What is the difference between Linux and Unix?

Unix is a proprietary OS developed at Bell Labs in the 1970s, while Linux is a free, open-source Unix-like OS kernel written from scratch by Linus Torvalds. Unix is certified and sold commercially (e.g., AIX, HP-UX, Solaris), whereas Linux is freely available under the GPL license. Both share the same design philosophy — everything is a file, small single-purpose tools, pipes — but Linux was not derived from Unix source code. Today, macOS is also Unix-certified (based on BSD). Linux dominates servers, cloud, and embedded systems due to its open nature and community development.