A shell is a program that users use to communicate with non user friendly machines. The shells that are comonly used on Unix are sophisticated interactive programs with several facilities for customization and speed up. In this assignment you will write a simplified shell, loosely modeled after sh. The BNF of the language is:
where < and > is redirection, | is pipe, & is asynchronous execution (the second process starts before the first is done), ; is synchronous execution, || is “or” (the second command is executed only if the first returns non zero status) and && is “and”. The parser for this language is given as well as a main program and a routine that prints out the evaluation tree. You have to write the eval routine that will create the processes, set up the pipes and do the synchronization. The cd and exit commands will not be executed in a separate process. Start by reading the following manual pages: fork, execvp, wait, kill, malloc, open, close, dup, dup2, pipe, strchr, chdir.
