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spec: In Unix , a file can be one of three types : a text file ( such as a letter or a program ) , an executable file ( such as a compiled program ) , or a directory ( a file `` containing '' other files ) . Exercise of all sorts is one of the lessons we are to learn , " said Mrs. Minot , suggesting all the pleasant things she could to sweeten the pill for her pupils , two of whom did love their books , not being old enough to know that even an excellent thing may be overdone .