UNIX is an operating system that was developed by Dennis Ritchie and Kenneth Thompson at Bell Labs more than 30 years ago. It allows multitasking and multiuser capabilities and offers portability with other operating systems. It comes with hundreds of programs that are of two types: integral utilites, such as the command line interpreter; and tools such as email, which are not necessary for the operation of UNIX but provide additional capabilities to the user. It is functionally organized at three levels: the kernel, which schedules tasks and manages storage; the shell, which connects and interprets user's commands, calls programs from memory, and executes them; and tools and applications, which offer additional functionality to the operating system, such as word processing and business applications. UNIX^® was registered by Bell Laboratories as a trademark for computer operating systems. Today, this mark is owned by The Open Group.