This thesis deals with the problems that arise when a human user interacts with a computer via a command language. It investigates how to make the computer behave like a helpful expert who watches over the user's shoulder and offers spontaneous help rather than simply error messages or no feedback at all, as is commonly the case. The Unix command shell has been used as an example domain of this kind of human computer dialogue. A prototype active help system called RESCUER (standing for REasoning System about Commands of Unix using Evidence Reasonably), incorporates the required additional reasoning and response strategy in the form of user modelling and advice generation components.
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