How to write to a relative path with PS actions

If you mean building an absolute path from a relative path, I can offer a method. You first need to build the relative paths relative to the very script you are gonna run with. Then do these: 

1. Use `$fileName` to get the full path with file name of the current script. We have a base now.

2. Use `new File($fileName)` or `File($fileName)` to get a file object, on which we can use `.parent` to get the absolute path.

3. Use ` '/' ` and `+` to connect the absolute path and your relative path. And use `new Folder` or `Folder` (if your relative path is a folder), or use `new File` or `File` (if you relative path contains the file name).

So it would look like this at last: `new File(new File($.fileName).parent +  relativePath (eg. '/../../Template/YourPSFile.psd') `. This gets you an absolute path at runtime, like you can use `app.open(new File(new File($.fileName).parent +  relativePath)` to open the file.

Please post a screenshot of the fully expanded Action in the Actions Panel and describe what you want it to achieve differently. 

This Script would save a jpg in the same Folder as the active Document (and if that has no path yet to the Desktop). 

Save the code as a txt-file, change the extension to .jsx and copy it into Photoshop’s Presets/Scripts-Folder; after restarting Photoshop it should be available under File > Scripts and can be assigned a Shortcut or used in an Action. 

// saves jpg into same folder;
// be advised: this  overwrites existing jpgs of the same name without prompting. 
// 2010, use it at your own risk;
#target photoshop;
if (app.documents.length > 0) {
var thedoc = app.activeDocument;
// getting the name and location;
var docName = thedoc.name;
if (docName.indexOf(".") != -1) {var basename = docName.match(/(.*)\.[^\.]+$/)[1]}
else {var basename = docName};
// getting the location, if unsaved save to desktop;
try {var docPath = thedoc.path}
catch (e) {var docPath = "~/Desktop"};
// jpg options;
var jpegOptions = new JPEGSaveOptions();
jpegOptions.quality = 9;
jpegOptions.embedColorProfile = true;
jpegOptions.matte = MatteType.NONE;
//save jpg as a copy:
thedoc.saveAs((new File(docPath+'/'+basename+'.jpg')),jpegOptions,true);
//that’s it; thanks to xbytor;
};

AFAIK, you don't/can't do so with an action... Where are you using the absolute path in the action? In a save step, elsewhere?

You can turn on the modal control for an interactive override in the action.

Or you can use the "override action step" in Batch for recorded open or save commands.

Or you can use the "Insert menu item" command to place a generic open or save step into an action.

Otherwise, this is available via scripting, however it depends on the context of the step in the action on where/how you would use a scripted step recorded into the action.

Are you familiar with JavaScript?