COMS W4118: Homework Assignment 1

COMS W4118 Spring 2008: Homework Assignment 1
Updated Feb 7 2008

Due Tuesday, February 12th at 9:10 AM (beginning of class)

This homework consists of two components: written assignment and programming assignment. Each is to be submitted via Courseworks.

Both components of the assignment are to be done alone, not in teams. Discussion is encouraged but collaboration is not allowed.

Written Assignment

Exercise numbers refer to the course textbook, Operating System Concepts with Java. Each problem is worth 5 points.

Exercise 1.3
Exercise 1.10
Exercise 1.11
Exercise 1.17
Exercise 2.3
Exercise 2.4
Exercise 2.8
Exercise 2.14

You should submit your assignment via Courseworks->Class Files->Shared Files. Upload your answers in a single ASCII text file. The filename should be HW1.<uni>.txt For example, if your uni is sa3152, your submission filename should be HW1.sa3152.txt

Programming Assignment: Shell implementation

Description

A shell is a piece of software which provides an interface for users. It interprets user commands and executes them. Some shells provide simple scripting terms such as if or while, and allow users to make a program which facilitates their computing life. There are two categories of shells: command line and graphical. Intuitively, command line shells provide command line user interface commonly used in Unix/Linux environments, while graphical shells provide graphic user interface(GUI) such as MS Windows. In this assignment, we will focus on command line shells.

One of the interesting features of shells provided in Unix/Linux is a pipeline:

$ cmd1 | cmd2 | cmd3

This command connects the standard output of cmd1 to the standard input of cmd2, and again connects the standard output of cmd 2 to the standard input of cmd3. Common usages of this feature are ls -al | more or who | wc -l.

In this assignment, you will implement your own shell.

Purpose

Introduce basic Linux programming environment
Familiarize students with Linux programming using several system calls

Programming Assignment

1. (10 points) Write a simple shell. Basically your shell should read the line from standard input, parse the line with command and arguments, and execute the command with arguments. You should use fork and exec system calls.

The shell should find the command in your current working directory first. If not found, it should search the directories in the shell's pathname list which will be explained in the part 2.
You are not allowed to use system function in the standard library.
You may assume that arguments are separated by white spaces. You do not have to deal with special characters such as ', ", \, <, >, etc. One exception is the pipeline character (|).

2. (20 points) Implement the following built-in commands.

exit: users can exit from the shell with the exit command.
cd: cd is a command to change directories. You will need to invoke the chdir system call.
path: path is a command not only to show the current pathname list, but also to append or remove several pathnames. You may keep an variable or data structure to deal with pathname list. This list helps searching for executables when users enter specific commands.

path (without arguments) displays the pathnames currently set. It should show pathnames seperated by colons. ex) "/bin:/usr/bin"
path + /foo/bar appends the pathname to the "path" variable.
path - /foo/bar removes the pathname from the "path" variable.

3. (20 points) Extend your shell with pipeline (|).

You should use following system calls: fork, exec*, pipe, dup2, wait, close.
The depth of pipeline is limited to 2. It means that you should deal with commands like cmd1 | cmd2 or cmd1 | cmd2 | cmd3. No bonus points for deeper pipeline implementation.

Special Note - Clarifications

For all the above programs you should not make use of execlp or execvp. You are free to use any sys call from the exec* family which does not make use of the native shell PATH environment variable.
We do not want to put a restriction on the system calls you may use. You are free to use any other system calls and any data-structures but you must justify it's use in your README file.
There are no restrictions on the length of a command line and number of arguments to a given command.

Requirements

You should submit your assignment via Courseworks->Class Files->Shared Files. Post a single archive file including all of your files. The filename should be HW1.<uni>.tar.gz. For example, if your uni is sa3152, your submission filename will be HW1.sa3152.tar.gz.

Use the C language. Other languages are not allowed. No exceptions.
Include all source code, a Makefile, and a descriptive README file.
The executable filename of your shell should be "myshell". Please do no include the binary as part of you archive

Make sure that your program compiles and runs correctly on the CS CLIC machines. To grade this assignment, we will extract your submission file, type make, and run myshell.

References

Programming in C

Standard C Library

The GNU C Library Manual

COMS W4118 Spring 2008: Homework Assignment 1 Updated Feb 7 2008