range

#!/bin/bash

set -euo 'pipefail'

usage() {
  local b=$(tput bold)
  local n=$(tput sgr0)

  echo "${b}Usage:${n} $(basename "$0") '[<lower>;]<upper>'"
  echo ""
  echo "${b}Description:${n}"
  echo "  Generates enumerated newlines from <lower> to <upper>"
  echo ""
  echo "${b}Arguments:${n}"
  echo ""
  echo "  <lower>     lower bound (integer), default to 0 if omitted"
  echo "  <upper>     upper bound (integer), mandatory"
  echo ""
  echo "${b}Examples:${n}"
  echo ""
  echo "  $(basename "$0") '10;20'"
  echo "  $(basename "$0") '300'"
}

main() {
  local range=${1:-null}

  if [[ "$range" == '-h' || "$range" == '--help' || "$range" == 'null' ]]; then
    usage
    exit 0
  fi
  if ! grep -E '([0-9]+;[0-9]+|^[0-9]+)' <<< "$range" > /dev/null; then
    >&2 echo "Error: invalid range!"
    usage
    exit 1
  fi
  local lower=$(grep -Eo '[0-9]+;' <<< "$range" | tr -d '\n')
  local upper=$(sed "s/${lower:-;}//g" <<< "$range" | tr -d '\n')

  lower=$(tr -d '\n;' <<< "$lower")
  lower=${lower:-0}

  for (( i = lower; i < upper; i++ )); do
    echo "$i"
  done

  exit 0
}

main "$@"