EditorApplication.currentScene

Description

The path of the Scene that the user has currently open (Will be an empty string if no Scene is currently open). (Read Only)

All paths are relative to the project folder. Like: "Assets/MyScenes/MyScene.unity".

using UnityEngine;
using UnityEditor;

public class Example : MonoBehaviour
{
    // saves the current Scene into a new temporary scene.
    [MenuItem("Example/Save temp Scene %s")]
    static void SaveTempScene()
    {
        string[] path = EditorApplication.currentScene.Split(char.Parse("/"));
        path[path.Length - 1] = "Temp_" + path[path.Length - 1];
        EditorApplication.SaveScene(string.Join("/", path));
    }
}