Bash script with flags

script.sh

#!/bin/bash
# usage:
#   ./script.sh -a <value for a> -b <value for b>

while getopts a:b: opt
do
  case $opt in
    a) VAR_A=$opt;;
    b) VAR_B=$opt;;
  esac
done

: ${VAR_A:?missing flag -a}
: ${VAR_B:?missing flag -b}

echo $VAR_A $VAR_B