Something i've always wondered about:

what's the difference and why some companies choose to use commercial unices instead of the free ones ? what can make solaris or hp-ux or god knows what else more attractive to someone in charge of the it dept than freebsd or linux debian ?

i mean this in terms of professionnal functionnality not in terms of "geek coolness". do they have something that the free ones do'nt have (on top of the price that is...) ???

jad,
The biggest thing that Sun or HP or IBM offer is the quality of support for their particluar unix. An IT manager usually thinks of the 'worst that could happen' scenarios where the inhouse IT dept cannot resolve a problem that has cropped up with a system. Problems such as these can be passed on to the manufacturers themselves to resolve.
Trust me, if a company does go for Linux for their IT needs then they will go in for a large company such as RedHat (who will charge a pretty packet for the support).
In terms of technology, there may not be much difference, but things like a tightly integrated logical volume manager, a local clustering solution and such, do help as well (esp. with HP-UX and IBM's AIX).

The primary reason is that commercially available systems are of way superior quality and capabilities as they better integrated with proprietary hardware. For example SUN and AIX systems allow to exchange or upgrade memory and processors without stopping system. They have storage solutions unheard in pc world.
The applications written for AIX and Solaris are of incomparably higher quality and offer range of technical abilities even not conceivable at pc level (backup libraries of thousands of tapes, multi-volume, multi-group, multi-span storage solutions).
Commercially available UNIXes have great level of integrity as the systems, hardware and integration come from the same hands so the general level of reliability, upgradeability and expandability is way above. All upgrades before been published are tested before on all standard configurations because the hardware is standard. As the hardware is limited by brand the drivers are of incomparably higher quality, offer superior communication modes and so on.
Sun systems may have hundreds of dynamically redistributed CPUs, hundreds of gigabytes of memory and hundreds of terabytes of disk space.

Let say the level of system solutions implemented in AIX circa 2000 is even not on the horizon of possibilities of Linux or Free BDS. They are good systems for what they are good, but they are not peer to the real stuff.

Ok, I got your points Amro, I want to ask few questions here, I hope these are not irrelevant. In spite of extra storage, hardware compatibility issues and many extra features like you mentioned hot-swapping capabilities.

1) Do all these UNiXES & GNU/LINUX differ at kernel level? Suppose if a small business wants to go for some standard solution like:

File Server, Domain Name Server, Mail Server or Web Server. Should they go for a commercial version or Fedora or any other free availble UNIX flavor will fulfil their requirements, because I dont see any difference

2) Does UNIX & GNU/LINUX kernel differ or Linus Torvald inherited same code from UNIX kernel or he designed his own code, I really dont know about that.

3) If original UNIX kernel code was written at Bell Labs then from where these free UNIX distributions have been evolved? Did they distribute their code to different Universities and organizations? Like BSD?

4) One stupid question, what was the first shell to interact with UNIX kernel, was it sh? or how inventors of UNIX and other experts were interacting with UNIX kernal during initial days?

Waiting for your valuable comments.

Regards,
Tayyab

You asked four questions that all effectively are the same question: is there is a difference as we have a kernel that is that cool in every way (for small business)?
Well, it depends... Sometimes if it is really small business they hardly need their own solutions, they may be better off using ISP provided DNS, and commercially available little NAS. If it is company that has real requirements they should buy a something else as GCO (general costs of ownership) with Linux is VERY HIGH. Red-Hat that is conceived as a standard Linux for enterprise is VERY expensive, I would say prohibitively expensive. For the money they ask for an installation and support (fairly crappy support over phone) one can buy complete Sun blade solution that is cool computer (and it comes with Solaris standard) and with two years of free warranty. Solaris has graphic system management console that allows to someone after some 3 days drill manage this system (for a small company), comparing to hiring someone for $30-60 per hour to do the same on Linux.

So here the difference: to have high quality hardware with warranty support and free OS on it or pay just for deficient OS and then buy crappy hardware for it.
It is not even a question, otherwise Sun, IBM and other would be long out of business.

The point who has the better kernel in to miss the point. It is as to ask: how is your "Aston Martin"'s car is better than my Kia: see I have pretty cool conditioner. Kernel is relative to the system, I do not pay for a kernel, I even do not care what kind of kernel I get (micro/mod/other), I pay for a system. One can't compare garage minded development with a state of the art technology. Linux is good for what is is good while it is free, as a system that is sale for money it can't hold water against commercially available cousins. See for example Apple's Server, that is WOW software, it is head and shoulders above anything in Linux, it is cheap, easy to install and runs on inexpensive Power Mac (some $1500-$3000) so that is example of the real quality, integrated system for little money. When it comes to business, we are not after the system that is free but for something dependable, manageable as our profits depend on that. It is all about money, if you make say $10000 per day, it is cheaper to pay $10000 for the real system rather than have "free" system that will brake more than 1 day a year.

Blowtorch has it right. It is mostly about support for the OS/hardware.

There are other factors that would affect the decision to go for one or another, but whatever those are, not many businesses will decide to adopt an unsupported OS, mainly because:
a) cost of maintenance - It is much cheaper to contract outside support than have in-house developers, and
b) accountability/time to resolve - Debian may be a better distribution than Fedora (IMHO), but if you have an issue with Debian, you'd email Ian and Deb and maybe they'd respond... or not... while with Red Hat, you'd maybe have a 2hr response support contract and have someone work on your issue right away.

There are yet other situations, like it is the case of SPARC/Solaris, where you go with the hardware for your particular need, and take whatever OS comes with it. If you need uptimes of 1000 days+ in extreme conditions (i.e., high heat, dust) , you are not going with Dell/Windows, that's for sure.