Completion is invoked when you type either of the two control characters. The completion function takes the path string to ...

Completion is invoked when you type either of the two control  characters.  The completion function takes the path string to the left  of the cursor appends a wild card character to it if none is already  present and builds up a list of paths that match.  It then displays the  first matching path.  If no paths match, it just beeps and leaves the  display alone.  Thereafter, repeated pressing of the same control  character will cycle through the list of matching paths.  Pressing the  Shift key with the control character will move through the list  backwards.  If you edit the line in any way and press the control  character again, the saved list of matching paths is discarded and a new  one generated.  The same occurs if you switch between file and directory  name completion.  The only difference between the two control characters  is the file completion character matches both file and directory names,  while the directory completion character only matches directory names.  If file completion is used on any of the built in directory commands  (CD, MD or RD) then directory completion is assumed.    The completion code deals correctly with file names that contain spaces  or other special characters by placing quotes around the matching path.  Also, if you back up, then invoke completion from within a line, the  text to the right of the cursor at the point completion was invoked is  discarded.    The special characters that require quotes are:              ()[]{}^=;!%'+,`~