#!/usr/bin/env bash
#
# Archive all files in source directory into destination directory.

set -euo pipefail

# Parse command-line arguments.
unset SRCDIR DESTDIR BASENAME

while getopts 's:d:n:' arg; do
  case "${arg}" in
    s) SRCDIR="${OPTARG}" ;;
    d) DESTDIR="${OPTARG}" ;;
    n) BASENAME="${OPTARG}-" ;;
    *) echo "Unknown argument '${arg}' given. Exiting." >&2 && exit 1 ;;
  esac
done

# Check and return true if source files are newer than any archive already created.
function archive-should-update() {
    local latest="${DESTDIR}/${BASENAME}latest.tar.gz"

    if ! test -e "${latest}"; then
        return 0
    elif test "$(find "${SRCDIR}" -type f -newer "${latest}" -print -quit | wc -l)" -gt 0; then
        return 0
    fi

    return 1
}

# Create compressed archive for source path into destination path for pre-defined, date-based name.
function archive-create() {
    local name="${BASENAME}$(date +%w%H)"

    # Create uncompressed archive for source path and compress separately to ensure minimal blocking.
    tar --verbose --create --file "${DESTDIR}/${name}.tar" --directory "${SRCDIR}" .
    gzip --force "${DESTDIR}/${name}.tar"

    # Point the latest archive to a "special" name for future use.
    (cd /backup && ln --symbolic --force "${name}.tar.gz" "${BASENAME}latest.tar.gz")
}

# Entry-point for script. Create tar archive for source directory into destination directory, checking
# for any changes since last run, and ensuring files are archived correctly for subsequent restore.
function main() {
    if archive-should-update "${SRCDIR}" "${DESTDIR}"; then
        archive-create "${SRCDIR}" "${DESTDIR}"
    fi
}

main "$@"