#!/bin/bash

function backup() {
    touch "$1"
    org="$(md5sum $1)"
    while [ 1 == 1 ]; do
        new="$(md5sum $1)"
        if [ "$org" != "$new" ]; then
            cp "$1" ".${1}_backup"
            org="$new"
        fi
        sleep "$2"
    done
}

backup test.txt 1