Then there is this method, but I guess that article refers to no redirection output.

If in background or via cron, I always redirect. But this is for UN*X type systems with a tail that supports '-f'

$ prog > /tmp/log.txt 2>&1 &

Then

$ tail -f /tmp/log.txt

Just so happens, I actually used this the other day for a long running process on OpenBSD :)

In practice: I boot into tty and manually start the graphical session (Wayland/Sway). I occasionally get (non-Sway) warnings when I return to tty (eg close the window manager). But the output is always scuffed, so I can't read the whole log. The lines get printed on top of each other or something.

Is there a way to read everything from tty, from within the tty?

Neither of the methods below work, because the warnings/errors aren't produced by Sway itself, but some other OS module/component.

$ sway |& tee /tmp/sway.log

$ tail -f /tmp/sway.log

https://news.ycombinator.com/item?id=46234678

(one login session, say over serial)

  -> ioTaskStdGet 0, 1
  value = 3 = 0x3
  -> taskIdSelf
  value = 13600784 = 0xcf8810

  -> ioTaskStdSet 0xcf8810, 1, 0x9
  value = 0 = 0x0

  -> printf "foo\n"
  -> taskIdSelf
  -> i

  -> foo
  value = 4 = 0x4
  value = 13600784 = 0xcf8810


     NAME         ENTRY       TID    PRI   STATUS      PC       SP     ERRNO  DELAY
  ----------  ------------ -------- --- ---------- -------- -------- ------- -----
  ...

Nor should he, IMO. Hacky workarounds are almost always a terrible idea that will bite you in the ass someday.