AnonyMonk suggested avoiding the shell for numerous very good reasons -- some of which would likely be those I would cite: if you use the shell and the external program, you've increased complexity; likely have opened some exploitable security gaps, and added overhead for your server. Unless you think the variety of conversions done by the external program outweigh all of those, heed AM's advice.
And, in any case, be aware you're reinventing the wheel! Is there some really valid reason to create and invent/install your own wheels, gears, and interfaces when a simple search -- for "unit conversions" or even "conversions" will turn up many existing (and mostly, free) services?
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