Prune list of backup files to be daily, weekly, monthly, and yearly depending on the backup age.

prune-backup.sh

#!/bin/sh
# usage: prune-backup [-f] files...
# By default the script prints files to be deleted. Use -f to actually delete.
# Assuming the file names have format of %Y-%m-%dT*

# exit on failure, disable glob
set -ef

filter_date() {
  format="$1"
  match="$2"
  op="${3:--eq}"

  while read file; do
    date="${file##*/}"
    date="${date%T*}"
    if [ $(date -d "$date" "+$format") "$op" "$match" ]; then
      echo "$file"
    fi
  done
}

if [ "$1" = "-f" ]; then
  dry=
else
  dry=1
fi

files="$(printf '%s\n' "$@")"
today=$(date -d $(date +%F) +%s)

last_week="$(expr "$today" - 604800)"
last_month="$(expr "$today" - 2592000)" # assume 30 day month
last_year="$(expr "$today" - 31536000)" # assume 365 day year

# finding files to prune by printing the ones to keep on top of file list then
# filter out the duplicates. The filters here are in parallel, in theory
# there's a probability of lines interrupting each other when the line length
# exceeds stdout buffer, but in practice file name sizes are restricted to be
# much smaller than stdout buffer size so it should be fine.
prune=$( (
  echo "$files" | filter_date %s "$last_week"  -ge;                        # in the past week
  echo "$files" | filter_date %s "$last_month" -ge | filter_date %w 1 -eq; # in the pat month and every monday
  echo "$files" | filter_date %s "$last_year"  -ge | filter_date %d 1 -eq; # in the pat montyear and first of every month
  echo "$files" | filter_date %j 1 -eq;                                    # first of every year
  echo "$files"
) | sort | uniq -u)

IFS='
'
if [ "$dry" ]; then
  printf '%s\n' $prune
else
  rm -fv $prune
fi