Writing a simple shell for unix is actually a common student project for CS students in their second or third year. If you know C at the level of K&R (especial

Writing a simple shell for unix is actually a common student project for CS students in their second or third year. If you know C at the level of K&R (especially the last few chapters on system calls), you can learn what you need to write a shell from a book like Stevens' Advanced Programming in the Unix Environment and/or Bryant and O'Halloron's Computer Systems: A Programmer's Perspective.