Another mapfile question.

[Steven W. Orr]

I never even noticed that mapfile exists. I read the section, I googled the
galaxy to look for example usage, and I am stuck as to why this is A Good Thing.

Is there an example of how to use this that uses the callback feature? And why 
is this preferable to using a while read loop? I'd just like to see why this 
is a useful thing.

I did find one comment from Chet: "It was requested and contributed by the 
bashdb folks.  I suppose Rocky could weigh in on it."

Is using mapfile supposed to be faster than using something like:
IFS=
aa=()
while read line
do
   aa+=("$line")
done < fn
vs.
IFS=$'\n'
aa=($(< fn))
vs.
mapfile -t aa < fn

Also, I'm sort of stumped at the notion of the callback getting the index of 
the element that is about to be assigned, versus getting the text that was 
just read.

If I could just get a clue of why this is useful... I can see the usefulness 
of -t -n -s, but the value of the callback eludes me.

TIA

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