Sometime in 2016 Linux will be 25 years old. Exactly when is a matter of opinion. We could consider Linux's 25th birthday to be August 25th. That's because on that date in 1991, Linus Torvalds made his announcement to the minix community to let them know that he was working on a modest new OS. He had started the work in April. By October 5th, he felt that his new OS was usable and ready for the community to start using it.
Whether you count the announcement (8/25/1991), its readiness for use/testing (10/05/1991), or Linus' initially getting his project off the ground (April 1991) as the official birthday, Linux turns 25 sometime this year. And what is most amazing is what has happened since. All manner of Linux OSes have come into being.
If I'm counting correctly, Wikipedia lists 258 distributions! Check out https://en.wikipedia.org/wiki/List_of_Linux_distributions. From Alpine to ZipSlack, Linux has proliferated faster than anyone imagined, capturing the excitement of some of the world's most creative programmers and gaining some of the most devoted followings that computing technologies have ever known.
In his August 1991 email, Linus announced his project with a very modest "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready." Little did anyone at that time imagine how popular and important Linux would become or how many directions it would take.
That "not big and professional" statement is obviously so remote from what Linux has become that it's almost hard to imagine just how Linux took off the way it did. Today, Linux resides on the tiniest and most limited systems as well as large, complex, servers. It sits on an uncountable number of desktops and has even led to the creation of the Android OS (based on Linux) which lives on so many of our phones and tablets.
Because Linux is, for the most part, not licensed, no one seems to have a good estimate of how many systems today are running some version of it. Some versions, like the "enterprise" versions of Red Hat and Suse, are licensed, but these add up to an extremely small percentage of the systems in use today. How many Linux systems are running today? No one knows. And no one has a very good guess either.
Even I had no idea, until I started checking and found the Wikipedia page, how many types and versions of Linux have come into being. Some have come and gone, but the number of distributions that are still viable and in use is quite amazing. According to the Wikipedia categories, we have these counts:
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