I was looking for a way to have a build number in Unity so that:

1. I want a “build” number to be dense, at least as dense as commits
2. it should auto increment on run
3. it should auto increment from whatever scene you are launching your game
4. it should be possible to decide to show it or not in every scene
5. it should totally take care of itself – so when I get feedback on a new build I can always ask the tester which build is she using, and I don’t go “damn it I forgot to increase the build number!!”

The idea is simply to have an editor script that changes a prefab (so that you can place it in every scene, but has a global state) each time you run. Here is my code, which you can fit to your needs (this also does the life-saving “save scene before run” which you may pick too):

// Author: Pietro Polsinelli - http://designAGame.eu
// Twitter https://twitter.com/ppolsinelli
// All free as in free beer

using TMPro;
using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;
using UnityEngine.SceneManagement;

namespace OL
{
    [InitializeOnLoad]
    public class AutoSaveOnRun
    {
        static AutoSaveOnRun()
        {
            //Thanks https://twitter.com/andrewlukasik for the "+=" fix.
            EditorApplication.playmodeStateChanged += () =>
            {
                if (EditorApplication.isPlayingOrWillChangePlaymode && !EditorApplication.isPlaying)
                {
                    GameObject build = AssetDatabase.LoadAssetAtPath("Assets/__Scripts/Tools/Components/Build/Build.prefab",
                        typeof(GameObject)) as GameObject;

                    if (build != null)
                    {
                        TextMeshProUGUI bn = build.GetComponent<TextMeshProUGUI>();
                        if (string.IsNullOrEmpty(bn.text))
                            bn.text = "1";
                        else
                        {
                            bn.text = (int.Parse(bn.text)+1).ToString();
                        }
                    }
                    
                    EditorSceneManager.SaveScene(SceneManager.GetActiveScene());
                    AssetDatabase.SaveAssets();
                }
            };
        }
    }
}

P.S. More approaches

In order to change the build on compile and not on save, I now use this code in the (also very useful) stop-play-on-compile class:

// Copyright Cape Guy Ltd. 2015. http://capeguy.co.uk.
// Provided under the terms of the MIT license -
// http://opensource.org/licenses/MIT. Cape Guy accepts
// no responsibility for any damages, financial or otherwise,
// incurred as a result of using this code.

// Modified by Pietro Polsinelli. 2017. https://twitter.com/ppolsinelli

public class ExitPlayModeOnScriptCompile
    {
        static bool increasedBuildForThisCompile;

        // Static initialiser called by Unity Editor whenever scripts are loaded (editor or play mode)
        static ExitPlayModeOnScriptCompile()
        {
            Unused(_instance);
            _instance = new ExitPlayModeOnScriptCompile();
        }

        ExitPlayModeOnScriptCompile()
        {
            EditorApplication.update += OnEditorUpdate;
        }

        ~ExitPlayModeOnScriptCompile()
        {
            EditorApplication.update -= OnEditorUpdate;
            // Silence the unused variable warning with an if.
            _instance = null;
        }

        // Called each time the editor updates.
        static void OnEditorUpdate()
        {
            if (EditorApplication.isCompiling && !increasedBuildForThisCompile)
            {
                increasedBuildForThisCompile = true;

                GameObject build = AssetDatabase.LoadAssetAtPath("Assets/__Scripts/Tools/Components/Build/Build.prefab",
                    typeof(GameObject)) as GameObject;

                if (build != null)
                {
                    TextMeshProUGUI bn = build.GetComponent<TextMeshProUGUI>();
                    if (string.IsNullOrEmpty(bn.text))
                        bn.text = "1";
                    else
                    {
                        bn.text = (int.Parse(bn.text) + 1).ToString();
                    }
                }
            }

            if (!EditorApplication.isCompiling)            
            {
                increasedBuildForThisCompile = false;
            }

            if (EditorApplication.isPlaying && EditorApplication.isCompiling)
            {
               Debug.Log("Exiting play mode due to script compilation.");
                EditorApplication.isPlaying = false;
            }
        }

        // Used to silence the 'is assigned by its value is never used' warning for _instance.
        static void Unused<T>(T unusedVariable)
        {
        }

        static ExitPlayModeOnScriptCompile _instance = null;
    }
}

Yet another approach is to identify the build number with a platform build (I don’t use that because I want density): https://gist.github.com/andrew-raphael-lukasik/36a30f0955d7cdc758e394dc4e7266bf.

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