In many ways, the Linux kernel forms the core of the Linux project, but other
components make up the complete Linux operating system. Whereas the Linux
kernel is composed entirely of code written from scratch specifically for the
Linux project, much of the supporting software that makes up the Linux
system is not exclusive to Linux but is common to a number of UNIX-like
operating systems. In particular, Linux uses many tools developed as part
of Berkeley's BSD operating system, MIT's X Window System, and the Free
Software Foundation's GNU project.