Example 1 (Without arguments):

#!/bin/bash
while getopts ":a" opt; do
  case $opt in
    a)
      echo "-a was triggered" >&2
      ;;
    \?)
      echo "Invalid option" >&2
      exit 1
      ;;
  esac
done

Example 2 (With arguments):

#!/bin/bash
while getopts ":o:" opt; do
  case $opt in
    o)
      if [ "$OPTARG" = "true" ]; then
          echo "Option -o trigged and argument is true..."
      else
          echo "Option -o trigged but argument is not true"
      fi
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    : )
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done