UNIX
Many ideas in Unix were new. Other operating systems copied them. Today, there are many operating systems that have some of the ideas of Unix in them. For this reason, some people talk about a "Unix philosophy" of doing things. Linux is one of these systems with many of the Unix ideas in it. Linux does not use code from UNIX. Linux only shares some of the ideas and the names of commands. So, Linux is not a UNIX operating system. Instead it is called "Unix-like".