I'm am curious to know which linux server is the most popular in the market and enterprise markets. Is it redhat linux enterprise, SUSE linux enterprise or Ubuntu Server?

Also, has linux overtaken the unix operating system as the preferred server solution?

As to your last question - it depends on how you count.

Linux has a few percentage points more the UNIX( about 6) for DNS/web/mail. But that doesn't include compute servers, back end servers, mainframe servers, supercomputers... And that is a LOT of other servers (most are not UNIX anymore). Then there are the linux guests on others (about 35% of Azure is used for Linux for instance). Also not necessarily included are things like web search engines (you might count them as compute/back end servers though. All of the ones I know of are Linux, with the exception of Bing and a few that that use bing behind the scenes).

As to which server (RH/Suse/ubuntu) Don't know. A lot of places started with Suse/RH, but customize/change it for their use. I'm not familiar with those that use ubuntu.

A search for server OS usage will turn up many articles. As jpollard points out, different surveys use different metrics, so you may want to look at several of them.

My guess would be that RHEL/CentOS are probably most used, if only because they are long established and extremely stable, but that's no more than an impression formed from reading and listening to lots of Linux news and podcast. SUSE is also well-established and stable; my impression is that it is better known in the enterprise east of the Atlantic.

Statistically insignificant, but I've never worked at a place that used anything other than RH/CentOS. I've been using SUSE for my work/personal computers for years because it offers a much better user experience IMO and works perfectly fine in RH environments.

We have a guy that tried deploying Gentoo on a few boxes and everyone hated it (and him.) I've never heard of anyone using anything Debian based for any back-end servers, but I'm sure its out there as well.

I'm thinking of training for a IT career. I want to focus my studies on the most used linux server distro. And it looks like RHEL is the winner. Since RHEL is not free, I have to settle for Centos for practice.

It isn't a case of "to settle". There is no difference between the two. CentOS removes the proprietary logos and a few proprietary applications. But that is the only difference.

CentOS is also used as a prototype testbed for RH now, taking over part/some/all? the role that Fedora used to do (Fedora lost between 2 and 3 million users with the crappy Gnome3 product).

There isn't just one. There are two general families of distro (not counting some outside those groupings). You'll need more than a passing familiarity with one APT-based distro as well as the RPM ones, in order to demonstrate sufficient breadth of knowledge. So if you have picked CentOS for the RPM side, you can look at Debian / Devuan, Ubuntu, or Linux Mint for the APT side. Much will be the same, it's just the package management that's different. Also, Red Hat and Canonical are now days pushing a 3rd and 4th packaging system as well. Though I'm not sure if they are heading in the right direction or behaving in the right way to get there.

+1 With RH being a commercially supported distro there's just a plethora of good documentation available that also applies to CentOS so that's my thinking on why they are ahead in (production) server environments.

In my corporation, RHEL if production customer-facing, CentOS is backend/test customer facing. Internally we use some Ubuntu server and SUSE. The executives who do not actually do the work love SUSE (it costs less), the green from (or in) India out-of-college crowd likes Ubuntu, but the hard-core Engineers who keep us in business prefer RHEL and CentOS.

Not everything cutting-edge works with RHEL, but you can pretty much count on it to work exactly as RH says it should ALWAYS. When you job depends on it, dependable is the way to go.

On a personal level, I prefer Debian (NOT Ubuntu) but come back to what RH has done (and continues to do) for the community and industry. I push Debian at home, but RHEL/CentOS for business, government, and military (I am USARND Retired).

And, in case you were wondering: RHEL was the first commercial OS to pass the newest military IT security and reliability standard for use on combat critical platforms (think ships, submarines, aircraft). Microsoft never has. Ubuntu never has. Suse never has (though clearly - to me - COULD). That should, by itself, probably not be a consideration for business use, but that diligence and focus might give one pause for thought.

Which operating systems did they test though? The ones you mentioned, were they tested and failed or never tested at all? Do you have public sources for this information? Just curious what they tested and how they tested it.

Also, this may help although it seems to point in the opposite direction of what everyone in this thread said with Debian and Slack taking top spots :-)
http://www.linuxquestions.org/questi...ar-4175562705/

Personal preferences of Linux admins often don't reflect the reality of what organizations choose for stability/support/tco.

I know RH 6 had finished evaluation. The major problem with the validation is the security model used may not be all that strong - for instance, it excludes testing of mandatory access controls, IPSec, containers, ...

Makes it easy for EAL 4+, but even Microsoft with its well known vulnerable Windows system can get EAL 4+. It all depends on what you test.