macOS Java Terminal Help!

[Awesomeness]

Okay, I have officially put myself to the test. I am making a TERMINAL EMULATOR IN JAVA! I've made the GUI, the mac menu, actionlisteners... everything... it's just the terminal unix part isn't goin' so well.

Here's my code for the command runner so far:

package programming;

import java.io.*;

public class Unix {

    public static String command(String cmd) {

        String s = null;
        String text = "";

        try {

	    // run the Unix "ps -ef" command

            Process p = Runtime.getRuntime().exec(cmd);

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

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

            // read the output from the command

            while ((s = stdInput.readLine()) != null) {
                text = text + s;
            }

            // read any errors from the attempted command

            while ((s = stdError.readLine()) != null) {
                text = text + s;
            }

        }
        catch (Exception e) {

        }
        return text;
    }
}

It works when I do stuff like ls, echo, and cd, but for unknown commands it doesn't say anything, other stuff it messes up, etc. For example, when I type in python, it freezes up.
PS: My dad fixed the computer somehow. If you're wondering what I'm talking about, look at my recent threads.

Anyways, how can I improve this code? I want to make it so that I can give it any command and it will return what it would spit back out at you in a real terminal. I want it to be fully functional!

Note, to see my program, change the extension of the .zip to a .jar, and open it. You can see how the terminal behaves.
EDIT:
Nooo! the jar didn't wooork!!! I'll upload a new one soon.

Ok, got the new jar. It works. I didn't set the main() in the old one.

AHHH! It auto-decompresses it! Darn... I'll try again...

OK!!! I MADE IT A .TXT! IT... WILL... WORK!!!