Setting up or Fixing File Permissions

In Symfony 3.x, you needed to do some extra work to make sure that your cache directory was writable. But that is no longer true! Since Symfony 4, everything works automatically:

• In the dev environment, umask() is used in bin/console and public/index.php so that any created files are writable by everyone.
• In the prod environment (i.e. when APP_ENV is prod and APP_DEBUG is 0), as long as you run php bin/console cache:warmup, no cache files will need to be written to disk at runtime. The only exception is when using a filesystem-based cache, such as Doctrine's query result cache or Symfony's cache with a filesystem provider configured.
• In all environments, the log directory (var/log/ by default) must exist and be writable by your web server user and terminal user. One way this can be done is by using chmod -R 777 var/log/. Be aware that your logs are readable by any user on your production system.