naive multi-processing backup script in BASH

backup.sh

#!/bin/bash
#
# Backup script that tries to utilize all available disc spindles and cores.
# For every target archive ("sink") create a file "sink.list"
#   which contains what to backup ("sources") linewise.
# (Prefer plzip to xz if you have the former at hand.)
#
# wrapper to: tar sort xargs xz
#
# run: ./backup.sh run *.list
#
# author: W-Mark Kubacki <wmark@hurrikane.de>

set -euo pipefail

: ${now:="$(date --iso-8601 --utc)"}
: ${spindles:=2}
: ${jobs:="$(nproc --ignore=1)"}

if (($# < 1)); then
  >&2 printf "argument expected, but not given\n"
  exit 2
fi

case "${1}" in
  compress)
    if (($# < 2)); then
      # xargs runs this even if there are no params
      exit 0
    fi
    if ! [[ -s "${2}" ]]; then
      >&2 printf "tarball not found: ${2}\n"
      exit 4
    fi

    exec xz "${2}"
    ;;
  tar)
    if (($# < 3)); then exit 0; fi
    : ${tarball="${2}/${3//.list}.tar"}

    if [[ -s "${tarball}.xz" ]]; then
      exit 0
    fi
    if [[ -s "${tarball}" ]]; then
      printf "${tarball}\n"
      exit 0
    fi

    tar -cf "${tarball}" "${3}" $(cat "${3}")
    printf "${tarball}\n"
    ;;
  do|run)
    mkdir -p "${now}/$(hostname)"
    shift

    for LISTFILE; do
      printf "${LISTFILE}\n"
    done \
    | sort -h \
    | xargs --max-args=1 --max-procs=${spindles} "${0}" tar "${now}/$(hostname)" \
    | xargs --max-args=1 --max-procs=${jobs} "${0}" compress
    ;;
  *)
    printf "${0} run [file...]\n"
    ;;
esac