two things on your question: openHAB isn’t “just an App” - it’s mereley and importantly a backend system, capable of connecting different hardware and systems and appliances. In your case it can connect to either sensor directly via so called bindings - or you can use custom made script, which import data and information into openHAB Backend.
The App is only one of many User Interfaces, which use the information present in the backend.
So, that being said: You need a way to Import your temperature into the Backend and then you can expose this item to a sitemap and then you can use the App to display the sitemap … and the item’s state.
For an external ressource like your bash script you can use the REST API (is linked in the user manual), which you can use from within the script and update item states. and yes, the exec-binding can trigger the script. and yes, you could easily write a so-called rule (which in turn can have a cron trigger) to read the bash output and put it directly in to a item.
But before doing one of those things, I highly recommend reading through the mentioned docs and get a better understanding of openHAB. You can of course come back here, if you have any questions.