资料来源 : Free On-Line Dictionary of Computing

input/output redirection
     
         In {Unix}, to send ouput from a {process}
        to different {file} or {device} or to another process via a
        {pipe}, or to have a process read its input from a different
        file, device or pipe.  Some other {operating systems} have
        similar facilities.
     
        To redirect input to come from a file instead of the keyboard,
        use "<":
     
        	myprog < myfile
     
        Similarly to redirect output to a file instead of the screen:
     
        	ls > filelist
     
        A pipe redirects the output of one process directly into the
        input of another
     
        	who | wc -l
     
        A common misuse by beginners is
     
        	cat myfile | myprog
     
        Which is more or less equivalent to "myprog < myfile" except
        that it introduces an extra unnecessary cat process and buffer
        space for the pipe.  Even the "<" is unnecessary with many
        standard Unix commands since they accept input file names as
        command line arguments anyway.
     
        Unix's concept of {standard input/output} and I/O redirection
        make it easy to combine simple processes in powerful ways and
        to use the same commands for different purposes.
     
        (1998-04-24)