This script will handily sort millions of mail files out of a mail inbox into a series of directories organized by month

mail-sort.sh

#!/usr/bin/env bash

maildir="$1"
if [[ "$maildir" == "" ]]; then
    echo "Usage: mail-sort.sh <maildir> <dest>"
    exit -1
fi
if [[ ! -d "$maildir" ]]; then
    echo "Please make sure '$maildir' is a directory!"
    exit -1
fi

destdir="$2"
if [[ "$destdir" == "" ]]; then
    echo "Usage: mail-sort.sh <maildir> <dest>"
    exit -1
fi
if [[ ! -d "$destdir" ]]; then
    echo "Please make sure '$destdir' is a directory!"
    exit -1
fi


# loop over all files in the directory; since there could be millions of files, disable sort and stat for performance
for filename in $(ls -U1 $maildir); do
    # first part of name contains unix timestamp, so grab that and format into yy-mm format
    unixtime="$(echo "$filename" | cut -d. -f1)"
    filesort="$(date -d @$unixtime +%Y-%m)"
    
    # create directory here (or comment out and manually create them for minor performance boost)
    # mkdir -p "$destdir/$filesort"
    
    # relocate file into the proper sort directory
    mv "$maildir/$filename" "$destdir/$filesort/$filename"
    
    # print new file path
    echo "$destdir/$filesort/$filename"
done