The STDIN file desciptor references the standard input to the shell. For a terminal interface the standard input is the keyboard. The shell receives input from the keyboard on the STDIN file desciptor and process each character you type in.when you use input redirect symbol (<) , linux replaces the standard input file desciptor with the file referenced by the redirection. It reads data and behaves like just user typed it in. (<<) Double input character stands for concatenation to the file.
cat command to input in file :
cat comand to input in itself::
input from commands :
error inputs in file as it transfers file descriptor `2` in input.: