Time Machine cleaner - remove backups not listed in `tmutil listbackups`

tmclean.sh

#!/bin/bash
set -eo pipefail
DIR=$(tmutil machinedirectory)
backups=$(tmutil listbackups)
ls "$DIR" | while read subdir; do
    if echo $subdir | grep -q '[^0-9-]'; then continue; fi
    if echo $backups | grep -q $subdir; then continue; fi
    dir="$DIR/$subdir"
    if [ "$1" = -n ]; then
        echo "$dir"
    else
        tmutil delete "$dir"
    fi
done

To run this in cron, give Full Disk Access permission to /usr/sbin/cron in Security and Privacy.

This doesn't work with APFS backup introduced in Big Sur.