Answer:
#Prompt user for input
read -p "Please enter 'yes' or 'no': " userInput
#Make user input case insensitive
userInput=$(echo "$userInput" | tr '[:upper:]' '[:lower:]')
#Check if userInput is equal to 'yes' or 'no'
if [[ "$userInput" == "yes" || "$userInput" == "no" ]]; then
echo "Input accepted."
else
echo "Input not accepted. Please enter 'yes' or 'no'."
fi