Asking for userinput with bash

There is a fine line between hardcoded values and a messy configurable script. Especially with bash scripts there is no tendency to use a config file, mainly because the standalone nature of scripts is defeated by an additional file to copy around. To have some default values set at the beginning of the script, but asking for user input to potentially overwrite these values is a user-friendly way to handle this. And maybe it makes a script a bit more self-documenting, too.

This will continue as soon as the key is pressed:

asksure() {
  if [ -z "$1" ]
    then echo -n "Please select [Y]es or [N]o: (Y/N)? "
  else
    echo "${1} (Y/N)"
  fi

  while read -r -n 1 -s answer; do
    if [[ $answer = [YyNn] ]]; then
      [[ $answer = [Yy] ]] && retval=0
      [[ $answer = [Nn] ]] && retval=1
      break
    fi
  done
echo
return $retval 
}

if asksure "Continue?"; then
  echo "Continuing.";
else
  echo "Not continuing."
  exit
fi

This will continue only when enter is pressed:

  while true; do
      read -p "Do you wish to install this program? [yn] " yn
      case $yn in
          [Yy]* ) echo "yes entered"; break;;
          [Nn]* ) echo "exiting"; exit;;
          * ) echo "Please answer yes or no.";;
      esac
  done

This suggests an editable default:

VERSION=6
read -e -p "Enter/edit the subnet: " -i "10.16.${VERSION}.0" SUBNET

if [[ "$SUBNET" =~ ^((1?[0-9]{1,2}|2[0-4][0-9]|25[0-5])\.){3}(1?[0-9]{1,2}|2[0-4][0-9]|25[0-5])$ ]]; then
  echo "${SUBNET} success"
else
  echo "${SUBNET} fail"
fi