About piping linux program output

Does anyone know how I can let the output of a linux program go to stdout and also a file output at the same time? This is because I want to see the progress on stdout (takes hours to run it), but at the same time I want to log the output in a text file too.

This is the OP. tried >>6 before but the output is buffered so it doesn't write to the file until the program exits. not keen to insert fflush command everywhere in the code. need help from EXPERT LINUX PROGRAMMERS.