Shell Programming and Scripting

#!/bin/bash

#Just trying to check if letters are in the user input. Any tips?
# I have tried regexp and using 0-9 etc, i cannot get this to work either in just an if statement or while in a loop.

echo "Please pick a number"
read num
if [ -z $num ]; then
	echo "Please enter a number"
	read num
elif [[ $num != [[:digit:]] ]]; then
	echo "That is not a number, try again!"
	read num
# I have tried [0-9] etc and does not work.

elif	[[ $num = [[:digit:]] ]]; then
	echo $num
	echo "This is a number"
fi

# i understand this would need to be in a loop to continuously check until a number is chosen
# as of now it is always returning "That is not a number, try again!"
# I have an entire guessing working until i put anything that tries to verify if user input has letters.  If letters are present, i will be prompting for them to enter numbers within the loop.

#!/bin/bash
echo "Please pick number"
read number

if [ -z $number ]; then
echo "Please input a number"
read number

elif [[ ! $number == [0-9] ]]; then
echo "Not a number, Try again!"
read number

elif
[[ $number == [0-9]; then
echo $number
echo "That is a number!"
fi

# I can only check the first digit, how can i check the entire $number variable for any letters?