Thank you very much.I think there are something wrong with my expression.
grep Download
When you type something, hitting grep will do a row match
The "grep Download" command has no file.So it is the case that"If no FILE is given, recursive
searches examine the working directory, and nonrecursive searches read standard
input. ".
When I type the command,the terminal will do two things which are 'recursive
searches Examine the working directory' and 'nonrecursive searches read
standard input'.In this case, Download should be used as a matching string, but
instead of going through all the files in the current directory, the terminal
waits for the user's input.
So I think it's a little different from what you wrote.