process.stdout
process.stdout 属性会返回连接到 stdout (文件描述符 1) 的流。
它是一个 net.Socket（也就是 Duplex 流），除非文件描述符 1 指向文件（在这种情况下它是一个 Writable 流）。
例如，拷贝 process.stdin 到 process.stdout：
process.stdin.pipe(process.stdout);process.stdout 与其他的 Node.js 流有重大区别。
详见进程 I/O 的注意事项。
The process.stdout property returns a stream connected to
stdout (fd 1). It is a net.Socket (which is a Duplex
stream) unless fd 1 refers to a file, in which case it is
a Writable stream.
For example, to copy process.stdin to process.stdout:
process.stdin.pipe(process.stdout);process.stdout differs from other Node.js streams in important ways. See
note on process I/O for more information.