Date: Tue, 05 Nov 1996 00:27:24 GMT Server: NCSA/1.5 Content-type: text/html Last-modified: Thu, 31 Oct 1996 21:38:52 GMT Content-length: 7405 CS 537 - Programming Assignment I

CS 537
Programming Assignment I

Due:

September 17 at the start of class

Contents


Introduction

The purpose of this assignment is to introduce you to Java programming. You are to implement a simple shell (command interpreter) that behaves similarly to the UNIX shell. When you type in a command (in response to its prompt), it will create a thread that will execute the command you entered. Multiple commands can be chained together on a single line, separated by `&' (ampersand) characters. Your shell will create a thread for each individual command and prompt for more user input when they have all finished.

Unlike the real shell, your program only has to deal with a handful of ``built-in'' commands:
cat file ... Print the contents of the named files to System.out one after the other.
cmp file1 file2 Check whether the two files have identical contents and print an appropriate message to System.out.
sort file Print the lines of file in sorted (lexicographic) order.
exit Terminate the program. You should also terminate on reaching end-of-file on System.in.
You needn't implement pipes or re-direction of standard input and standard output, but you must be able to handle an arbitrary number of commands per line -- each with an arbitrary number of arguments separated by arbitrary amounts of white space (blanks or tabs) (although only cat takes more than two arguments).

Suggestions

Your public static void main() procedure in your primary class will be quite simple. It will be an infinite loop that prints a prompt, reads a line (in other courses, a program with an infinite loop is considered a bad thing, but in Operating systems, it's the norm!), parses it (breaks it up into its constituent commands), starts a new thread to handle each of the different commands, and then waits for all the threads to finish before printing the next prompt.

Scanning

For scanning, you may find it easier to read the entire line into a String object. The System.in object is of type InputStream, so it can read either single bytes or arrays of bytes. You could represent an input line as an array of bytes, but you will find it much easier to use a String instead. You may want to look into the class DataInputStream to figure out how to read a line into a String. Tokenizing a String is made almost trivial using the StringTokenizer class found in java.util .

Commands

For the cat command, you should look at the class FileInputStream to see how to read data from a file. cmp will be similar to cat, but this time the contents of the two files will be compared. For sort, an efficient sorting algorithm is not required; anything that works is ok. You might want to adapt the insertion sort used to introduce Java in the discussion sections. Some of the classes that might help you here are the DataInputStream class and the Vector class.

Using Threads

Your primary class will read a command from a user and then will create a thread to carry out the command. It will then wait until the thread has finished before continuing its own execution. There are two ways to start threads in Java. The first is to derive your class from the Thread class and then override its run() function (see pp. 161-162 in the text). The second is to use the Runnable interface (pp. 177-178). Here you create a class that implements this interface. You then pass a reference to this class into the constructor of a new thread object. The former is perhaps easier to understand conceptually but the latter is more general. You may choose either method for this assignment.

Exceptions

Java requires you to place within a try block any methods that might cause an exception. Following the try block is a catch clause (or catch clauses) that will be used to catch any exceptions that have been thrown (see chapter 7 for more details on the syntax of these statements). Your code should deal with exceptions in an appropriate manner. For example, exceptions such as attempting to open a file that does not exist should result in a message to the user and the continuation of the program. More serious exceptions may require an error message followed by program termination (using System.exit()).

Grading

Hand in your source program and a transcript of a terminal session which demonstrates your shell's ability to perform as specified (see script(1)). Be sure that you use test data adequate to exercise your program's capabilities. You should follow all the principles of software engineering you learned in CS 302 and CS 367, including top-down design, good indentation, meaningful variable names, modularity, and helpful comments. You will be graded not only on the basis of correctness, but also programming style and completeness of test data.

Other Stuff

For those of you writing your programs on the Solaris machines, you might consider using a makefile to aid in compilation. Those of you using DOS may want to look into doskey, a little program that provides for easy command manipulation. Feel free to send any other work-saving methods you find to the ta, and he will distribute them to the class.
solomon@cs.wisc.edu
Thu Oct 31 15:38:52 CST 1996

Copyright © 1996 by Marvin Solomon. All rights reserved.