Java program is not able to execute terminal command

Question

import java.io.*;
import java.io.IOException;
import java.util.Scanner;

public class AutoStart{
    public static void main(String[] args){
        while(true){
            Runtime r = Runtime.getRuntime();
            try{
                Process p = r.exec("ps -ef >> services.txt");
                try{
                    p.waitFor();
                } catch(InterruptedException e){
                    e.getStackTrace();
                }   

                Scanner txtscan = new Scanner(new File("services.txt"));
                int running = 0; //0 means not running and 1 means running
                while(txtscan.hasNextLine()){
                    String str = txtscan.nextLine();
                    if(str.indexOf("red5") != -1){
                        running = 1;
                    }
                }

                if(running == 0){
                    //red5 is not running so start it now
                    //code to start it goes here
                }

                //at the end remove services.txt file
                //code to remove that file goes here.


            } catch(IOException e){
                e.getStackTrace();
            }

            try {
                Thread.sleep(1000);                 //1000 milliseconds is one second.
            } catch(InterruptedException ex) {
                Thread.currentThread().interrupt();
            }

        }
    }
}

On line 10 I am trying to create a text file which contains list of all the running programs but my java program is not able to create it.

This program is not able to create services.txt file and I don't get any error at all so I am confused what's the problem. Can you help me figure out the problem? Thank you.

Answer 1

InuThe Process class will not throw an exception if your command returns a non-zero exit code (usually indicating failure). You have to dig into it yourself.

Here is a basic change to your code that will print the error and output stream (command line tools may print to either) to the console upon receiving a non-zero exit code from the process.

Hopefully this helps you figure it out:

import java.io.*;
import java.io.IOException;
import java.util.Scanner;

public class AutoStart{
    public static void main(String[] args){
        while(true){
            Runtime r = Runtime.getRuntime();
            try{
                Process p = r.exec("ps -ef >> services.txt");
                try{
                    p.waitFor();
                } catch(InterruptedException e){
                    e.getStackTrace();
                }

                if (p.exitValue() != 0){

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

                    System.out.println("Error Stream:");

                    String line;

                    while ((line = br.readLine()) != null){
                        System.out.println(line);
                    }

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

                    System.out.println("Output Stream:");

                    while ((line = br.readLine()) != null){
                        System.out.println(line);
                    }

                    System.exit(1);
                }

                Scanner txtscan = new Scanner(new File("services.txt"));
                int running = 0; //0 means not running and 1 means running
                while(txtscan.hasNextLine()){
                    String str = txtscan.nextLine();
                    if(str.indexOf("red5") != -1){
                        running = 1;
                    }
                }

                if(running == 0){
                    //red5 is not running so start it now
                    //code to start it goes here
                }

                //at the end remove services.txt file
                //code to remove that file goes here.


            } catch(IOException e){
                e.getStackTrace();
            }

            try {
                Thread.sleep(1000);                 //1000 milliseconds is one second.
            } catch(InterruptedException ex) {
                Thread.currentThread().interrupt();
            }

        }
    }
}

Answer 2

This calls a subprocess without relying on any shell mechanism, catching the resulting standard output.

public static void main( String[] args ) throws Exception {
  try { 
    ProcessBuilder pb = new ProcessBuilder( "/bin/ps", "-ef" );
    Process process = pb.start();
    InputStream is = process.getInputStream();
    Reader rdr = new InputStreamReader( is );
    LineNumberReader lnr = new LineNumberReader(rdr);
    String line;
    while( (line = lnr.readLine()) != null ){
      if( line.contains( "skype" ) ){
        System.out.println( "skype is running" );
      }
    }
    process.waitFor();
  } catch( Exception e ){

  } catch( Error e ){

}