Runtime.exec hangs even If I drain output.

Question

Hi Everyone,
I'm trying to make a program that would give me access to a command line on a remote server. So what I'm trying to do is use Runtime.exec("cmd") to open a command line, then reading from it and printing to it normally.
The problem is the same very common problem anyone using exec encounters: it hangs. The thing is, however, I do drain the input and error streams immediately, but it still hangs, and here's a curious thing:
When I use a BufferedReader and use its readLine() method, it only manages to read the first two lines before it hangs, giving me an output such as this:
+Microsoft Windows XP [Version 5.1.2600]+
+(C) Copyright 1985-2001 Microsoft Corp.+
Of course I placed the System.out.println inside the loop that reads lines, cause if I put it outside the loop, I don't get any output at all because the subprocess hangs before the loop is finished.
Now if I don't use a buffer, and use the input's stream's read() method, it reads all the way through the output, but hangs at the end where it's supposed to read -1, and never reads it.
I thought maybe I could bypass that, and break the loop at the last character in the output (the character '>'). Now while this works the first time, sending the output to the client like this:
+Microsoft Windows XP [Version 5.1.2600]+
+(C) Copyright 1985-2001 Microsoft Corp.+
F:\eclipse workspace\RemCommand>
However it doesn't work after that, when I try to type any command. I send my command to the subprocess normally using a PrintWriter, and then try to read the output, but that fails because apparently the subprocess is still stuck at the -1 from the previous read operation.
I really don't know what the problem is, is it possible that the read possibly doesn't return -1 at all, maybe returns a different character to signify the end or something, and my loop keeps trying to read or what?
I'm at my wits end.
Any help?
Thanks in advance.

Answer 1

Here is the code again. Check the somehow improved detection of the default Windows prompt.
Check the lines in comments that allow you to define your own, perhaps more reliable prompt.
import java.io.*;
import java.util.*;
public class Cmd
  public static void main( String[] args ) throws Exception
    ProcessBuilder pb = new ProcessBuilder( "cmd" );
//    Map<String, String> env = pb.environment();
//    env.put( "PROMPT", ".,.,." );
    pb.redirectErrorStream( true );
    Process p = pb.start();
    InputStream is = p.getInputStream();
    OutputStream os = p.getOutputStream();
    PrintWriter pw = new PrintWriter( os, true );
    readToPrompt( is );
    pw.println( "dir" );
    readToPrompt( is );
    pw.println( "ipconfig" );
    readToPrompt( is );
    pw.println( "netstat" );
    readToPrompt( is );
  private static void readToPrompt( InputStream is ) throws IOException
    String s = "";
    for (;;)
      int i = is.read();
      if ( i < 0 )
        System.out.println();
        System.out.println( "EOF" );
        System.exit( 0 );
      s += new String( new byte[] { ( byte )i } ).charAt( 0 );
      if ( s.endsWith( "\r\n" ) )
        System.out.print( s );
        s = "";
//      if ( s.equals( ".,.,." ) )
      if ( s.length() > 2 && s.charAt( 0 ) >= 'A' && s.charAt( 0 ) <= 'Z' && s.charAt( 1 ) == ':' && s.endsWith( ">" ) )
        System.out.print( s );
        break;
}Edited by: baftos on Sep 10, 2008 11:27 AM