Bash snippet for a confirmation prompt

confirm.sh

function confirm() {
  confirmed=
  read -p "$1 (y/n): " choice
  while [[ -z $confirmed ]]; do
    case "$choice" in
      y|Y)
        confirmed=true
        ;;
      n|N)
        confirmed=false
        ;;
      *)
        read -p "Invalid choice; answer y for yes or n for no (y/n): " choice
        ;;
    esac
  done

  echo "$confirmed"  
}

usage.sh

if [ $(confirm 'Are you sure?') = true ]; then
  echo 'OK, I'll do it!'
else
  echo 'OK, nevermind then.'
fi