Replaces/modifies the Unity behavior templates using a handful of regexes.

UnityTemplateFixer.cs

using System.Diagnostics;
using System.IO;
using System.Text.RegularExpressions;
using UnityEditor;
using Debug = UnityEngine.Debug;

/// <summary>
/// Unity template fixer by grapefrukt (m@grapefrukt.com)
/// Runs a series of regexes on 
/// </summary>

public class UnityTemplateFixer : Editor {
	static readonly string[] TemplateFiles = {
		"81-C# Script-NewBehaviourScript.cs.txt"
	};

	readonly struct ReplaceTuple {
		public readonly string regex;
		public readonly string replacement;

		public ReplaceTuple(string regex, string replacement = "") {
			this.regex         = regex;
			this.replacement   = replacement;
		}
	}

	static readonly ReplaceTuple[] FindReplacePairs = {
		// replaces the comment before Start()
		new ReplaceTuple( @"\w*// Start is called before the first frame update$"),
		// replaces the comment before Update()
		new ReplaceTuple( @"\w*// Update is called once per frame$"),
		// braces on line 4-ever
		new ReplaceTuple( @"\s*{", " {"),
	};

	[MenuItem("Tools/Fix Class Templates")]
	static void FixTemplates() {
		foreach (var file in TemplateFiles) {
			FixTemplate(file);
		}
	}

	static void FixTemplate(string templateFile) {
		var path = Path.GetDirectoryName(EditorApplication.applicationPath);
		path = Path.Combine(path, "Data", "Resources", "ScriptTemplates", templateFile);

		if (!File.Exists(path)) {
			Debug.LogError("Could not find script template at expected location.");
			return;
		}

		var template = File.ReadAllText(path);
		foreach (var tuple in FindReplacePairs) {
			var regex = new Regex(tuple.regex, RegexOptions.Multiline);
			template = regex.Replace(template, tuple.replacement);
		}
		
		// read the template from disk again, compare it with the modified version
		// if they're the same, we're done
		var onDisk = File.ReadAllText(path);
		if (template == onDisk) {
			Debug.Log("Template is already as desired.");
			return;
		}

		// write the modified template to a temp path
		var newPath = FileUtil.GetUniqueTempPathInProject();
		newPath = Path.GetFullPath(newPath);
		File.WriteAllText(newPath, template);

		// start a new process that runs copy in the command prompt
		// we need to elevate the privileges of this to be able to write to the program files directory
		// where unity keeps its templates
		var startInfo = new ProcessStartInfo {
			FileName  = "cmd.exe",
			Arguments = $"/c copy /y \"{newPath}\" \"{path}\"",
			Verb      = "runas",
		};
		
		var process = new Process { StartInfo = startInfo };
		process.Start();
		
		// we wait for the process to finish before we check the file on disk again
		process.WaitForExit();
		
		onDisk = File.ReadAllText(path);
		if (template == onDisk) {
			Debug.Log("Template replaced successfully.");
		} else {
			Debug.LogError("Template replacement failed.");
		}
	}
}