Question

I’m writing a Java program that needs to extract a 32 character key from the system’s /dev/urandom interface. For this, I’m using the following procedure:

public String generateKey() {
            try {
                    Runtime run = Runtime.getRuntime();
                    Process pr = run.exec("tr -cd '[:alnum:]' < /dev/urandom | fold -w30 | head -n1;echo;");
                    pr.waitFor();
                    BufferedReader buf = new BufferedReader(new InputStreamReader(pr.getInputStream()));
                    StringBuffer sb = new StringBuffer();
                    String line = "";
                    while ((line = buf.readLine())!= null ) {
                            sb.append(line);
                    }
                    return sb.toString();
            } catch (Exception e) {
                    e.printStackTrace();
                    return "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
            }
    }

(If it can’t do it then it will return a bunch of null characters)

For some reason it’s returning nothing (or throwing an exception and not printing the stack trace). I know this because I print it out to the console (nothing) and when I give it to my RC4 engine it throws a divide by zero error. Why is the command not working and how could I make it work?

Thank you for the help.

Edit: Not two minutes after I posted this I have deduced it’s not returning the null characters because I set it to return the word “lemons” and it did the same thing so it’s not throwing an exception.

Edit 2: As per a suggestion in the comments I tried to read right from the file with the same results but I think I may be doing this wrong…

File file = new File("/dev/urandom");
            FileReader freader = null;
            StringBuffer sb = new StringBuffer();
            int x = 0;
            try {
                    freader = new FileReader(file);
                    BufferedReader reader = new BufferedReader(freader);
                    for (int i=0;(i<32||(x=reader.read())==-1);i++) {
                            sb.append((char) x);
                    }
            return sb.toString();
            } catch(Exception e) {
                    e.printStackTrace();
                    return "a\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
            }

Edit 3: This may be helpful to those helping:
Using the above code and piping the output to od I found it’s just reading 0s. (Again, it’s not throwing an exception otherwise it wouldn’t be all zeros:)

# ./run.sh | od
0000000 000000 000000 000000 000000 000000 000000 000000 000000
*

Answer 1

As already indicated by user Miserable Variable, try to run your commands inside an actual Unix shell; and make sure the shell gets invoked as an interactive shell (sh -i -c 'cmd1 | cmd2').

Otherwise the UNIX piping mechanism seems to get blocked because tr keeps reading from /dev/urandom but for some reason refuses to write to its stdout.

Executed in Terminal.app though the following command runs without any hiccups:

sh -c 'LC_ALL=C tr -cd [:alnum:] < /dev/urandom | fold -w30 | head -n1; echo;'

Another approach would be to explicitly limit the number of bytes read from /dev/urandom (see code below).

/*

# cat GenerateKey.java
javac GenerateKey.java
java GenerateKey

# For more information see the "Java exec and Unix pipes" comment in:
# "Java exec - execute system processes with Java ProcessBuilder and Process (part 3)",
# http://alvinalexander.com/java/java-exec-processbuilder-process-3

*/

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

public class GenerateKey {
    public static void main(String[] args) throws Exception {
        Runtime run = Runtime.getRuntime();

        // does not work on Mac OS X 10.6.8
        //String[] cmd = { "/bin/sh", "-c", "LC_ALL=C tr -cd '[:alnum:]' < /dev/urandom | fold -w30 | head -n1; echo;" };  

        // works
        String[] cmd = { "/bin/sh", "-i", "-c", "LC_ALL=C tr -cd '[:alnum:]' < /dev/urandom | fold -w30 | head -n1; echo;" };  
        //String[] cmd = { "/bin/sh", "-c", "head -c 3000 < /dev/urandom | LC_ALL=C tr -cd '[:alnum:]' | head -c 30" }; 
        //String[] cmd = { "/bin/sh", "-c", "dd if=/dev/urandom bs=3000 count=1 2>/dev/null | LC_ALL=C tr -cd '[:alnum:]' | fold -w30 | head -n1" }; 
        Process pr = run.exec(cmd);

        pr.waitFor();
        BufferedReader buf = new BufferedReader(new InputStreamReader(pr.getInputStream()));
        String line;
        while ((line = buf.readLine())!= null ) {
            System.out.println(line);
        }
    }
}