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Incompatible Timesharing System
History
ITS was created by hackers dissatisfied with the directions taken by the Multics project. A central theme of ITS is openness of information: there are no file permissions or really restrictions of any kind. Anyone can (without logging in!) read and modify other users' files, spy on other terminals, update system documentation, patch the kernel, and halt the whole system. Until more and more guests started flooding in from ARPAnet, ITS had no password authentication mechanism. Even after passwords were implemented, they weren't required for users connecting locally.
These conditions resulted in a very particular culture surrounding the system. ITS hackers were fiercely dedicated to exploration and free access to information. Logging in with a username, while entirely unnecessary for most interactions, was seen as polite. To stop people from digging for crash-causing bugs, a command was created to halt the system with no effort.
A number of important applications originated on ITS, among them Zork, info, Emacs, Maclisp, Macsyma, MDL, and Scheme. Emacs is of particular interest, as several of its default keybindings were derived from associated ITS behaviors (e.g., C-g to quit, C-z to suspend and return to the console).
MIT shut down their last ITS system in 1990. The operating system survives through the https://github.com/PDP-10/its repository, which will build a complete system from source.