#!/usr/bin/env bash

set -o errexit
set -o errtrace
set -o nounset

function trap_err()
{
    echo Exiting because error occurred in script.
}
trap trap_err ERR


HASH_TOOL="sha256sum"
INTERNEL=32
TARGET_DIR=""
FILE=""


function show_usage()
{
    echo "$0 usage:"
    echo "  --target-dir    Directory to store the backup files"
    echo "  --file          The file to monitor"
    echo "  --hash-tool     Tools to calculate hash of file, default sha256sum"
    echo "  --internel      Seconds between checking file, default 32s"
}

function my_log()
{
    echo "$(date --iso-8601=ns) -- $*"
}

function backup_the_file()
{
    local the_file=$1
    local target="${the_file}_$(date --iso-8601=seconds --reference $the_file)"
    my_log "backuping the file to $target"
    
    if [ -f "$TARGET_DIR/$target" ]
    then
        my_log "$target already exists, skipped."
    else
        cp --no-target-directory $the_file $TARGET_DIR/$target
    fi
}


function main()
{
    while [ "$#" -gt 1 ]
    do
        case "$1" in
            "--target-dir")
                TARGET_DIR=$2
                shift
                ;;
            "--file")
                FILE=$2
                shift
                ;;
            "--hash-tool")
                HASH_TOOL=$2
                shift
                ;;
            "--internel")
                INTERNEL=$2
                shift
                ;;
            *)
                echo unknown option: $1
                show_usage
                exit 1
                ;;
        esac
        shift
    done

    if [ -z "$FILE" -o -z "$TARGET_DIR" ]
    then
        echo "option(s) --file or --target-dir is unspecified."
        exit 1
    fi
    hash_prev=""
    while [ "true" ]
    do
        hash_curr=$(eval "$HASH_TOOL $FILE")
        if [ "$hash_prev" != "$hash_curr" ]
        then
            my_log "the hash now: $hash_curr"
            backup_the_file $FILE
            hash_prev=$hash_curr
        fi

        sleep "$INTERNEL"
    done
}

main "$@"