Random执行本地命令报异常

public class RuntimeExecDemo {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Runtime rt=Runtime.getRuntime();
        try{
            Process pr=rt.exec("help");
            InputStream is=pr.getInputStream();
            BufferedReader br=new BufferedReader(new InputStreamReader(is));
            String str=null;
            while((str=br.readLine())!=null) {
                System.out.println(str);
            }
        }catch(IOException e) {
            e.printStackTrace();
        }        
    }
}

代码跟书上的例子完全一样，执行结果出异常

java.io.IOException: Cannot run program "help": CreateProcess error=2, 系统找不到指定的文件。
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:1048)
    at java.lang.Runtime.exec(Runtime.java:593)
    at java.lang.Runtime.exec(Runtime.java:423)
    at java.lang.Runtime.exec(Runtime.java:320)
    at RuntimeExecDemo.main(RuntimeExecDemo.java:15)
Caused by: java.io.IOException: CreateProcess error=2, 系统找不到指定的文件。
    at java.lang.ProcessImpl.create(Native Method)
    at java.lang.ProcessImpl.(ProcessImpl.java:458)
    at java.lang.ProcessImpl.start(ProcessImpl.java:139)
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:1029)
    ... 4 more

执行的是这个命令，你用的是window还是mac linux?