Ruby进程（2） Process创建和回收 －－－ Process.fork, Process.wait和Process.detach
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Process.fork{}
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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.?
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# Process.fork 会运行两次，第一次返回子进程pid，第二次返回nilpid = Process.forkputs "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
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Process.wait()
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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).
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Process.detach()
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Process.detach(pid) to register disinterest in their status; otherwise, the operating system may accumulate zombie processes.
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