Continuously Scan Directory and Perform Script on New Items - bash

Question

First, please forgive me and be easy on me if this question seems easy; the first time I tried posting a question about another subject, I didn't provide enough information a few months ago. My apologies.
I'm trying to scan my incoming media folder for new audio files and convert them to my preferred format into another folder, without removing the originals.
I've written the script below and while it seems to work for one-offs, I can't seem to get it to create the destination directory name based off the source directory name; and I can't seem to figure out how to keep it looping, "scanning", for new media to arrive without processing what it's already processed.
I hope this makes sense...
#! /bin/bash
srcExt=$1
destExt=$2
srcDir=$3
destDir=$4
opts=$5
# Creating the directory name - not currently working
# dirName="$(basename "$srcDir")"
# mkdir "$destDir"/"$dirName"
for filename in "$srcDir"/*.flac; do
basePath=${filename%.*}
baseName=${basePath##*/}
ffmpeg -i "$filename" $opts "$destDir"/"$baseName"."$destExt"
done
for filename in "$srcDir"/*.mp3; do
basePath=${filename%.*}
baseName=${basePath##*/}
ffmpeg -i "$filename" $opts "$destDir"/"$baseName"."$destExt"
done

Answer 1

there are different ways of doing this, the easiest way might just be to look at the "modification date" of the file and seeing if it has changed, something like:
#! /bin/bash
srcExt=$1
destExt=$2
srcDir=$3
destDir=$4
opts=$5
# Creating the directory name - not currently working
# dirName="$(basename "$srcDir")"
# mkdir "$destDir"/"$dirName"
for filename in ` find "$srcDir" \( -name '*.mp3' -o -name '*.flac' \) -mmin -10`; do
basePath=${filename%.*}
baseName=${basePath##*/}
ffmpeg -i "$filename" $opts "$destDir"/"$baseName"."$destExt"
done

Answer 2

Consider using mkdir -p which will a) create all necessary intermediate directories, and b) not complain if they already exist.
If you want the new items to be processesd immediately they arrive, look at inotify or fswatch on macOS. In general, if less urgent, schedule your job to run every 10 minutes under cron, maybe prefixing with nice so as not to be a CPU "hog".
Decide which files to generate by changing directory to the source directory and iterating over all files. For each file, work out what the corresponding output file should be according to your rules, test if it already exists, if not create it.
Don't repeat all your for loop code like that, just do:
cd "$srcDir"
for filename in *.flac *.mp3 ; do
GENERATE OUTPUT FILENAME
if [ ! -f "$outputfilename" ] ; then
mkdir -p SOMETHING
ffmpeg -i "$filename" ... "$outputfilename"
fi
done