Easiest way to build a one-off iPhone "App" (probably JS on a local web page)? At work, we have a certain esoteric calculation that needs to be made from time to time. There is a simple two-line bash script available that does it, but this requires a computer and I often need to do this calculation on the workshop floor or in the field where a laptop might not be immediately to hand. It would be very convenient to have something like an iPhone app instead. It must be usable without an Internet connection as I sometimes work in places with no wifi or phone signal.

Of course I could learn Objective C, set up XCode, get whatever Apple dev account I need for this, and so on - but this seems like a disproportionate amount of work to replace two lines of bash script on one phone for my own use. 

So, question, what is the simplest way to make something that looks roughly like an app on my home screen? If it weren't for the offline requirement then a Safari bookmark to an HTML/Javascript page on the Web somewhere would be ideal - is there any equivalent to a file:// URL on iOS?
I asked this as a genuine question, but I'm now answering it myself since I found a technique that works for me and it might help others.

I wrote a basic old-school HTML + Javascript page that did the required calculation, then uploaded it to my web server along with the manifest file described here. Having visited the page once, my phone has cached it and can now re-open it (including re-running the Javascript to give the new result) even without Internet access.