Shell Programming and Scripting

I want a shell script that accepts user input simultaneously when performing other tasks. Example: A shell script should echo some messages on the console and when the user presses some keys it should respond to that action.
say, when user presses the key
A - more information should be printed on the console
M - minimum information should be printed on the console
P - pause the script

How to achieve it ?

Code:

#!/usr/bin/bash

while true
do
  echo "im in infinite loop.."
done

#simultaneously below logic should be handled

  if [ key "A" is pressed ]
  then
      echo "hello you pressed A while infinite loop is running"
   fi

  if [ key "P" is pressed ]
  then
      echo "pausing the script"
   fi