The TC (tcsh) and Bourne Again (bash) shells have filename completion turned on by default, while the C (csh) and Korn (ksh) shells can activate this feature. If you are typing in the name of a file (which can be the name of a command), and you hit the ``tab'' key, the shell will try to complete the typing of the filename for you based on context. If it can't, it will beep at you.