2 Welcome, New User! Like Russian Roulette with Six Bullets Loaded Ken Thompson has an automobile which he helped design. Unlike most automobiles, it has neither speedometer, nor gas gauge, nor any of the other numerous idiot lights which plague the modern driver. Rather, if the driver makes a mistake, a giant “?” lights up in the center of the dashboard. “The experienced driver,” says Thomp- son, “will usually know what’s wrong.” —Anonymous New users of a computer system (and even seasoned ones) require a certain amount of hospitality from that system. At a minimum, the gracious com- puter system offers the following amenities to its guests: • Logical command names that follow from function • Careful handling of dangerous commands • Consistency and predictability in how commands behave and in how they interpret their options and arguments • Easily found and readable online documentation • Comprehensible and useful feedback when commands fail