Time Sharing Option (TSO) is an interactive command line interpreter for IBM mainframe operating systems such as MVS/ESA, OS/390 and z/OS. It fills the same purpose as a command shell like Bash on Unix, or cmd on Windows. TSO is normally used by mainframe system administrators and programmers. For more information see the book MVS TSO by Doug Lowe.
So named since when it was originally introduced in the 1960s, time-sharing was considered an "optional feature", and hence TSO was offered as an optional feature of OS/360.
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