Group files in a year each x months (default 4)

group-each.sh

#!/usr/bin/env bash
#
# Usage: group.sh target_directory 4
# Result:
#   target_directory/part1of3/...files from January till April (inclusive)
#   target_directory/part2of3/...files from May till August
#   target_directory/part3of3/...files from September till December
#

set -e

target="$(cd "$(dirname "$1")"; pwd)/$(basename "$1")"
each=${2:-'4'}
parts=$((12 / each))

# list files only, with the year
# example:
#   01 image.jpg
#   02 some_filename with space.jpg
files="$(ls -l --time-style=+%m $target | grep '^-' | tr -s ' ' | cut -d " " -f 6- | awk NF)"

while IFS= read -r line; do
    IFS=" " read month filename <<< "$line"
    # remove leading 0
    month=$(echo $month | sed 's/^0*//')

    part=$(((month + each - 1)/each))
    part="part${part}of$parts"
    dir="$target/$part"
    file="$target/$filename"

    printf "%s <- %s" $dir $file

    mkdir -p "$dir"
    mv "$file" "$dir"

    printf " $\n"
done <<< "$files"