Trim or prune your backup directories in $prefix_yyyymmdd[hhmm] format. See http://blog.sahal.info/post/110947605246/trimming-backups for more information.

trimbackups.sh

#!/bin/bash
# ./trimbackups.sh [prompt]
# desc: Use this for trimming backup directories stored as $prefix_yyyymmdd[hhmm] in $backup dir
# todo: implement getopts :x
#
# Copyright (c) 2015 Sahal Ansari github@sahal.info
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

backup="/full/path/to/backup/dir"
prefix="www_"
#$offset is the number of chars in string $prefix plus 2 for "./", which comes from find
# i use this later to determine  year, month, day, [hour, minute] quickly using bash parameter expansion
offset=$(( ${#prefix} + 2 ))

# save filenames to delete here
todelete="$backup/to-delete"

# overwrite todelete each time this is run for [insert reason here]
echo > "$todelete"

#tmp array to store items to delete (will be written to file $todelete later)
deletelist=()

# bash supports associative arrays
# this one converts month number to month name (in Mmm format)
# https://stackoverflow.com/questions/1494178/how-to-define-hash-tables-in-bash
declare -A months
months=( ["01"]="Jan" ["02"]="Feb" ["03"]="Mar" ["04"]="Apr" ["05"]="May" ["06"]="Jun" ["07"]="Jul" ["08"]="Aug" ["09"]="Sep" ["10"]="Oct" ["11"]="Nov" ["12"]="Dec" )

# current month, day, year
cyear="$(date +%Y)"
cmonth="$(date +%m)"
cweek="$(date +%U)"
cday="$(date +%d)"

# finds dirs matching $prefix_yyyymmdd[hhmm] (e.g. www_201502130922, sf-slowfruit_20150213) in $backup dir
cd "$backup"
dirnames=( $(find . -regextype grep -regex ".*/$prefix[0-9]\{8,12\}" -type d -print | sort -r) )

# define functions
function ask_delete { # this is used by the prompt function below
    echo " -- delete?"
    select yn in "Yes" "No"; do
    case "$yn" in
        Yes ) echo "$1" >> "$todelete"; break;;
        No ) break;;
    esac
    done
}

function chknprint {

    #checks if the date is already listed in $todelete, if so skips it

    grep "${1:-}" "$todelete" > /dev/null 2>&1
    if [ "$?" -eq "0" ]; then
        continue
    fi

    # print the date in a pretty format
    # i've selected Mmm dd, yyyy [hh:mm] b/c 'MERICA
    year="${1:$offset:4}"
    month="${1:$offset+4:2}"
    day="${1:$offset+6:2}"
    week="$(date --date=$year$month$day +%U)"

    echo -ne "${1:-}"":  "
    echo -ne "${months["$month"]}"" ""$day"", ""$year"

    # check if there are 8 or 12 digits after prefix (hour+minute was set)
    date_length=$(( ${#1} - $offset ))
    if [ "$date_length" = "12" ]; then
        hour="${1:$offset+8:2}"
        minute="${1:$offset+10:2}"
        echo -ne " ""$hour"":""$minute"
    fi

}

# print a list of directories in human readable format
# also check for multipe entries per day (if current month) or multiple entries per month (if otherwise)
# todo: if date is 365 days in the past, set to delete
for (( i=0; i<"${#dirnames[@]:-}"; i++))
do

    chknprint "${dirnames[i]:-}"

    daysago="$((( $(date +%s) - $(date -d $year$month$day +%s) ) / 86400 ))"
    if [ "$daysago" -gt "365" ]; then
        echo " x"
        deletelist+=("${dirnames[i]:-}")
        continue
    fi

    if [ "$i" -ne "0" ]; then

        lyear="${dirnames[i-1]:$offset:4}"
        lmonth="${dirnames[i-1]:$offset+4:2}"
        lday="${dirnames[i-1]:$offset+6:2}"
        lweek="$(date --date=$lyear$lmonth$lday +%U)"

        # if month is equal to the month before it, then it's interesting
        if [ "$lmonth" -eq "$month" ]; then

            # if month is equal to the current month
            if [ "$month" -eq "$cmonth" ]; then

                # if week is equal to the week before it, then it's interesting
                if [ "$lweek" -eq "$week" ]; then

                    # if week is equal to the current week
                    if [ "$cweek" -eq "$week" ]; then

                        if [ "$lday" -eq "$day" ]; then
                            echo " *"
                            deletelist+=("${dirnames[i]:-}")
                        else
                            echo
                        fi
                
                    else
                        echo " ."
                        deletelist+=("${dirnames[i]:-}")
                    fi
                else
                    echo #"weeks do not match."
                fi
            else
                echo " **"
                deletelist+=("${dirnames[i]:-}")
            fi

        else
            echo
        fi

    else
        echo
    fi
done

if [[ "$1" == "prompt" ]]; then

    # print hr
    echo "*************************************************************"

    for (( i=0; i<"${#dirnames[@]:-}"; i++))
    do

        chknprint "${dirnames[i]:-}"

        ask_delete "${dirnames[i]:-}"

    done

else

    # check each item in array $deletelist to see if it exists in file $todelete
    # if not, add it to the file
    for (( i=0; i<"${#deletelist[@]:-}"; i++))
    do

        grep "${deletelist[i]:-}" "$todelete" > /dev/null 2>&1
        if [ "$?" -eq "0" ]; then
            continue
        fi

        echo "${deletelist[i]:-}" >> "$todelete" 2>&1

    done

fi