How to let a shell script do not proceed until "a" is input

#!/bin/bash
# Initialize the variable a
a=""
# Use a while loop to read input until a is set
while [ -z "$a" ]; do
    echo "Enter a value for 'a'."
    read a
done

# Print the value of a
echo "You entered: $a"

DO NOT use "read -p"!

• In bash: "-p" means add a prompt
• In zsh: "-p" means input from a coprocess

If you in a terminal uses zsh and source a shell script containing "read -p":

read:2: -p: no coprocess

In summary, just use "echo" before read a variable from the input.