Creating a Shell Interface Using Java
This assignment consists of modifying a Java program so that it serves as a shell interface that accepts user commands and then executes each command in a separate process external to the Java virtual machine.
A shell interface provides the user with a prompt, after which the user enters the next command. The example below illustrates the prompt jsh> and the users next command: cat Prog.java. This command displays the file Prog.java on the terminal using the UNIX cat command.
jsh> cat Prog.java
Perhaps the easiest technique for implementing a shell interface is to have the program first read what the user enters on the command line (here, cat Prog.java) and then create a separate external process that performs the command. We create the separate process using the ProcessBuilder() object, as illustrated in the section labeled Process Creation in Java found later in this assignment description. In our example, this separate process is external to the JVM and begins execution when its run() method is invoked.
A Java program that provides the basic operations of a command-line shell is supplied in the code of Figure 1 on the next page. The main() method presents the prompt jsh> (for java shell) and waits to read input from the user. The program is terminated when the user enters < Control>< C>.
This assignment is organized into three parts: (1) creating the external process and executing the command in that process, (2) modifying the shell to allow changing directories, and (3) adding a history feature.
import java.io.*;
public class SimpleShell
{
public static void main(String[] args) throws java.io.IOException
{
String commandLine;
BufferedReader console = new BufferedReader
(new InputStreamReader(System.in));
// we break out with < control>< C>
while (true)
{
// read what the user entered
System.out.print("jsh>");
commandLine = console.readLine();
// if the user entered a return, just loop again
if (commandLine.equals("")) continue;
/** The steps are:
(1) parse the input to obtain the command and any parameters
(2) create a ProcessBuilder object
(3) start the process
(4) obtain the output stream
(5) output the contents returned by the command */
}
}
}
The first part of this assignment is to modify the main() method in Figure 1 so that an external process is created and executes the command specified by the user. Initially, the command must be parsed into separate parameters and passed to the constructor for the ProcessBuilder object. For example, if the user enters the command
jsh> cat Prog.java
the parameters are (1) cat and (2) Prog.java, and these parameters must be passed to the ProcessBuilder constructor. Perhaps the easiest strategy for doing this is to use the constructor with the following signature:
public ProcessBuilder(List< String> command)
A java.util.ArrayList, which implements the java.util.List interface, can be used in this instance, where the first element of the list is cat and the second element is Prog.java. This is an especially useful strategy because the number of arguments passed to UNIX commands may vary (the cat command accepts one argument, the cp command accepts two, and so forth.)
If the user enters an invalid command, the start() method in the ProcessBuilder class throws an java.io.IOException. If this occurs, your program should output an appropriate error message and resume waiting for further commands from the user.
The next task is to modify the program in Figure 1 so that it changes directories. In UNIX systems, we encounter the concept of the current working directory, which is simply the directory you are currently in. The cd command allows a user to change current directories. Your shell interface must support this command. For example, if the current directory is /usr/tom and the user enters cd music, the current directory becomes /usr/tom/music. Subsequent commands relate to this current directory. For example, entering ls will output all the files in /usr/tom/music.
The ProcessBuilder class provides the following method for changing the working directory:
public ProcessBuilder directory(File directory)
When the start() method of a subsequent process is invoked, the new process will use this as the current working directory. For example, if one process with a current working directory of /usr/tom invokes the command cd music, subsequent processes must set their working directories to /usr/tom/music before beginning execution. It is important to note that your program must first make sure the new path being specified is a valid directory. If not, your program should output an appropriate error message.
If the user enters the command cd, change the current working directory to the users home directory. The home directory for the current user can be obtained by invoking the static getProperty() method in the System class as follows:
System.getProperty("user.dir");
Many UNIX shells provide a history feature that allows users to see the history of commands they have entered and to rerun a command from that history. The history includes all commands that have been entered by the user since the shell was invoked. For example, if the user entered the history command and saw as output:
0 pwd
1 ls -l
2 cat Prog.java
the history would list pwd as the first command entered, ls -l as the second command, and so on.
Modify your shell program so that commands are entered into a history. (Hint: The java.util.ArrayList provides a useful data structure for storing these commands.)
Your program must allow users to rerun commands from their history by supporting the following three techniques:
1. When the user enters the command history, you will print out the contents of the history of commands that have been entered into the shell, along with the command numbers.
2. When the user enters !!, run the previous command in the history. If there is no previous command, output an appropriate error message.
3. When the user enters !< integer value i>, run the ith command in the history. For example, entering !4 would run the fourth command in the command history. Make sure you perform proper error checking to ensure that the integer value is a valid number in the command history.
1. The driver class is the class SimpleShell as shown in Figure 1 and it should only contain only one method, main.
2. This assignment used UNIX commands in its description. You might assume your program would only be able to execute in Linux, UNIX, or MacOS. But, your program can also run in Microsoft Windows. If you test your program in Linux, UNIX, or MacOS then type UNIX commands at the java shell (jsh>) prompt. If you test your program in Microsoft Windows then type DOS prompt commands at the java shell (jsh>) prompt.
Regrettably, for the external process to execute DOS prompt commands, if your program executes within Microsoft Windows, then your program always has to make the first two elements of the list passed to the ProcessBuilder constructor the following strings: "cmd.exe" and "/c". For example, if the user enters the command
jsh> type Prog.java
the parameters are the strings "type" and "Prog.java". The list object passed to the ProcessBuilder constructor has its first element the string "cmd.exe", its second element the string "/c", its third element the string "type", its fourth element the string "Prog.java".
Luckily, the above situation is not necessary if your program runs within Linux, UNIX, or MacOS. But, you must configure your program to work whether it executes within Linux, UNIX, MacOS or Microsoft Windows. Therefore, your program has to detect the operating system it is running within. Actually, it is easy to determine through the use of the following line of code:
String OSName = System.getProperty("os.name");
If the string OSName contains Windows then your program is running within Microsoft Windows; otherwise, it is running within Linux, UNIX, or MacOS.
3. Tip: Make your program as modular as possible, not placing all your code in one .java file. You can create as many classes as you need in addition to the class described above. Methods should be reasonably small following the guidance that "A function should do one thing, and do it well." You will lose a lot of points for code readability if you dont make your program as modular as possible. But, do not go overboard on creating classes and methods. Your common sense should guide your creation of classes and methods.
4. Do NOT use your own packages in your program. If you see the keyword package on the top line of any of your .java files then you created a package. Create every .java file in the src folder of your Eclipse project
5. Do NOT use any graphical user interface code in your program!
When a Java program begins execution, an instance of the Java virtual machine is created. On most systems, the JVM appears as an ordinary application running as a separate process on the host operating system. Each instance of the JVM provides support for multiple threads of control; but Java does not support a process model, which would allow the JVM to create several processes within the same virtual machine. Although there is considerable ongoing research in this area, the primary reason why Java currently does no support a process model is that it is difficult to isolate one processs memory from that of another within the same virtual machine.
It is possible to create a process external to the JVM , however, by using the ProcessBuilder class, which allows a Java program to specify a process that is native to the operating system (such as /usr/bin/ls or C:\WINDOWS\system32\mspaint.exe). This is illustrated in Figure 2 on the next page. Running this program involves passing the name of the program that is to run as an external process on the command line.
We create the new process by invoking the start() method of the ProcessBuilder class, which returns an instance of a Process object. This process will run external to the virtual machine and cannot affect the virtual machine and vice versa. Communication between the virtual machine and the external process occurs through the InputStream and OutputStream of the external process.
import java.io.*;
public class OSProcess
{
public static void main(String[] args) throws IOException
{
if (args.length != 1)
{
System.err.println("Usage: java OSProcess < command>");
System.exit(0);
}
// args[0] is the command that is run in a separate process
ProcessBuilder pb = new ProcessBuilder(args[0]);
Process process = pb.start();
// obtain the input stream
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
// read the output of the process
String line;
while ( (line = br.readLine()) != null)
System.out.println(line);
br.close();
}
}
Figure 2 Creating an external process using the Java API.