Left 4 Dead 2 VScripts are server-side scripts that are run in an in-game virtual machine. They are written in Squirrel, a compiled scripting language similar to Lua.
The official Squirrel documentation can be found here[squirrel-lang.org]

Generally, VScripts can be divided into 4 types, according to their usage:
The most common use of VScripts in Left 4 Dead 2 is to influence the behavior of the AI Director. These scripts can range from simple adjustments of infected spawning and prohibiting boss infected, to custom events like onslaughts and gauntlets, and even complex staged panic events and fully custom finales. Most of the events in the official campaigns are mainly implemented this way.

Director Scripts work mainly by writing overriding values of the various variables used by the Director into a DirectorOptions table.

Only one Director Script can be running at a time. Executing a new one will terminate any previous running one and remove any values it set in DirectorOptions.

Another common use is to attach a script to an entity. The script provides easy access to read and modify many of the entity's properties, and even to write new KeyValues. This allows for controlling entities in ways that would be very complicated or impossible to with the entity I/O system in Hammer.

Any entity is capable of running a script and has the ability to set a specified think function to run every 0.1 seconds, as well as executing script code as an entity output.

Some entities also have specialized functions for VScripts, with the most prominent being point_script_use_target, which allows for turning other entities into fully programmable timed buttons.

(This guide will not cover this kind of VScripts)

Scripts can also be made to run on map load, based on game mode and optionally map name. These scripts are commonly used to create scripting and modify global Director options for mutations, but can also be used in custom maps.

Global scripts can have script hook functions added that get called from the game at certain events, like when players/objects take damage.

There are many utility functions and features readily available for these scripts, including a resource and building system, and custom panic wave spawning. We will focus on this kind of scripts later.

All scripts can access functions for many features, including spawning entities either from precompiled lists or programmatically, spawning infected, a HUD system, storing data across levels and to disk, and much more.

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The scripts are loaded from text files with the file extensions .nut and .nuc, where .nuc denotes encryption of plain text .nut. Custom scripts are read from \left 4 dead 2\left4dead2\scripts\vscripts\, as well as \scripts\vscripts\ when packed into a .vpk file.

Note: Browse and extract VPK files with third-party programs like GCFScape.

• left 4 dead 2/left4dead2/pak01_dir.vpk
  
• left 4 dead 2/left4dead2_dlc1/pak01_dir.vpk
  
• left 4 dead 2/left4dead2_dlc2/pak01_dir.vpk
  
• left 4 dead 2/left4dead2_dlc3/pak01_dir.vpk
  
• left 4 dead 2/update/pak01_dir.vpk
  
• left 4 dead 2/sdk_content/scripting/scripts/vscripts/

These official scripts are coming with encrypted .nuc files, you can download the plaintext format here: Decompiled VScripts[www.dropbox.com]

VSLib is a simple, powerful VScript library for L4D2, which packs many APIs of VScript. This library will make your modding so much easier. Create superior mutations, interactive maps, and much more. Actually, the famous Admin System is developed using VSLib.

Download VSLib here: L4D2Scripters/vslib[github.com]

You can choose an editor for editing .nut files, as you like. Here I am using Sublime Text 3, which has a plug-in that supports Squirrel language highlighting:

• Navigate to Left 4 Dead 2/left4dead2/addons/ (just where you store some addon campaigns), creating a folder named mymutation;
  
• Head into mymutation, this will be used as a reference to later relative paths;
  
• Create directories: modes/、scripts/vscripts/, and decompress VSLib to the later;

To avoid ambiguity, I will describe it in Shell Script syntax:

Until now, your scripts/vscripts/ should be like this:


--------------------------------------

• Create file: addoninfo.txt in current folder;
  
• Create file: mymutation.txt under modes/
  
• Create file: mymutation.nut under scripts/vscripts/

Write following contents to ./addoninfo.txt:

Write following contents to modes/mymutation.txt:

This file: scripts/vscripts/mymutation.nut, is where we will write code into.

Previously, I published a simple Mutation to Workshop: Malicious Boomer.
You can get the source code on my Github: typowritter/L4D2-Mutations[github.com]

To make it short, the rule is:

1. No common infected, normally you can only see Boomers.
   
2. Boomer vomits can attract a mob of Special Infected, which includes all SI but Boomer themselves and witches

Now, I will demonstrate how to code it from scratch:

At the very first, we must include VSLib:

Then we configure the Director:

The code above writes some data to table MutationOptions. At map loding, the Director will check DirectorOptions first, then MutationOptions, so the configurations in later can override former ones.

I designed 3 game states to switch between them:

• Normal：Only Boomers will respawn.
  
• Peak：Get vomited，stopping the Boomer respawn and starting other SI respawn, and holding 20 seconds.
  
• Relax：After 20 seconds' Peak, enter Relax for 10s, stop all SI respawn. During this 10s the Boomer vomit will not trigger any panic. 10s later go back to Normal state.

Then we configure the Director:

We will need some global variables to store the current state, and Timers to control them:

Now the write functions to set the Timer and act as the callback:

Thus now we can switch between our states, we need a function to control SI respawn:

OK, we are almost done. Now we just need to bind SpawnInfected to a repeatly running Timer:

Done! This is all the code we just wrote:

In order for our Mutation mode to be loaded like a normal add-on, we need to package it into a common VPK format:

Find Left 4 Dead 2\bin\vpk.exe，drag&drop mymutation folder to vpk.exe, it will the generate a mymutation.vpk in the same directory as mymutation (thus addons in our example).

Just open your game and play! You can start a campaign by typing this in the console:

Please check my another Guide: Linux Dev Toolchain Collection, in which there are some useful tools to do the work.

As we can see, the game has loaded our Mutation:

Valve Developer Community: Left 4 Dead 2 Level Creation

VSLib Online Docs: Simple, powerful VScript Library for L4D2[l4d2scripters.github.io]

Guides by Rayman1103: Rayman1103 - My Content[www.gamemaps.com]

Mutation Creator by me: l4d2-mutation-creator[github.com], with a demonstration video

Discord Group: Dead4Mods[discord.gg]