Here's my method using GDB from many years ago: https://gist.github.com/zaius/782263

Is there a way to read from present tty?

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

This prompted me to ask the crowd about a similar use-case of editing your command line as it's already running your command

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

vxworks 6.x:

(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
  ----------  ------------ -------- --- ---------- -------- -------- ------- -----
  ...

I assume roughly the same caveats would apply, though? Buffering might be set wrong (and have no mechanism to be updated because the program never checks again), etc.

Thanks, links saved.

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 :)