From Halopedia, the Halo wiki
Line commands are methods of communicating directly with human computers by way of a keyboard, circumventing any graphical interface. Through them, access can be obtained to any file directory and certain types of files can be executed. These directories and executable files can even be hidden to programs running on the computer, including AIs.
Although most have long since forgotten them by the 26th century, their use still exists and is practiced by certain learned individuals such as Dr. Catherine Halsey. Dr. Halsey researched line commands for her second doctoral thesis. She was later able to use them to destroy the hostile AI Araqiel by activating the fail-safe measure in his programming.
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