I'm a bit wary of when people say "no really, I honestly need to cram an unlimited number of results into one single variable and it won't work any other way". This is usually just a bad programming habit, not an actual need.

Consider:

Code:

while read EXT
do
        find ../FILES/ -name "*.${EXT}" | while read FILE
        do
                mv -i "${FILE}" "/path/to/destination/folder"
        done
done < extlist.txt

This lets you process it one name at a time so you don't need to worry about shell limits.

Cramming unlimited numbers of arguments is a bad idea for many reasons:
Using pipes to process names one at a time avoids these both.

Actually, you can simplify that using the -exec feature of find:

Code:

while read EXT
do
        find ../FILES/ -name "*.${EXT}" -exec mv -i '{}' "/path/to/destination/folder" ';'
done < extlist.txt

...and if you're using Linux, you can use the -t feature of mv and the + feature of find, which will let it move multiple files with one command very quickly:

Code:

while read EXT
do
        find ../FILES/ -name "*.${EXT}" -exec mv -t "/path/to/destination/folder" -i '{}' '+'
done < extlist.txt

---------- Post updated at 10:40 AM ---------- Previous update was at 10:20 AM ----------

find works. Something else is wrong.

Have you tried one of my solutions?