How to connect to Linux using Java code?

[Anam Ghouri]

Hi,

I need to connect to Linux using java code from my Windows machine and execute a linux command. Following is the code I am using:

import java.io.*; 
import java.net.Socket; 

public class RunCommand { 
    public static void main(String args[]) { 
        String s = null; 
        try { 
        // run the Unix "grep" command 

            Socket s1=new Socket("ip address",port);   
             PrintWriter wr=new PrintWriter(new   
             OutputStreamWriter(s1.getOutputStream()),true);  
             wr.println("Hi Server...");  
             wr.flush();  

             BufferedReader br=new BufferedReader(new  InputStreamReader(s1.getInputStream()));   

             System.out.println(br.readLine());  

            Process p = Runtime.getRuntime().exec("grep info filepath"); 

             BufferedReader stdInput = new BufferedReader(new  
                 InputStreamReader(p.getInputStream())); 


            BufferedReader stdError = new BufferedReader(new  
                 InputStreamReader(p.getErrorStream())); 

            // read the output from the command 

            System.out.println("Here is the standard output of the command:\n"); 
            while ((s = stdInput.readLine()) != null) { 
                System.out.println(s); 
            } 

            // read any errors from the attempted command 

            System.out.println("Here is the standard error of the command (if any):\n"); 
            while ((s = stdError.readLine()) != null) { 
                System.out.println(s); 
            }   

            System.exit(0); 

        } 

        catch (IOException e) { 
            System.out.println("exception happened - here's what I know: "); 
            e.printStackTrace(); 
            System.exit(-1); 
        } 

    } 

} 

I am getting following error while executing this code:

java.io.IOException: Cannot run program "grep": CreateProcess error=2, The system cannot find the file specified
at java.lang.ProcessBuilder.start(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at macys.RunCommand.main(RunCommand.java:45)
Caused by: java.io.IOException: CreateProcess error=2, The system cannot find the file specified
at java.lang.ProcessImpl.create(Native Method)
at java.lang.ProcessImpl.<init>(Unknown Source)
at java.lang.ProcessImpl.start(Unknown Source)
... 5 more
exception happened - here's what I know:

I think it is not executing linux command because it is still in Windows operating system. When I include the following code in my program, i can see that is still in Windows.

 String os = System.getProperty("os.name");
         
              if ( os != null && os.startsWith("Windows"))
              {
                 System.out.println ("I am windows");
              }
              else{
            	  System.out.println ("I am Linux");
                 
        
              }

Output: "I am windows"

I need to connect to Linux and execute a Linux command from Windows. How do I do it using Java code? Can somebody please help me out in this asap?

[Tim Moores]

Oh boy. With all due respect, I think you have quite limited understanding of network computing and remote access. Runtime.exec executes on the local machine; where else would it execute? Do you see anything in that method call that could be construed as telling it to do otherwise (if that were even possible, which it is not)?

If you want to execute commands on a remote machine, you need to open a Telnet or SSH session. Telnet has fallen out of favor since it's less secure than SSH, so the target machine may not support it; if it does, then you can use the Telnet client code that's part of the Apache Commons Net library.

If SSH is available, check out JSch; it's a Java client library for that.

Lastly, using readLine and println with sockets is bound to cause you grief. This is a good article on that issue: http://www.fenestrated.net/~macman/mirrors/Apple%20Technotes%20%28As%20of%202002%29/tn/tn1157.html

[Anam Ghouri]

Here is the code for someone who needs it:

import com.jcraft.jsch.*;
import java.io.*;

public class Exec{
  public static void main(String[] arg){
    try{
      JSch jsch=new JSch();  

      String host=null;
      if(arg.length>0){
        host=arg[0];
      }
      else{
        host="username@ipaddress"; // enter username and ipaddress for machine you need to connect
      }
      String user=host.substring(0, host.indexOf('@'));
      host=host.substring(host.indexOf('@')+1);

      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=  "grep 'INFO' filepath"; // enter any command you need to execute

      Channel channel=session.openChannel("exec");
      ((ChannelExec)channel).setCommand(command);

     
      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();
      session.disconnect();
    }
    catch(Exception e){
      System.out.println(e);
    }
  }

  public static class MyUserInfo implements UserInfo{
    public String getPassword(){ return passwd; }
    public boolean promptYesNo(String str){
    	str = "Yes";
		return true;}
  
    String passwd;

    public String getPassphrase(){ return null; }
    public boolean promptPassphrase(String message){ return true; }
    public boolean promptPassword(String message){
        passwd="password"; // enter the password for the machine you want to connect.
        return true;
    }
    public void showMessage(String message){
   
    }
 
    }
  }