Jsch - One Session Multiple Channels

Question

I managed to execute a single command through ssh with Jsch but when i try to execute a second command it fails

For debugging i brought this problem down to this lines:

import java.io.IOException;
import java.io.InputStream;

import com.jcraft.jsch.Channel;
import com.jcraft.jsch.ChannelExec;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;

public class Exec {

    public static void test(Session session) throws Exception {
        Channel channel = session.openChannel("exec");
        ((ChannelExec) channel).setCommand("pwd");

        channel.setInputStream(null);

        ((ChannelExec) channel).setErrStream(System.err);

        InputStream in = channel.getInputStream();

        channel.connect();

        byte[] tmp = new byte[1024];
        while (true) {
            while (in.available() > 0) {
                int i = in.read(tmp, 0, 1024);
                if (i < 0)
                    break;
                System.out.print(new String(tmp, 0, i));
            }
            if (channel.isClosed()) {
                System.out.println("exit-status: " + channel.getExitStatus());
                break;
            }
            try {
                Thread.sleep(1000);
            } catch (Exception ee) {
            }
        }
        channel.disconnect();
    }

    public static void main(String[] arg) {
        try {
            JSch jsch = new JSch();         
            Session session = jsch.getSession("nck", "127.0.0.1", 22);          
            session.setPassword("asd");         
            session.setConfig("StrictHostKeyChecking", "no");           
            session.connect();

            test(session); // This one succeeds with exit-status: 0         
            test(session); // This one fails with exit-status: 255

            session.disconnect();
        } catch (Exception e) {
            //
        }
    }
}

This is mostly the official Exec example but this gives me this output:

/home/nck
exit-status: 0
exit-status: 255

The first command is executed succesfully the second doesnt.

Any Ideas ?

Answer 1

Here is a complete working Java class based on @krishna's answer:

/**
 * Run several ssh commands in a single JSch session
 */
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import java.io.*;

class SshMultiCommands
{
   public static void main(String[] args) throws Exception
   {
      JSch jsch = new JSch();
      String user = "user";            //CHANGE ME
      String host = "192.168.222.157"; //CHANGE ME
      String passwd = "password";      //CHANGE ME
      int port = 22;    
      Session session = jsch.getSession(user, host, port);
      session.setPassword(passwd);

      session.setConfig("StrictHostKeyChecking", "no");

      session.connect();

      Channel channel = session.openChannel("shell");
      OutputStream ops = channel.getOutputStream();
      PrintStream ps = new PrintStream(ops, true);

      channel.connect();
      InputStream input = channel.getInputStream();

      //commands
      ps.println("ls -lag");
      ps.println("cd /etc");
      ps.println("ls");
      ps.println("exit");
      ps.close();

      printResult(input, channel);

      channel.disconnect();
      session.disconnect();
   }

   /**
    * @param input
    * @param channel
    */
   private static void printResult(InputStream input,
                                   Channel channel) throws Exception
   {
      int SIZE = 1024;
      byte[] tmp = new byte[SIZE];
      while (true)
      {
         while (input.available() > 0)
         {
            int i = input.read(tmp, 0, SIZE);
            if(i < 0)
               break;
             System.out.print(new String(tmp, 0, i));
         }
         if(channel.isClosed())
         {
            System.out.println("exit-status: " + channel.getExitStatus());
            break;
         }
         try
         {
            Thread.sleep(300);
         }
         catch (Exception ee)
         {
         }
      }
   }
}

Answer 2

Hey I have exactly the same problem with jsch and Ubuntu. (How) did you solve it? Making a new session for every execute eats too much time? At the moment I catch the jsch exception and search for "session isn't down" then I reconnect the session and execute command again. That works but is not a good solution.

Edit: I was on the wrong way I solved my problem for now by using

channel = session.openChannel("shell");

instead of "exec".