Question

I’m trying to run commands straight through the CMD on Windows (Terminal on Linux). I have the following code. It’s acting very strangely. First, when run the program does not print anything. Second, when debugged and stepped through, the program prints the default CMD text and input line. Finally, I can’t seem to write to the CMD so it’ll execute commands. Here’s my code. I’m guessing it may be a threading issue, but I’m not familiar with Runtime.

    Runtime r = Runtime.getRuntime();
    try {
        Process p = r.exec("cmd");
        InputStream iStream = p.getInputStream();
        BufferedReader sReader = new BufferedReader(new InputStreamReader(iStream));
        while(sReader.ready()) {
            System.out.print((char)sReader.read());
        }

        OutputStream oStream = p.getOutputStream();
        BufferedWriter sWriter = new BufferedWriter(new OutputStreamWriter(oStream));
        sWriter.write("mkdir test");
        sWriter.newLine();

        while(sReader.ready()) {
            System.out.print((char)sReader.read());
        }
    } catch(Exception e) {
        e.printStackTrace();
    }

And my output when debugged and stepped through, (no output displayed when its run)

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

D:\workspaces\Maven\Command Line>

Answer 1

There could be a couple of things going on. One is that arguments are not getting passed correctly the way you are executing the process. The other, like @zacheusz mentioned, is that output may be coming out from the error stream instead of the input stream.

ProcessBuilder helps with both these things, allowing easy construction of a list of commands and the ability to merge input and error stream. I recommend trying using it:

Process p = null;
try {
    List<String> cmd = new LinkedList<String>();
    cmd.add("executable");
    cmd.add("-arg1");
    cmd.add("value1");
    cmd.add("-arg2");
    ProcessBuilder pb = new ProcessBuilder(cmd);
    pb.redirectErrorStream(true);
    p = pb.start();
    BufferedReader input = new BufferedReader(new InputStreamReader(p.getInputStream()));
    while (input.ready()) {
        String line = input.readLine();
        System.out.println("From process: "+line);
    }
    input.close();
} catch (IOException e) {
    this.logMessage("IOException caught: "+e.getMessage());
    e.printStackTrace();
}