Linux began in 1991 as a personal project by Finnish student Linus Torvalds to create a new free operating system kernel. The resulting Linux kernel has been marked by constant growth throughout its history. Since the initial release of its source code in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to the 4.15 version in 2018 with more than 23.3 million lines of source code, not counting comments,1 under the GNU General Public License v2 with a syscall exception meaning anything that uses the kernel via system calls are not subject to the GNU GPL.234
Not to be confused with Linux kernel version history.