I was asked to clarify the question. I'm not asking about any specific program's behavior, but I used ffmpeg as an example of the behavior I'm asking about. To restate the question:
When a program's stdout is piped to another program, how does that program also produce terminal output. Is stderr the only option in terms of output streams?
Original question:
I'm a long-time Windows developer slowly learning my way around Linux to support various electronics and programming hobbies. I'm working with some headless Raspberry Pi's, so I only interact with them through an ssh terminal. I had thought terminal output was just stdout, and indeed when I control a child process launched from a .NET Core program via the Process class, the terminal output is what my program receives when it intercepts the stdout stream.
But I need to launch bash so that I can pipe ffmpeg to VLC, and I realized when I do this from the terminal "by hand", ffmpeg writes processing details to the terminal while simultaneously piping data to VLC. I had thought a command-line pipe redirects stdout to another program's stdin.
I was having trouble with bash (it doesn't seem to pass stdin data from my program to the programs launched with the bash -c "ffmpeg ... | cvlc ..." switch), so I was considering using two Process instances and handling the pipe that way.
Then it occurred to me to wonder about the relationship of terminal output versus pipe output, and what's really going on behind the scenes.
Edit: When I wrote that, I forgot that I typically capture and output both stdout and stderr using the Process class. Under Windows, stderr is rarely used in my experience, is it perhaps routine in Unix to use stderr for non-error terminal output? Just a guess...
There are two different approaches for a program to send output that is separate from its standard output.
One is to output to standard error, and as you suspect, this might be more common on Unix-style environments than on Windows; see Do progress reports/logging information belong on stderr or stdout? for some discussion of this. Standard error can be redirected; see What are the shell's control and redirection operators?
The other is to output directly to the terminal the program is running on, if any, by using /dev/tty. See How does `less` take data from stdin while still be able to read commands from user? for a discussion of this (on input, but similar aspects apply to output).
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