Gmail – Once saved, is there a way to change the subject line

As long as you don't need to parse the contents of the subject line, you can do this with If This Then That.

You'll need the Gmail and Google Drive channels. Specifically:

Trigger

Depending on how you're going to identify the incoming message, you'll use one of the following triggers:

• Any new email in inbox
• New email in inbox from
• New starred email in inbox
• New email in inbox labeled
• New email in inbox from search

"Subject" is one of the ingredients available, which you'll obviously need. There's other useful things, like "ReceivedAt".

Action

You'll want Add row to spreadsheet where you'll defined the name of the spreadsheet, what goes in the row, and the folder path in Google Drive where it's located.

There are several recipes already shared that might service. This recipe, for instance, saves a new row for every received message.

You can use this Greasemonkey script from lifehacker

Copying it here for the record:

// ==UserScript==
// @name            Show Editable Subject
// @description  Always show editable subject field when replying to a message in Gmail.
// @include         http://mail.google.com/*
// @include         https://mail.google.com/*
// @author          Gina Trapani
// @namespace   http://lifehacker.com
// @version         0.1

// @tab Compose
// @homepage http://lifehacker.com/
// ==/UserScript==


// Version 0.1: Released

// Borrowed some keyhandler code from http://bitterpill.org/gmail_tinyurl/
// Repurposed some code from http://userscripts.org/scripts/review/20887
// Based on  mathmike's the Show Details  user script http://userscripts.org/scripts/show/13700
// mathmike: Most of the functions below were borrowed from Gmail Macros (New)


var CLICK_TO_SHOW = "MRoIub";
var INNER_HTML = "Edit Subject"
var KEYCOMBO_ONLY=false;
var THIRD_KEYCODE=66; 

var gmail = null;

window.addEventListener('load', function() {
  if (unsafeWindow.gmonkey) {
    unsafeWindow.gmonkey.load('1.0', function(g) {
      gmail = g;

    if (KEYCOMBO_ONLY) {
        getDoc().defaultView.addEventListener('keydown', keyHandler, false);
    } else {
        gmail.registerViewChangeCallback(handleView);
        handleView();
    }
    });
  }
}, true);

function getDoc() {
 return gmail.getNavPaneElement().ownerDocument;
}

function keyHandler(event) {
    GM_log(event.ctrlKey + ' ' + event.shiftKey + ' ' + event.keyCode);
    if (event.ctrlKey == true && event.shiftKey == true && event.keyCode == THIRD_KEYCODE) { 
         clickSpan();
    }
}

function handleView() {
     //if (gmail.getActiveViewType() == "co") clickSpan();  //Compose view
      if (gmail.getActiveViewType()=="cv") tryToClickSpan();  //Conversation view, user may hit Reply link
}

function clickSpan() {
  var nodes = 
    getNodesByTagNameAndClass(gmail.getActiveViewElement(), "span", CLICK_TO_SHOW);
  if (!nodes) return false;
  for (var i in nodes) {
      GM_log("Node " + i + " HTML is " + nodes[i].innerHTML );
      if (nodes[i].innerHTML == INNER_HTML){
        simulateClick(nodes[i], "click");
        return true;
      }
    }
}

function simulateClick(node, eventType) {
  var event = node.ownerDocument.createEvent("MouseEvents");
  event.initMouseEvent(eventType,
                       true, // can bubble
                       true, // cancellable
                       node.ownerDocument.defaultView,
                       1, // clicks
                       50, 50, // screen coordinates
                       50, 50, // client coordinates
                       false, false, false, false, // control/alt/shift/meta
                       0, // button,
                       node);

  node.dispatchEvent(event);
}

function getNodesByTagNameAndClass(rootNode, tagName, className) {
  var expression = 
      ".//" + tagName + 
      "[contains(concat(' ', @class, ' '), ' " + className + " ')]";

  return evalXPath(expression, rootNode);
}

function evalXPath(expression, rootNode) {
  try {
    var xpathIterator = rootNode.ownerDocument.evaluate(
      expression,
      rootNode,
      null, // no namespace resolver
      XPathResult.ORDERED_NODE_ITERATOR_TYPE,
      null); // no existing results
  } catch (err) {
    GM_log("Error when evaluating XPath expression '" + expression + "'" +
           ": " + err);
    return null;
  }
  var results = [];

  // Convert result to JS array
  for (var xpathNode = xpathIterator.iterateNext();
       xpathNode;
       xpathNode = xpathIterator.iterateNext()) {
    results.push(xpathNode);
  }

  return results;
}

function tryToClickSpan()  {
        if (!clickSpan() ) {
            setTimeout(tryToClickSpan, 500);
        }
}