIX 7.
Solaris was formerly a unix system but Oracle no longer cares about maintaining the standard.
*nix is a better term for describing the various unices and unix-like operating systems in the market.
By the way, unix on the whole is irrelevant now. Linux already dominates the server and supercomputer space, and outside of esoteric uses like mainframes (which are also a dying breed) and databases (AIX), only BSD has a small general purpose following, because it is used by almost every manufacturer under the sun to avoid the legal implications of the GPL.