opening a shell and interacting with its I/O in java

Question

I am trying to open a shell (xterm) and interact with it (write commands and read the shell's output)

Here is a sample of code which won't work:

public static void main(String[] args) throws IOException {
    Process pr = new ProcessBuilder("xterm").start();
    PrintWriter pw = new PrintWriter(pr.getOutputStream());
    pw.println("ls");
    pw.flush();
    InputStreamReader in = new InputStreamReader(pr.getInputStream());
    System.out.println(in.read());
}

When I execute this program an "xterm" window opens and the "ls" command is not entered. Only when I close the window I get a "-1" printed and nothing is read from the shell

IMPORTANT-

I know I can just use:
Process pr = new ProcessBuilder("ls").start();

To get the output, but I need the "xterm" opened for other uses

Thanks a lot

Answer 1

Your problem is that the standard input and output of the xterm process don't correspond to the actual shell that is visible in the terminal window. Rather than an xterm you may have more success running a shell process directly:

Process pr = new ProcessBuilder("sh").start();

Answer 2

Here a full java main example of how to interact with shell on java 8 (is really simple do that on java 4,5,6 whatever)

Example of output

$ javac Main.java
$ java Main
echo "hi"
hi

The code

import java.io.*;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;


public class Main {

    public static void main(String[] args) throws IOException, InterruptedException {

        final List<String> commands = Arrays.asList("/bin/sh");
        final Process p = new ProcessBuilder(commands).start();

        // imprime erros
        new Thread(() -> {
            BufferedReader ir = new BufferedReader(new InputStreamReader(p.getErrorStream()));
            String line = null;
            try {
                while((line = ir.readLine()) != null){
                    System.out.printf(line);
                }
            } catch(IOException e) {}
        }).start();

        // imprime saida
        new Thread(() -> {
            BufferedReader ir = new BufferedReader(new InputStreamReader(p.getInputStream()));
            String line = null;
            try {
                while((line = ir.readLine()) != null){
                    System.out.printf("%s\n", line);
                }
            } catch(IOException e) {}
        }).start();

        // imprime saida
        new Thread(() -> {
            int exitCode = 0;
            try {
                exitCode = p.waitFor();
            } catch(InterruptedException e) {
                e.printStackTrace();
            }
            System.out.printf("Exited with code %d\n", exitCode);
        }).start();


        final Scanner sc = new Scanner(System.in);
        final BufferedWriter bf = new BufferedWriter(new OutputStreamWriter(p.getOutputStream()));
        final String newLine = System.getProperty("line.separator");
        while(true){
            String c = sc.nextLine();
            bf.write(c);
            bf.newLine();
            bf.flush();
        }

    }

}