Process.fork{}

当block为空的时候，fork会返回2次结果，一次是在父进程中，返回子进程的pid，一次是在子进程中，返回nil。

Creates a subprocess. If a block is specified, that block is run in the subprocess, and the subprocess terminates with a status of zero. 

Otherwise, the fork call returns twice, once in the parent, returning the process ID of the child, and once in the child, returning nil. 

# Process.fork 会运行两次，第一次返回子进程pid，第二次返回nil
pid = Process.fork
puts "current pid :#{pid}"
if pid.nil? then
  puts "# In child"
  puts exec('ls ~')
else
  puts "# In Parent"
  puts "Sub Process ID:#{Process.wait(pid)}"
end

 Source: http://stackoverflow.com/questions/806267/ruby-how-to-fire-and-forget-a-subprocess

Process.wait()

当进程退出时会保留一些关于退出状态的信息。这些状态信息并不能清除，直到它被父进程使用Process.wait来使用。

Process.wait(pid)   Waits for a child process to exit, returns its process id, and sets $? to a Process::Statusobject containing information on that process.

Process.wait(-1)  Waits for any child process (the default if no pid is given).

Process.detach()

Process.detach(pid) to register disinterest in their status; otherwise, the operating system may accumulate zombie processes.