如何设置免费的SSL证书？

Question

我有以下关于如何引导网站托管环境的说明

#!/bin/bash

apt update -y
apt upgrade ufw sshguard unattended-upgrades wget curl git zip unzip tree -y
ufw --force enable
ufw allow 22,25,80,443

apt install lamp-server^ -y
a2enmod http2 deflate expires

apt install phpmyadmin php-mbstring php-gettext -y

cat <<-EOF >> "$HOME"/.profile
    set -x
    complete -r
    export web_application_root="/var/www/html"
    export -f full_apache_restart
    full_apache_restart() {
        find "$web_application_root"/* -type d -exec chmod 755 {} \+
        find "$web_application_root"/* -type f -exec chmod 644 {} \+
        chown -R www-data:www-data "$web_application_root"/
        systemctl restart apache*
    }
EOF
source "$HOME"/.profile 2>/dev/null

# Set DB credentials with PHPMyAdmin
# Set Apache virtual host
# Set SSL certificate

如何免费或无偿地执行最后一条指令“设置 SSL 证书”，而无需付费？