From the manual for grep, the general form of grep usage is:grep options pattern input_file_namesIn particular, it is stated There can be zero or more input file names.What files are candidates for the search, if there are zero file names? Ex. grep mystr
Zero input_file_names in Grep
grep
From man grep:   file   A pathname of a file to be searched for the patterns. If no file          operands are specified, the standard input shall be used.So it will wait for you type some text if there's no pipeline or redirection involved.