stderr 是标准错误流，显示命令中的错误输出。
stderr is the standard error stream, which displays error output from commands.
让标准错误流写入到multilog进程的管道，这个管道已经连接到了标准输出流。
Makes the standard error stream write to the pipe to the multilog process that is already connected to the standard output stream.
有一点不是很重要但却很有帮助，初始打开每个应用程序时，都具有三个文件描述符，从0到2，分别表示标准输入、输出和错误流。
As a bit of helpful trivia, each application is initially opened with three file descriptors, 0 through 2, for the standard input, output, and error streams, respectively.
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