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Tip of the Day
Language: Java
Expertise: Beginner
Mar 22, 2000

Running Another Program from Java

You can use Java to execute another program and display its output (via a BufferedReader). Here's the code:

import java.io.*;
import java.util.ArrayList;

public class ExecDemo
{
   static public String[] runCommand(String cmd) 
      throws IOException
   {  
      // The actual procedure for process execution:
      runCommand(String cmd);
      
      // Create a list for storing  output.
      ArrayList list = new ArrayList(); 

      // Execute a command and get its process handle
      Process proc = Runtime.getRuntime().exec(cmd); 

      // Get the handle for the processes InputStream
      InputStream istr = proc.getInputStream(); 

      // Create a BufferedReader and specify it reads 
      // from an input stream.
      BufferedReader br = new BufferedReader(
         new InputStreamReader(istr));
      String str; // Temporary String variable

      // Read to Temp Variable, Check for null then 
      // add to (ArrayList)list
      while ((str = br.readLine()) != null) list.add(str);
      
      // Wait for process to terminate and catch any Exceptions.
      try { proc.waitFor(); } 
      catch (InterruptedException e) {
        System.err.println("Process was interrupted"); }

      // Note: proc.exitValue() returns the exit value. 
      // (Use if required)

      br.close(); // Done.

      // Convert the list to a string and return
      return (String[])list.toArray(new String[0]); 
   }

   // Actual execution starts here
   public static void main(String args[]) throws IOException 
   {
      try
      {
         // Run and get the output.
         String outlist[] = runCommand(args[0]); 

         // Print the output to screen character by character.
         // Safe and not very inefficient.
         for (int i = 0; i < outlist.length; i++)
            System.out.println(outlist[i]);
      }
      catch (IOException e) { System.err.println(e); }
   }
}

The comments explain the code, but briefly:

String[] runCommand(String cmd) is used to run the program:

The code creates an InputStream and "hooks" it to the executed program's output stream. A BufferedReader then reads the output and saves it to a list of strings.

After all the output has been read, waitFor is called to wait on the program to terminate. The exitValue method may be called if desired to get the program's return value.

You can use this technique to execute programs on a server and display the output. In that case, one usually would have to convert this to an applet (for use with the web) and it would be better to replace the runCommand(args[0]); line with with an arbitrary command such as runCommand("ls -a -l"); or runCommand("dir");, (depending on the OS you're running under and your specific purpose) to prevent misuse. (Imagine runCommand("rm ."))

Of course the programs you run have to be in the search path (JDK 1.2.2), but a safer approach would be to use an arbitrary path and filename, for example:

runCommand("/usr/bin/diskspace");

Note that the programs that are being run may not be as portable between systems as this Java application itself.

Jeremy Chen
 
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