Exercise 3: Which Session and Window Is This Process In? — Possible Solution
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Find it:

  $ pgrep -af "sleep 2000"
  61008 sleep 2000

Proof 1 — its ancestors:

  $ pstree -s -p 61008
  systemd(1)───SCREEN(61007)───sleep(61008)

The SCREEN(61007) ancestor matches session 61007.jobs in screen -ls.
(This shows the session, but not the window number.)

Proof 2 — its environment:

  $ tr '\0' '\n' < /proc/61008/environ | grep -E '^(STY|WINDOW)='
  STY=61007.jobs
  WINDOW=0

Screen set these variables when it started the program in window 0 of
session 61007.jobs.

WHY THIS WORKS AS AN ANSWER
------------------------------
It uses two independent sources — the process tree kept by the kernel
and the environment screen gave the process — and notes which one
gives the window number. As the chapter warns, the environment method
only works for processes that were started inside screen.