Links

Tools

Web documents markup validation
https://validator.w3.org

SSL

Diagnose problems with SSL certificates.
Qualys SSL Labs
SSL Shopper

Mozilla SSL Configuration Generator
https://ssl-config.mozilla.org/

JSON

JSON tools on MDN

JSFiddle

My examples

Documentation

Linux

Linux
Advanced Bash-Scripting Guide

Scripting

GNU Awk user guide
sed, a stream editor

WooCommerce

WooCommerce Documentation
WooCommerce Action and Filter Hook Reference

PHP Coding Standards

Two scripts: phpcs and phpcbf
https://github.com/squizlabs/PHP_CodeSniffer

A set of rules for the Squizlabs PHP Codesniffer
https://github.com/WordPress/WordPress-Coding-Standards

Something else altogether
https://github.com/FriendsOfPHP/PHP-CS-Fixer

Commercial

https://wpsandbox.io/