Comment by yjftsjthsd-h

> It would be nice to have an agreed-upon protocol for progress reporting. For instance, imagine of we had file descriptor 3 as "stdprogress". Programs could dump specially formatted messages there which the parent job control shell could intercept and turn into a progress display (with multiple rows for pipeline elements and such).

Different thing, but I am reminded of the BSDs having SIGINFO.

> It's not bad; but fluff like this can't be in band with the output, or at least not by default.

FWIW, I would describe stderr as being out of band. IIRC that's how e.g. pv works; `<file.img pv | dd of=/dev/foo` sends data along stdout while giving status info to the user since stderr isn't being piped.