Executing a .sql file via a command prompt Ubuntu PostgreSQL

For the past hours, I have been trying to make it work but after trying many options I cannot simply execute this file. While following this tutorial, I got to the point where I had to create a .sql file and then execute it from command but it does not work for me.

Each time I try: \i mysql.sql it mentions that no such directoy or file exists. Probably because the pathname was not mentioned but then again, when I did it did not work either.