Write a shell in C that implements a command line interface similar to well-known shells like bash. The shell will have an interactive input prompt and parse the command line input into semantic tokens. It will implement parameter expansion, shell special parameters ($$, $?, and $!), tilde (~) expansion, two shell built-in commands (exit and cd), redirection operators (< and >), and the & operator to run commands in the background. The shell will also handle custom behavior for SIGINT and SIGTSTP signals. The shell will run in an infinite loop and will terminate when the built-in exit command is executed or on EOF on stdin. The assignment is expected to test the Unix process API, signal handling, and I/O redirection.