[DOCS] Scripting walkthrough

docs/source/config_reference/scripting/index.rst

@@ -2,11 +2,131 @@
 Scripting
 =========
 
-Work in progress! Explanation of how to define scripts/variables will be added here.
+``ytdl-sub`` fields (file-names, tags, etc) are defined using variables and scripts. The links below
+contain reference documentation for each built-in variable and scripting function.
 
 .. toctree::
   :maxdepth: 1
 
   entry_variables
   override_variables
-  scripting_functions
+  scripting_functions
+
+How it Works
+------------
+
+Fields in the config that support ``formatters`` mean they support scripting, and will
+*format* the field using its defined script.
+
+In its most basic form, a script is a string comprised of variables and/or functions.
+
+Static String
+~~~~~~~~~~~~~
+
+The following example sets ``ytdl-sub``'s output directory. It is
+considered *static* because it does not depend on anything from an entry.
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "Custom YTDL-SUB TV Show"
+
+Static Variables
+~~~~~~~~~~~~~~~~
+
+``ytdl-sub`` offers a few built-in static variables, including ``subscription_name``.
+We can use this instead of hard-coding it above:
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "{subscription_name}"
+
+The syntax for variable usage is brackets with the variable name within it. Assuming
+our subscription is actually named "Custom YTDL-SUB TV Show", then ``ytdl-sub``
+will actually write to that directory.
+
+Entry Variables
+~~~~~~~~~~~~~~~
+
+For context, an *entry* is a video or audio file downloaded from ``yt-dlp``.
+*Entry variables* are variables that are derived from an entry's ``info.json`` file. This file
+comes from ``yt-dlp`` and contains every piece of metadata that it scraped.
+
+These variables are not considered static since they change per entry download. There are a
+few fields in ``ytdl-sub`` (i.e. ``output_directory``) that must be static. For others,
+we are free to use values that derive from an entry.
+
+Suppose we want to customize the name of an entry's output file and thumbnail to include its
+title in its name. We can do that using entry variables:
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "{subscription_name}"
+     file_name: "{title}.{ext}"
+     thumbnail_name: "{title}.{thumbnail_ext}"
+
+Sanitizing Variables
+~~~~~~~~~~~~~~~~~~~~
+
+For experienced ``yt-dlp`` scrapers, you may be thinking:
+
+- "what if the title has characters that do not play nice with my operating system?"
+
+``ytdl-sub`` is able to *sanitize* any variable, meaning it strips any bad characters out
+and can be used for file names. We can ensure our file names and directories by using:
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "{subscription_name_sanitized}"
+     file_name: "{title_sanitized}.{ext}"
+     thumbnail_name: "{title_sanitized}.{thumbnail_ext}"
+
+Simply add a ``_sanitized`` suffix to any variable name to make it sanitized.
+
+Creating Custom Variables
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Suppose we want to include the date in our file names. This means we'd need to update
+both the ``file_name`` and ``thumbnail_name`` fields to include it.
+
+Instead, we can create a custom *override variable*. This is ``ytdl-sub``'s method
+for creating and overriding custom variables.
+
+These are created in the ``overrides`` section. Let's take our above example and create
+a ``custom_file_name`` variable to use for the entry file and thumbnail fields:
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "{subscription_name_sanitized}"
+     file_name: "{custom_file_name}.{ext}"
+     thumbnail_name: "{custom_file_name}.{thumbnail_ext}"
+
+   overrides:
+     custom_file_name: "{upload_date_standardized} {title_sanitized}"
+
+Using Scripting Functions
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Let's suppose you are an avid command-line user, and like all of your file names to be
+``snake_cased_with_no_spaces``. We can use *scripting functions* to create and use a snake-cased
+title.
+
+.. code-block:: yaml
+
+   output_options:
+     output_directory: "{subscription_name_sanitized}"
+     file_name: "{custom_file_name}.{ext}"
+     thumbnail_name: "{custom_file_name}.{thumbnail_ext}"
+
+   overrides:
+     snake_cased_title: >-
+       {
+         %replace( title, ' ', '_' )
+       }
+     custom_file_name: "{upload_date_standardized} {snake_cased_title_sanitized}"
+
+You will notice that we use `>-`. This is YAML's way to say "allow a string to be multi-lined