Question

I am trying to run command line commands from Java and a quick sanity check made me realize that the reason I am having trouble is that I can’t get the pr.waitFor() call below to work. This programs ends in less than 30s and prints nothing after “foo:”. I expected it to take just over 30 s and print a random number after “foo:”. What am I doing wrong?

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Sample {

    public static void main(String[] args) throws Exception {
        Runtime rt = Runtime.getRuntime();
        Process pr = rt.exec("sleep 32; echo $RANDOM");
        pr.waitFor();

        BufferedReader input = new BufferedReader(
                                   new InputStreamReader(pr.getInputStream()));
        String line=null;

        StringBuilder s = new StringBuilder();
        while((line=input.readLine()) != null) {
            System.out.println(s);
            s.append(line);
        }
        System.out.println("foo: " + s.toString());
    }
}

Answer 1

You should try this:

Process pr = rt.exec(new String[]{ "bash","-c", "sleep 32; echo $RANDOM" });

This launches a shell process and then within the shell runs your commands. The shell doesn’t return till AFTER the commands have completed.

Let me know if that works for you.