An editor script for Unity 3D to open shaders in Notepad++ or the default text editor

OpenShaderInNotepad.cs

using System.Diagnostics;
using System.IO;
using UnityEditor;
using UnityEditor.Callbacks;
using UnityEngine;

public class OpenShaderInNotepad
{
	private const string NPP1 = @"C:\Program Files\Notepad++\notepad++.exe";
	private const string NPP2 = @"C:\Program Files (x86)\Notepad++\notepad++.exe";

	[OnOpenAsset]
#if UNITY_2019_2_OR_NEWER
	public static bool OpenInNotepad( int instanceID, int line, int column )
#else
	public static bool OpenInNotepad( int instanceID, int line )
#endif
	{
		Object asset = EditorUtility.InstanceIDToObject( instanceID );
		if( asset != null && !asset.Equals( null ) && asset is Shader && AssetDatabase.Contains( asset ) )
		{
			string shaderPath = Path.GetFullPath( AssetDatabase.GetAssetPath( instanceID ) );

			string args = "\"" + shaderPath + "\"";
			if( line >= 0 )
				args += " -n" + line;
#if UNITY_2019_2_OR_NEWER
			if( column >= 0 )
				args += " -c" + column;
#endif

			if( File.Exists( NPP1 ) )
				Process.Start( NPP1, args );
			else if( File.Exists( NPP2 ) )
				Process.Start( NPP2, args );
			else
				Process.Start( shaderPath );

			return true;
		}

		return false;
	}
}

How To

Simply create a folder called Editor inside your Project window and add this script inside it.