A script to identify which backups can be deleted based on a Towers of Hanoi exponential retention backup rotation policy.
Given a list of archive names ending in a timestamp,
prints a list of archive names to delete.
Archive names need to end in the format _%Y-%m-%d_%H-%M-%S.
Rather than recording a track and sequence number with each backup, this infers that information from the timestamp on each backup.
Multiple backups on the same day are considered to be a "single" backup -- that is, if day 2020-12-28 is to be retained, all backups for that day are retained.
The default configuration is to treat the expected minimum backup frequency as daily, and to keep two from each track.