CS 0449 – A Shell

The Basics In this assignment you are to implement a Unix shell program. A shell is simply a program that conveniently allows you to run other programs. Read up on your favorite shell (such as “bash”) to see what it does. The requirements for completing this assignment successfully are described below. The Input A shell, at its simplest, is a program that reads input from the user and tries to do the commands. The commands for our shell will be of one of two categories:

1. Internal commands the shell knows how to do itself, and
2. Other programs on the system to execute.

Your shell should read in a line of input using the fgets() command. You then need to tokenize it and interpret it according to the features we are supporting described in the section below. To tokenize the input, use the C standard library function strtok(). This function behaves oddly, so make sure you read about the right way to invoke it. Our delimiter set will be all whitespace characters (space, tab, newline) and the characters:

()<>|&;

We won’t be using them all in this assignment, but they are all useful in the real UNIX/Linux shell. The Details Your shell must support the following:

1. The internal shell command "exit" which terminates the shell.

   Example: exit

   Concepts: shell commands, exiting the shell

   System calls: exit()

2. The internal shell command "cd" which changes the present working directory

   Example: cd private

   Details: This command takes a relative or absolute path and changes the present working directory to that path

   Concepts: present working directory, absolute and relative paths

   System calls: chdir()

3. Any UNIX command, with or without arguments

   Example commands: ls, pico, pwd,ls –l, wc –l, ps -a

   Details: Your shell must block until the command completes and, if the return code is abnormal, print out a message to that effect. Argument 0 is the name of the command

   Concepts: Forking a child process, waiting for it to complete, synchronous execution, Command- line parameters

   System calls: fork(), execvp(), exit(), wait()

4. Any UNIX command, with or without arguments, whose output is redirected to a file

   Example: ls -l > foo

   Details: This takes the output of the command and put it in the named file Concepts: File operations, output redirection

   System calls: freopen()

5. Any UNIX command, with or without arguments, whose output is appended to a file

   Example: ls -l >> foo

   Details: This takes the output of the command and appends it to the named file Concepts: File operations, output redirection

   System calls: freopen()

6. Any UNIX command, with or without arguments, whose input is redirected from a file

   Example: more < foo

   Details: This takes the named file and makes it the standard input of the program Concepts: File operations, input redirection

   System calls: freopen()

Note: You must check and correctly handle all return values. This means that you need to read the man pages for each function to figure out what the possible return values are, what errors they indicate, and what you must do when you get that error. Please note that the UNIX commands in the list above are examples. Do not hard code support for them in. If it is not a built-in shell command, you should be attempting to run whatever program the user has typed as the first word on the commandline. For example, ls –l should work just as well as ls –a, pico, cat, pwd, etc.