Finding The Number Of Open File Descriptors Per Process On Linux And Unix

How does one find the number of open file descriptors being used by any given process?

The question is a bit of a trick, in and of itself, since some folks define "open file descriptors" as the number of files any given process has open at any given time. For our purposes, we'll be very strict, and make the (usually fairly large) distinction between "files open" and "open file descriptors."
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