Java Shell

Question

As part of my course (Computer Science), I have been asked to write a Unix shell in java. Unfortunately, I have only just started my course on programming in java, and have never used UNIX before, so am finding it quite hard.
I have got most of the program to work, although when I run a thread (with valid commands ie finger, who, clear etc.), I get the message "java.lang.ArrayIndexOutOfBoundsException" at UnixThread.run(UnixThread.java, compiled code).
I imagine the problem is that I am trying to run Unix commands from within the constraints of my program. The only way I can see to get around this is by creating some sort of batch file/script which loads when my shell quits, and then re-loads my shell as its last line. Unfortunately I do not know how to do this, and even if it did work, it would not allows background execution.
Any help would be appreciated
-=Jonny B=-

Answer 1

Hmm - I don't think that's the problem. Here's my code for the project - incase you can tell more from seeing the actual code in question.
cheers,
-=JB=-
/*project needs:
     jsh.java (this file)
     UnixThread.java
     EasyIn.java
//jsh.java:
import java.util.StringTokenizer;
class jsh{
     public static void main(String [] args){
          boolean exit = false;
          String command = "";
          while (exit==false){/*until user quits*/
               System.out.print("jsh> ");
               command = EasyIn.getString();
               while(command.indexOf(";;") > -1){/*removes any double ;'s which may cause errors*/
                    command = command.substring(0, (command.indexOf(";;")+1) ) + command.substring((command.indexOf(";;")+2), (command.length()+1));
               StringTokenizer commands = new StringTokenizer(command,";");
               UnixThread runThread = new UnixThread(commands.countTokens());
               while (commands.hasMoreTokens()) {
                    command = commands.nextToken();
                    if (command.equalsIgnoreCase("exit") && commands.hasMoreTokens()){
                         System.out.println("EXIT must be the final command in the thread. This command has been ignored.");
                         EasyIn.pause("Press a key to continue...");
                         System.out.println();
                    }else{
                         runThread.addCommand(command);
                    /*System.out.println(command);*/
               if (command.equalsIgnoreCase("exit")) {
                    exit = true;
               }else{
                    runThread.start();
//UnixThread.java
import java.io.*;
class UnixThread extends Thread{
     String[] allCommands;
     int commandNo = 0;
     UnixThread(int arraySize){ /*constructor used to initialise allCommands array to correct size*/
          allCommands = new String[arraySize];
     public void addCommand(String command){/*accessor method to add commands to the array*/
          allCommands[commandNo] = command;
          commandNo += 1;
     public void run(){/*executes the thread.*/
          Runtime go = Runtime.getRuntime();
          Process proc;
          for (int counter=0;counter < commandNo; counter++){
               try{
                    proc = go.exec(allCommands[counter]);
                    InputStreamReader fromScreen = new InputStreamReader(proc.getInputStream());
                    BufferedReader fromUnix = new BufferedReader(fromScreen);
                    if(allCommands[commandNo].charAt(allCommands[commandNo].length() - 1) != '&'){//if not background job
                         System.out.println(fromUnix.readLine());
               }catch(IOException error){
                    System.out.println("Bad command or Filename");
                    System.out.print("jsh> ");/*re-displays command prompt*/
// EasyIn.java
import java.io.*;
public abstract class EasyIn
static String s = new String();
static byte[] b = new byte[512];
static int bytesRead = 0;
public static String getString()
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
s=s.trim();
ok = true;
catch(IOException e)
System.out.println(e.getMessage());
     return s;
public static int getInt()
int i = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
i = Integer.parseInt(s.trim());
ok = true;
catch(NumberFormatException e)
System.out.println("Make sure you enter an integer");
catch(IOException e)
System.out.println(e.getMessage());
return i;
public static byte getByte()
byte i = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
i = Byte.parseByte(s.trim());
ok = true;
catch(NumberFormatException e)
System.out.println("Make sure you enter a byte");
catch(IOException e)
System.out.println(e.getMessage());
return i;
public static short getShort()
short i = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
i = Short.parseShort(s.trim());
ok = true;
catch(NumberFormatException e)
System.out.println("Make sure you enter a short integer");
catch(IOException e)
System.out.println(e.getMessage());
return i;
public static long getLong()
long l = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
l = Long.parseLong(s.trim());
ok = true;
catch(NumberFormatException e)
System.out.println("Make surre you enter a long integer");
catch(IOException e)
System.out.println(e.getMessage());
return l;
public static double getDouble()
double d = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
d = (Double.valueOf(s.trim())).doubleValue();
ok = true;
catch(NumberFormatException e)
System.out.println("Make sure you enter a decimal number");
catch(IOException e)
System.out.println(e.getMessage());
return d;
public static float getFloat()
float f = 0;
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
f = (Float.valueOf(s.trim())).floatValue();
ok = true;
catch(NumberFormatException e)
System.out.println("Make sure you enter a decimal number");
catch(IOException e)
System.out.println(e.getMessage());
     return f;
public static char getChar()
char c = ' ';
boolean ok = false;
while(!ok)
try
bytesRead = System.in.read(b);
s = new String(b,0,bytesRead-1);
if(s.trim().length()!=1)
System.out.println("Make sure you enter a single character");
else
c = s.trim().charAt(0);
ok = true;
catch(IOException e)
System.out.println(e.getMessage());
return c;
public static void pause()
boolean ok = false;
while(!ok)
try
System.in.read(b);
ok = true;
catch(IOException e)
System.out.println(e.getMessage());
public static void pause(String messageIn)
boolean ok = false;
while(!ok)
try
System.out.print(messageIn);
System.in.read(b);
ok = true;
catch(IOException e)
System.out.println(e.getMessage());
}