Read file to standard in for parser

Question

I am trying to implement a shell program in a linux environment. The part I am having trouble with is reading a setup_file inside of a shell before running the shell, to do things like set environment variables.

Currently the shell has a parser_results = parse() function which does a "getchar" and waits until the user types something into stdin, then does an execute(parser_result) which executes the command using the output of the parser.

What I want to do is to read the setup_file which has commands inside of it, have the parser read them in and give me the data structures I need. Then I can run execute.

My question is is how do I redirect the contents of the file to stdin? And how do I call the parser to parse this redirected input? I have been playing with dup and dup2 to no avail.

Answer 1

Short answer (to the question 'how do I redirect the contents of the file to stdin') is "You Don't".

You revise your input function to read from a given file stream instead of stdin, and then for reading from the file, you open it and pass that file pointer to your parsing code (and close when the parsing code is done), and then when you're ready for user input, you call the parsing code with stdin instead of the file. That saves fiddling with stdin.