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z/OS UNIX System Services fundamentals > Dub and undub - Pg. 10

z/OS UNIX has two shells, the z/OS shell and the tcsh shell. They are collectively called the z/OS UNIX shells. z/OS shell The z/OS shell is modeled after the UNIX System V shell with some of the features found in the Korn shell. As implemented for z/OS UNIX services, this shell conforms to POSIX standard 1003.2, which has been adopted as ISO/IEC International Standard 9945-2: 1992. The z/OS shell is upward-compatible with the Bourne shell. tcsh shell The tcsh shell is an enhanced but completely compatible version of the Berkeley UNIX C shell, csh . It is a command language interpreter usable both as an interactive login shell and a shell script command processor. It includes a command-line editor, programmable word completion, spelling correction, a history mechanism, job control, and a C-like syntax. 1.2.1 Dub and undub Dub is a term that means to make an MVS address space known to z/OS UNIX System Services. Once dubbed, an address space is considered to be a "process". Address spaces created by the fork() function are automatically dubbed when they are created; other address spaces become dubbed if they invoke a z/OS UNIX service. Dubbing also applies to MVS tasks. A dubbed task is considered to be a "thread". Tasks created by pthread_create() are automatically dubbed threads; other tasks are dubbed if they invoke a z/OS UNIX service. Undub is a term that means the inverse of dub. Normally, a task (dubbed a thread) is undubbed when it ends. An address space (dubbed a process) is undubbed when the last dubbed thread ends.