process.stdin
process.stdin 属性返回连接到 stdin (文件描述符 0) 的流。
它是 net.Socket（也就是 Duplex 流），除非文件描述符 0 指向文件，在这种情况下它是 Readable 流。
The process.stdin property returns a stream connected to
stdin (fd 0). It is a net.Socket (which is a Duplex
stream) unless fd 0 refers to a file, in which case it is
a Readable stream.
For details of how to read from stdin see readable.read().
As a Duplex stream, process.stdin can also be used in "old" mode that
is compatible with scripts written for Node.js prior to v0.10.
For more information see Stream compatibility.
In "old" streams mode the stdin stream is paused by default, so one
must call process.stdin.resume() to read from it. Note also that calling
process.stdin.resume() itself would switch stream to "old" mode.
