Before you read that post, I encourage you to go through my two previous posts (here and here), which are about working with files. They describe basic approach and explain why that method is not recommended. Instead of this, in the following post I am proposing more standard, popular and convenient way to work with files in the VHDL.
TextIO is a package created to simplify working with files. It works in the same way on every operating system with almost every type available in the VHDL.
VHDL Guide 