First of all, is it right? Then why?
1. Is Linux based on UNIX?
Linux is a UNIX-like system, so what is a UNIX-like system?
A UNIX-like system refers to a system that has evolved from the design style of UNIX.
A UNIX-like system is a system that looks like UNIX but is not actually UNIX;
In fact, it essentially borrows the interface and features of the UNIX system (multi-user, multi-tasking, etc.), but does not directly copy the source code of others. After all, they have copyright restrictions, so it is more of an inheritance of ideas.
BSD, MINIX systems, and Linux systems are all UNIX-like systems. Due to the high price of the UNIX standard certification, the only one that has been certified by the UNIX standard is Apple's MACOS system.
The main differences between the Linux system and the UNIX system are as follows:
- Open source : UNIX is closed source, Linux is open source.
- Cross-platform : UNIX systems are mostly matched with hardware, and can only run on workstations or minicomputers, and are expensive; while Linux can run on a variety of hardware platforms.
Although often compared, Linux and Unix are not related . In other words, the difference between Linux and Unix may be greater than the difference between Linux and windows in the implementation of some details of the operating system.
Linux is open source free software, and Unix is commercial software.
From the pedigree chart, the Linux branch and the rightmost minx are actually separated separately without any connection.
2. Why does UNIX charge
In the book "Legend of UNIX", the highlight moment of the UNIX system is mentioned. The sci-fi adventure movie "Jurassic Park" released in 1993 believes that many people must still be impressed by this old movie; one of the famous scenes , Professor Hammond's granddaughter is operating the UNIX system, closing the floodgate, thus saving a group of people.
This story tells us that learning one more operating system may save your life at a critical moment.
UNIX is completed by using assembly language. Assembly language is not powerful enough for programming and does not have good portability. So in 1971, Dennis Rich developed , UNIX was also rewritten in C language, and then 4, 5, and 6 versions of UNIX were released.
Since then, UNIX has been noticed by government agencies, research institutions, enterprises, and universities, and has gradually become popular; with the widespread popularity of UNIX systems, the C language has also become one of the most popular languages, and it has continued to this day.
At the beginning, AT&T didn't take UNIX seriously . After all, it was not a formal project. Besides, it had suffered a lot on the operating system (multics), so it didn't want to sell it for money. Therefore, it was provided to universities for free, and even Directly give the source code of the V7 version for research.
Therefore, in the following 10 years, UNIX has been widely used in various academic institutions, and even many institutions have improved on the basis of this source code. One of the most famous variants is the BSD (Berkeley Software Distribution) product developed by the University of California, Berkeley. ), and on this basis, three branches were born: FreeBSD, OpenBSD and NetBSD, and even the NextSETP used by Apple Computer's kernel Darwin is also a derivative version of BSD.
AT&T saw, oh, UNIX is so popular, so it realized the huge commercial value of UNIX , no longer authorized the source code of UNIX to academic institutions, and declared the copyright of the previous UNIX and its variants, which triggered various protracted Copyright disputes, this is something about UNIX.
3. From MINIX to Linux
Time came to the 1980s. With AT&T closing the source UNIX system, there were very few operating systems for students in schools;
In 1987, American Andy Tanenbaum (Andrew S. Tanenbaum), who was a professor at the Vrije University in Amsterdam in the Netherlands, wrote a UNIX-like system based on the source code of BSD in order to let students better understand the principles of the operating system . , named MINIX , which means mini UNIX, and open all the codes for university teaching and research;
Since it is a MINI, its code volume is relatively small, with a total of about 12,000 lines of code, and it is only a teaching tool , without any practical application value.
In 1991, our protagonist Linus Torvalds (Linus for short) began to have a strong interest in UNIX while he was at the University of Helsinki, Finland;
During school, because Linus often used his terminal emulator (Terminal Emulator) to access the newsgroups and emails on the university host, he was dissatisfied with the fact that MINIX was only allowed to be used in education, and also for the convenience of reading, writing and downloading files. , he began to write his own UNIX-like system;
During the day and night development during a summer vacation, the first kernel of Linux (version 0.02) was finally developed and named Linus' Minix, which was later renamed Linux.
In October 1991, Linus released a message in the Minix newsgroup, announcing the birth of the Linux kernel and disclosing the source code of the kernel ; after the release, Linux was well received as soon as it was released because of its clear structure and concise functions; later, with the help of many enthusiastic supporters , after many iterations of version upgrades, finally in March 1994, Linux 1.0 was officially released .
Just as Thompson and Rich did not expect the success of the UNIX system, Linus also did not expect that the kernel system he spent a summer vacation to play with could run on billions of devices around the world in the form of a commercial operating system. .
4. Is Linux free?
The Linux system we often talk about is actually more about the many distributions of Linux in a broad sense, because you don't go directly to the kernel of the operating system. 
The release version refers to some organizations or manufacturers that package the Linux kernel, various software, software package managers, etc., and provide the system installation interface, system configuration, and desktop environment, etc., to form a Linux release version.
There are many distributions of Linux, which can be roughly divided into two categories:
- Commercial versions maintained by commercial companies , represented by the famous Red Hat;
- Free versions maintained by community organizations , represented by Debian and Ubuntu.
Some students may start to wonder, didn't the above say that Linux is open source? Why is there still a commercial version? Yes, the Linux kernel is open source, but open source does not mean free. The commercial version charges for its commercial services and support.
For example, although Red Hat uses open source software, it has spent a lot of labor to integrate thousands of open source software into a system, and ensure the compatibility and stability of software, and provide follow-up support, maintenance and upgrade services, so it is charged;
If you spend enough money, such as buying their premium services, you can even have Red Hat engineers come over to solve the problem for you on the spot.
Many people may still think that open source charges are not understandable, but in fact, if you try to compile multiple open source software, or encounter some inexplicable errors in the operating system, and then you spend a few days searching github, stackoverflow and Google, there is no answer. When you find a problem and are distressed, you will feel that it is a very happy thing if someone can help you solve it.
What's more, enterprise projects often pursue fast launch when they are running, and the timing is based on days or even hours. At this time, it is very important to solve problems quickly;
This fee is very cost-effective for enterprises.
This incident tells us that there is no free lunch in the world, and free is often the most expensive.