using_getopts.sh

#!/bin/bash

# A script that outputs each number from start to end
# -s 1 to start at one
# -e 20 to end at 20
# -b to count backwards
# final argument is prefix to all the numbers
# e.g. count -s 1 -e 20 -b NUM_

reverse=0
declare -i start=1
declare -i end=10

# You will get an error message if one of the options is missing its arguments, could redirect to /dev/null or use the : at the beginning to put getopts in silent mode
while getopts ":s:e:b" opt; do
  case $opt in
    b)
      reverse=1
      ;;
    s)
      [[ "${OPTARG}" =~ ^[0-9]+$ ]] || { echo "Invalid option ${OPTARG} given for -s" >&2; exit 1; }
      start="${OPTARG}"
      ;;
    e)
      [[ "${OPTARG}" =~ ^[0-9]+$ ]] || { echo "Invalid option ${OPTARG} given for -e" >&2; exit 1; }
      end="${OPTARG}"
    ;;
    :)
      # in silent mode, when an option is not given an argument the $opt variable is : and OPTARG is the option name (e.g. e)
      echo "Option -${OPTARG} is missing an argument"
      exit 1
    ;;
    
    # In non-silent mode if an option is missing arguments, the $opt variable will be ?
    # In silent mode, if we have an unknown argument then $opt will be ?
    \?)
      echo "Unknown option -${OPTARG}"
      exit 1
    ;;
  esac
done

echo You have chosen to start at $start and end at $end

echo "getopts chomped through all args up to $OPTIND"

echo "Shifting arguments"
shift $(( $OPTIND - 1 ))

echo "Now the first argument is $1"

[[ $1 ]] || { echo "No prefix given"; exit 1; }

if [[ $reverse -eq 0 ]]; then
  for (( i=$start; i<$(($end+1)); ++i )); do
    echo "${1}$i"
  done
else
  for (( i=$end; i>$(($start-1)); --i )); do
    echo "${1}$i"
  done
fi