I recently started running but hate running both in the quiet and with music, so i've been listening to podcasts. Unfortunately, I've caught up with all my favorites and so I've started listening to some iTunesU courses, but unfortunately (again) very few of them have audio-only versions.

I wrote a shell script to copy any recently added iTunesU video files to another folder as m4a files (via ffmpeg) but since it's the first such script that i've ever written I am slightly worried to set it to run every morning ...

Anyway, I would appreciate some feedback from you expert types of what potential problems there might be with how I have it working now or suggestions of how I might improve part of it. Also, as you'll see, I tried to set up a function to do the extension check but couldn't get it to work that way, is there a way to do that?

#!/bin/bash

oldIFS="$IFS" # allow for files/directories with spaces in the names
IFS=$'\n'

is_video_file () 
{
    ext=$1

    if ([ $ext = "m4v" ] || [ $ext = "mp4" ]); 
        then
            return 1
        else
            return 0
    fi  
} # couldn't get this to work right...

iTunesU_Source=~/Music/iTunes_U/
iTunesU_Destination=~/Music/iTunesU_audio/

cd $iTunesU_Source

for sub in */; do
    echo;
    echo $sub

    dest_sub=${iTunesU_Destination}/${sub}

    # create the sub folder if it doesn't exist
    does_not_exist=false
    [ -d $dest_sub ] || does_not_exist=true

    if $does_not_exist;
        then
            mkdir $dest_sub
    fi

    # get the modified times for the two subfolders
    source_sub_time=$(stat -f "%m" -t "%s" ${sub})
    dest_sub_time=$(stat -f "%m" -t "%s" ${dest_sub})

    if $does_not_exist || (( $source_sub_time > $dest_sub_time )); 
        then
            cd $sub
    
            for file in *; do
                filename=$(basename $file)
                extension="${filename##*.}"
                filename="${filename%.*}"

                dest_file=$dest_sub$filename".m4a"

#               if ! [[ -f $dest_file ]] && (is_video_file $extension = 1);
                if ! [[ -f $dest_file ]] && ([ $extension = "m4v" ] || [ $extension = "mp4" ]);
                    then

                    ffmpeg -i $file -vn -acodec copy $dest_file
            fi
            done; # each file in source sub_folder
    
            cd ..
    fi # source sub_folder is new or recently modified

done

IFS="$oldIFS"

Thanks