Exercise 2: Finding and Rescuing a Job Started Outside Screen — Possible Solution
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Terminal A (not in screen):

  $ yes > /dev/null

Terminal B:

  $ pgrep -af "^yes"
  43301 yes

Prove it's not in screen:

  $ pstree -s -p 43301
  systemd(1)───...───bash(43250)───yes(43301)

  No SCREEN process appears among its ancestors.

Move it into screen:

  $ screen -S rescue
  $ reptyr 43301            # or: sudo reptyr 43301 if ptrace_scope is 1

(Terminal A's prompt usually returns once the process has moved.)

Prove it is now in screen:

  Ctrl-a d                  # detach
  $ pstree -s -p 43301
  systemd(1)───SCREEN(43320)───bash(43321)───...───yes(43301)

  The SCREEN(43320) ancestor matches the session in screen -ls.

Finally, stop it: screen -r rescue, then Ctrl-C (it now listens to
this window's keyboard), and exit.

WHY THIS WORKS AS AN ANSWER
------------------------------
It uses pstree -s as before-and-after evidence, rather than assuming
the move worked. `yes > /dev/null` is harmless but busy, so remember
to stop it. The exact tree shape reptyr produces can vary.