Invoking unix shell commands from java.

[Ashish Gupta]

Hi All,
I looked around a lot in this forum and on the web but could not exactly get an answer I was looking for. Maybe I did not searched much or in the right direction.

Here is what I want to achieve. From my java code (running on windows pc) I want to
1. connect to a linux box
2. invoke a command, say 'ls -l' to list all the files and directories
3. pass back the result (may be as an array, or a stream) back to my java program.

Could you please help me with this. Code snippets, especially for connecting to the linux box, will be very helpful.

Thanks
Ashish

[Ian Darwin]

The trick here is that you need some "protocol" that will allow you both to *connect" and
to *authenticate" to the UNIX system.

The most widely used implementation of such protocols used to be rsh and telnet, but these are generally regarded now as hopelessly insecure. The cookbook has examples of connecting to a Telnet server.

These protocols have largely been replaced by the SSH protocol (secure shell), see OpenSSH for a free, open-source implementation of ssh... in the C language.

For a Java implemenation, you want JSch. Download the code; it comes with examples.

Ian

[Ashish Gupta]

Hi Ian,
Thanks for the pointer to JSch. It was quite useful. I downloaded and tried the Exec.java example. In absence of any doc for JSch api, I am still strugling with 2 questions. Hopefully you or some one on this forum might have an answer.

1. When I execute the Exec.java, at session.connect(); I am prompted with a warning 'authenticity of host <host name> could not be established. RSA fingerprint .....'. Why is this warning coming? How can this be corrected.
2. Even after the unix command is executed, java program is still running. Could not figure out why this happens.
3. Output of the exec command is printed on standard output at the end. I want to capture this output in a text file or something similar.

Please help me in solving these issues.

Thanks
Ashish

[Layne Lund]

Hi Ian,
Thanks for the pointer to JSch. It was quite useful. I downloaded and tried the Exec.java example. In absence of any doc for JSch api, I am still strugling with 2 questions. Hopefully you or some one on this forum might have an answer.

1. When I execute the Exec.java, at session.connect(); I am prompted with a warning 'authenticity of host <host name> could not be established. RSA fingerprint .....'. Why is this warning coming? How can this be corrected.

From what I know, this doesn't have anything to do with Java but is a response from the SSH protocol when a host is not recognized. Every SSH tool I use issues a similar warning when I connect to a host for the first time. So the only way to "correct" this error message is clicking yes. The program should write some kind of config file that identifies hosts that are safe to use.

2. Even after the unix command is executed, java program is still running. Could not figure out why this happens.

I'm not sure about this. I would need to see the code in Exec.java.

3. Output of the exec command is printed on standard output at the end. I want to capture this output in a text file or something similar.

You can redirect the output to a file by doing something like this:

C:\> java Exec > output.txt

Please help me in solving these issues.

Thanks
Ashish[/QB]

[ October 16, 2004: Message edited by: Layne Lund ]

[Ashish Gupta]

Hi Layne

From what I know, this doesn't have anything to do with Java but is a response from the SSH protocol when a host is not recognized. Every SSH tool I use issues a similar warning when I connect to a host for the first time. So the only way to "correct" this error message is clicking yes. The program should write some kind of config file that identifies hosts that are safe to use.

I tried using the windows command prompt (using telnet) and got connected to linux host without any warning/error. I would hope to connect from java code as well without being prompted.

Exec.java code

  
import java.io.*;
 
public class Exec{
  public static void main(String[] arg){
 
    String xhost="127.0.0.1";
    int xport=0;
 
    try{
 
      JSch jsch=new JSch();  
String user = "username";
String host="dummy.hostname.com";
 
      Session session=jsch.getSession(user, host, 22);
 
      // username and password will be given via UserInfo interface.
      UserInfo ui=new MyUserInfo();
      session.setUserInfo(ui);
      session.connect();
      //String command=JOptionPane.showInputDialog("Enter command", "set|grep SSH");
  String command = "ls";
  
      Channel channel=session.openChannel("exec");
      ((ChannelExec)channel).setCommand(command);
      channel.setXForwarding(true);
      channel.setInputStream(System.in);
      channel.setOutputStream(System.out);
      //FileOutputStream fos=new FileOutputStream("/tmp/stderr");
      //((ChannelExec)channel).setErrStream(fos);
      ((ChannelExec)channel).setErrStream(System.err);
      channel.connect();
      /*
      byte[] tmp=new byte[1];
      InputStream in = channel.getInputStream();
      while(true){
if(channel.isEOF()) break;
while (in.available()>0){
  in.read(tmp, 0, 1);
  System.out.print(new String(tmp, 0, 1));
}
      }
      */
    }
    catch(Exception e){
      System.out.println(e);
    }
  }
 
  public static class MyUserInfo implements UserInfo{
//    public String getPassword(){ return passwd; }
public String getPassword(){ return "dummy"; }
 
    public boolean promptYesNo(String str){
      Object[] options={ "yes", "no" };
      int foo=JOptionPane.showOptionDialog(null, 
             str,
             "Warning", 
             JOptionPane.DEFAULT_OPTION, 
             JOptionPane.WARNING_MESSAGE,
             null, options, options[0]);
       return foo==0;
    }
  
    String passwd;
    JTextField passwordField=(JTextField)new JPasswordField(20);
 
    public String getPassphrase(){ return null; }
    public boolean promptPassphrase(String message){ return true; }
    
    public boolean promptPassword(String message){
      Object[] ob={passwordField}; 
      int result=
  JOptionPane.showConfirmDialog(null, ob, message,
JOptionPane.OK_CANCEL_OPTION);
      if(result==JOptionPane.OK_OPTION){
passwd=passwordField.getText();
return true;
      }
      else{ return false; }
    }
    
    public void showMessage(String message){
      JOptionPane.showMessageDialog(null, message);
    }
  }
}

[Ashish Gupta]

Hi Layne

From what I know, this doesn't have anything to do with Java but is a response from the SSH protocol when a host is not recognized. Every SSH tool I use issues a similar warning when I connect to a host for the first time. So the only way to "correct" this error message is clicking yes. The program should write some kind of config file that identifies hosts that are safe to use.

I tried using the windows command prompt (using telnet) and got connected to linux host without any warning/error. I would hope to connect from java code as well without being prompted.

Exec.java code

  
import java.io.*;
 
public class Exec{
  public static void main(String[] arg){
 
    String xhost="127.0.0.1";
    int xport=0;
 
    try{
 
      JSch jsch=new JSch();  
String user = "username";
String host="dummy.hostname.com";
 
      Session session=jsch.getSession(user, host, 22);
 
      // username and password will be given via UserInfo interface.
      UserInfo ui=new MyUserInfo();
      session.setUserInfo(ui);
      session.connect();
      //String command=JOptionPane.showInputDialog("Enter command", "set|grep SSH");
  String command = "ls";
  
      Channel channel=session.openChannel("exec");
      ((ChannelExec)channel).setCommand(command);
      channel.setXForwarding(true);
      channel.setInputStream(System.in);
      channel.setOutputStream(System.out);
      //FileOutputStream fos=new FileOutputStream("/tmp/stderr");
      //((ChannelExec)channel).setErrStream(fos);
      ((ChannelExec)channel).setErrStream(System.err);
      channel.connect();
      /*
      byte[] tmp=new byte[1];
      InputStream in = channel.getInputStream();
      while(true){
if(channel.isEOF()) break;
while (in.available()>0){
  in.read(tmp, 0, 1);
  System.out.print(new String(tmp, 0, 1));
}
      }
      */
    }
    catch(Exception e){
      System.out.println(e);
    }
  }
 
  public static class MyUserInfo implements UserInfo{
//    public String getPassword(){ return passwd; }
public String getPassword(){ return "dummy"; }
 
    public boolean promptYesNo(String str){
      Object[] options={ "yes", "no" };
      int foo=JOptionPane.showOptionDialog(null, 
             str,
             "Warning", 
             JOptionPane.DEFAULT_OPTION, 
             JOptionPane.WARNING_MESSAGE,
             null, options, options[0]);
       return foo==0;
    }
  
    String passwd;
    JTextField passwordField=(JTextField)new JPasswordField(20);
 
    public String getPassphrase(){ return null; }
    public boolean promptPassphrase(String message){ return true; }
    
    public boolean promptPassword(String message){
      Object[] ob={passwordField}; 
      int result=
  JOptionPane.showConfirmDialog(null, ob, message,
JOptionPane.OK_CANCEL_OPTION);
      if(result==JOptionPane.OK_OPTION){
passwd=passwordField.getText();
return true;
      }
      else{ return false; }
    }
    
    public void showMessage(String message){
      JOptionPane.showMessageDialog(null, message);
    }
  }
}

Thanks for any helpful suggestions that can be made.

Thanks
Ashish