#!/bin/bash

read -p "Please enter your username: " username

if [[ -z "$username" ]]; then
    echo "You must enter a username"
    exit 1
fi

badChar=false

for ((i=0; i<${#username}; i++)); do
    char="${username:i:1}"
    if ! [[ "$char" =~ [A-Za-z] ]]; then
        badChar=true
        break
    fi
done

firstChar="${username:0:1}"
validFirstLetter=false

if [[ "$firstChar" =~ [A-Za-z] ]]; then
    validFirstLetter=true
fi

if ! $badChar && (( ${#username} <= 20 )) && $validFirstLetter; then
    echo "The username $username is valid"
else
    echo "The username is not valid"
fi