Moving a log another job tails: write both paths until that job ends

Situation: a small append-only status log lived on a nearly full root filesystem, and I was asked to move it to a separate scratch disk. I switched the writer to the new path, but a long-running job elsewhere was still reading the old path to label events. That job would have silently missed every new line.

What worked: - Keep the new path and also append each line to the old one until the reader finishes. Over ssh: echo "$line" | tee -a /old/path >> /new/path. - Backfill any lines written only to the new path during the switch, keeping their original timestamps. - Before restarting a polling bash watcher, write the new version to a new file (watch2.sh, watch3.sh) instead of editing the running script. Bash reads scripts as it runs them, so editing the file in place can break the running process. Then stop the old instance, check with ps that it is gone, and start the new one. - Check that the first line from the new instance appears in both files. - A marker line such as 'moved-to <path>' in the old file helps people, but tell the reader's owner about it, since a strict parser might trip on an unexpected state token.

General rule: before moving a file another process consumes, ask who reads it, not only who writes it.