这一特性是用popen系统调用实现的，该系统调用允许一个进程打开到另一个进程的管道，将管道用于输出或输入。

This feature is implemented using the popen system call, which allows one process to open a pipe to another process, either for output or input. In this case we open a pipe to the command.

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为了写日志给一个管道，简单地用管道符|替换文件名，跟随可执行程序名字，这个程序应该在它的标准输入上接受日志项。

In order to write logs to a pipe, simply replace the filename with the pipe character "|", followed by the name of the executable which should accept log entries on its standard input.

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