Open a File for reading and associate with Standard Input (STDIN)
Use the current input stream as Standard Input for the program until “token” is seen. This is used in scripting:
Open a file for writing and truncate it and associate it with Standard Output:
Open a file for writing and seek to the end and associate it with Standard Output.It is used to append text to a file using a redirection operator.
Redirect FD n to the same place as FD m :
This means send STDERR to the same place that STDOUT is going to.