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 execute 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 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()

6. The internal shell command “time” that reports the user and system time of the program that is specified.
Example: time ls -l
Details: If the first token is “time”, execute the program that follows, waiting for it to complete. When it finishes, report the user and system time spent executing the program.
Concepts: user and system time
System calls: times() (use the command “man -s 2 times” to learn more)

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.

Requirements and Hints

You must read in input using fgets()
You must tokenize the input using strtok()
You do not need any special environment to run or execute this program (beyond using thoth as always). A shell can run another shell inside of it.
You may not use system() as this call uses the existing shell. You are writing your own shell.

Submission

You need to submit:

Your myshell.c file

Make a tar.gz file named USERNAME-project4.tar.gz

Copy your archive to the submit directory:

~jrmst106/submit/449/