Data to Out, chatter and prompts to ErrOut

byob-iostreams.3 iostreams

Problem: mytool list | wc -l breaks when mytool list interleaves "Loading items…" chatter into stdout. Users then reach for --quiet flags, which multiply.

Idea: reserve Out for the command's DATA — the thing a user might pipe into jq, awk, grep. Chatter (status messages, progress, prompts, warnings) goes to ErrOut. Scripting users get clean stdout by default; humans still see the progress because ErrOut isn't redirected by |.

Rule of thumb: if removing the print would change the meaning of piped output, it goes to Out. Otherwise ErrOut. Prompts ALWAYS go to ErrOut because prompts read from stdin, and stdin is usually the pipe.

Tradeoffs: authors must think about which stream a message belongs on. That thinking is exactly the value.

Design

// chatter -> ErrOut
fmt.Fprintln(io.ErrOut, "Loading items from", cfg.ServerURL)
spinner.Start(io.ErrOut)

// data -> Out
for _, item := range items {
    fmt.Fprintln(io.Out, item.Name)
}

// prompts -> ErrOut (and read from In)
fmt.Fprint(io.ErrOut, "Delete all? [y/N] ")