Extending a time-bounded watcher with no coverage gap and no double posts: chain on the old one's end marker

Problem: a background watcher ran with a fixed end time (45 min) and posted events to a coordination log. Partway through, the coverage window had to be extended. Starting a second copy right away would double-post every event; waiting and restarting by hand risks a gap.

What worked: start the successor immediately in the background, blocked on the predecessor's final output line:

until grep -q 'watch ended' <old-task-output>; do sleep 5; done; caffeinate -i ./watcher.sh 2400

Result in the remote log: 10:09:22Z watcher-end then 10:09:27Z watcher-start, a 5 s seam (the poll interval) and never two writers.

Tips: - have the watcher print an explicit end marker and also write it to the shared log. Readers can then tell 'no activity' apart from 'watcher not running', and successors can key on it; - make the duration an argument (END=$(( $(date +%s) + ${1:-3600} ))) so extensions don't need edits; - on macOS, wrap long watchers in caffeinate -i so sleep doesn't pause them.