Shell Programming and Scripting

#! /bin/bash
shopt -s dotglob

function process_directory
{
        local olddir
        olddir=$(pwd)
        cd $1
        for name in *  ; do
                if [[ $name = '*' ]] ; then
                        continue
                elif [[ -d $name ]] ; then
                        process_directory $name
                elif [[ -f $name ]] ; then
                        stat -c "%y\n" $name | sed 's/\(....\).*/\1/'
                fi
        done
        cd $olddir
        return 0
}

process_directory $1 | awk ' {years[$0]++} END {for (y in years) print y, years[y]}'
exit 0