concat multiple mp4 into a single one, extract audio as mp3

ffmpeg-concat.sh

#!/bin/bash

# default group size (no grouping)
group_size=-1

# required to deal with file names containing blanks
OIFS="$IFS"
IFS=$'\n'

# parsing the -g command line argument
for i in "$@"
do
case $i in
    -g=*|--groups=*)
    group_size="${i#*=}"
    shift # past argument=value
    ;;
    -h|--help)
    echo "Usage: ffmpeg-concat.sh [-g=<n>|--groups=n]
  The groups parameter allows to concatenate groups of n videos"
    exit 0
    shift # past argument=value
    ;;
    *)
            # unknown option
    ;;
esac
done

# removing preexisting temporary files
rm -f list-a*
rm -f all.txt*

# ffmpeg input file containing all the .mp4 file names to be converted
for f in `ls *.mp4`; do echo "file '$f'"; done > all.txt

# default input file name
input_files='all.txt'

if [ "$group_size" -gt 0 ]
then
    echo "Converting videos in groups of $group_size"
    split -l 10 all.txt list-
    input_files='list-*'
else
    echo "Converting all the videos"
fi

echo "converting files from $input_files"

for l in `ls $input_files`
do 
   echo "concatenating files from $l";
   ffmpeg -f concat -y -loglevel error -safe 0 -i $l -c copy "$l.mp4"
   echo "extracting audio files";
   ffmpeg -loglevel error -i "$l.mp4" -vn "$l.mp3"
done