Question

I’m trying to execute a new process and read from its input stream in Java. I have successfully used Runtime.getRuntime().exec(String) to start and receive input from several processes. However, when I try to use exec on some other processes, the input stream’s read method blocks, and it appears that there is no input. What might be causing the input stream to be empty for some of these processes? Specifically, I am wondering why bash.exe is not outputting anything.

I have written a JUnit test case to demonstrate this issue:

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import junit.framework.TestCase;

public class TestExec extends TestCase {

    public void testExec() throws IOException {
        List<InputPrinter> threads = new ArrayList<InputPrinter>();

        // Create a process for each of the commands and make sure that
        // it outputs at least one line to its input stream.
        threads.add(testExec("cmd"));
        threads.add(testExec("java"));
        threads.add(testExec("C:/cygwin/bin/vim-nox.exe"));

        // These bottom two fail, even though executing these
        // commands in cmd.exe results in immediate output
        threads.add(testExec("javac"));
        threads.add(testExec("C:/cygwin/bin/bash.exe"));

        // Give the threads a second to execute
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
            fail();
        }

        // Test that each command had input to read
        for(InputPrinter ip : threads) {
            assertTrue(ip.command + " has not read any input", ip.hasRead);
        }
    }

    // Starts a process for the given command and returns an
    // InputPrinter that can be used to check if the process
    // has had an input to read.
    public InputPrinter testExec(String command) throws IOException {
        Process proc = Runtime.getRuntime().exec(command);
        InputStream in = proc.getInputStream();

        InputPrinter ip = new InputPrinter(in, command);
        new Thread(ip).start();

        return ip;
    }

    // Simple Runnable to read from an InputStream. hasRead will be
    // true if at least one input has been read from the stream
    private class InputPrinter implements Runnable {
        InputStream in;
        String command;
        boolean hasRead;

        public InputPrinter(InputStream in, String command) {
            this.in = in;
            this.command = command;
            this.hasRead = false;
        }

        // Loop indefinitely while printing any received input
        public void run() {
            try {
                final byte[] b = new byte[1024];
                while (true) {
                    int n = in.read(b);
                    if (n > 0) {
                        System.out.print(new String(Arrays.copyOf(b, n)));
                        hasRead = true;
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
                fail();
            }
        }
    }

}

EDIT:

As far as I know, if a program isn’t using stdout or stderr, I shouldn’t see anything in the windows command prompt. What I am expecting to see when I start the bash process is “bash-3.2$,” the same thing I see when I open the command prompt and run “bash.exe”:

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\cygwin\bin>bash.exe
bash-3.2$

Answer 1

Regardless of Java, as far as I know you can pipe output (or input) from/to bash only when it is running as a script, not when it is running as an interactive shell (in which case you can only pass cmd parameters to it).

In other words, when you run bash from cmd as you mention in the comment, you see output, but it is contained in the bash process, it is not output that bash sends back to the parent cmd process.

Regarding the javac process, it is actually sending the output to the error stream. Try running from cmd javac 1>null and javac 2>null and you’ll see the difference.
Have you looked at the api here? You can try to use ProcessBuilder and redirect the error stream back to the primary input stream, it’ll be much easier to work with the processes this way.