How to copy input/output streams of the Process to their System counterparts?

Question

This is a follow up to this question. The answer suggested there is

to copy the Process out, err, and input streams to the System versions

Answer 1

Process.getOutputStream() returns a BufferedOutputStream, so if you want your input to actually get to the subprocess you have to call flush() after every write().

You can also rewrite your example to do everything on one thread (although it uses polling to read both System.in and the process' stdout at the same time):

import java.io.*;

public class TestProcessIO {

  public static boolean isAlive(Process p) {
    try {
      p.exitValue();
      return false;
    }
    catch (IllegalThreadStateException e) {
      return true;
    }
  }

  public static void main(String[] args) throws IOException {
    ProcessBuilder builder = new ProcessBuilder("bash", "-i");
    builder.redirectErrorStream(true); // so we can ignore the error stream
    Process process = builder.start();
    InputStream out = process.getInputStream();
    OutputStream in = process.getOutputStream();

    byte[] buffer = new byte[4000];
    while (isAlive(process)) {
      int no = out.available();
      if (no > 0) {
        int n = out.read(buffer, 0, Math.min(no, buffer.length));
        System.out.println(new String(buffer, 0, n));
      }

      int ni = System.in.available();
      if (ni > 0) {
        int n = System.in.read(buffer, 0, Math.min(ni, buffer.length));
        in.write(buffer, 0, n);
        in.flush();
      }

      try {
        Thread.sleep(10);
      }
      catch (InterruptedException e) {
      }
    }

    System.out.println(process.exitValue());
  }
}