Get cmd command output from java code

Question

I have a program in which I was able to successfully execute cmd commands from my code, but I want to get output from cmd command. How can i do this?

So far my code is:

Second.java:

public class Second {
    public static void main(String[] args) {
        System.out.println("Hello world from Second.java");
    }
}

and Main.java

public class Main {
    public static void main(String[] args) {
        String filename = args[1].substring(0, args[1].length() - 5);
        String cmd1 = "javac " + args[1];
        String cmd2 = "java " + filename;

        Runtime r = Runtime.getRuntime();
        Process p = r.exec(cmd1); // i can verify this by being able to see Second.class and running it successfully
        p = r.exec(cmd2); // i need to see this output to see if 

        System.out.println("Done");
    }
}

I can verify that the first command works successfully by checking Second.class, but what if this class generated some error, how can I see this error?

Answer 1

You need the OutputStream (InputStream) of your process (and you must use the ProcessBuilder) ... so

public static void main(String[] args) {
  String filename = args[1].substring(0, args[1].length() - 5);
  String cmd1 = "javac " + args[1];
  String cmd2 = "java " + filename;

  try {
    // Use a ProcessBuilder
    ProcessBuilder pb = new ProcessBuilder(cmd1);

    Process p = pb.start();
    InputStream is = p.getInputStream();
    BufferedReader br = new BufferedReader(new InputStreamReader(is));
    String line = null;
    while ((line = br.readLine()) != null) {
      System.out.println(line);
    }
    int r = p.waitFor(); // Let the process finish.
    if (r == 0) { // No error
       // run cmd2.
    }
  } catch (Exception e) {
    e.printStackTrace();
  }
}

Answer 2

A common example to get a return from a command:

 Process p = null;
    try {
        p = p = r.exec(cmd2);
        p.getOutputStream().close(); // close stdin of child

        InputStream processStdOutput = p.getInputStream();
        Reader r = new InputStreamReader(processStdOutput);
        BufferedReader br = new BufferedReader(r);
        String line;
        while ((line = br.readLine()) != null) {
             //System.out.println(line); // the output is here
        }

        p.waitFor();
    }
    catch (InterruptedException e) {
            ... 
    }
    catch (IOException e){
            ...
    }
    finally{
        if (p != null)
            p.destroy();
    }

Answer 3

see here: Extracting process exit code in case of ThreadInterrupted

You need to get a return code ... you have to wait for it.