Concatenate several srt file into one file with renumbering all the entries and shifting all the timestamps

concatSrts.sh

#!/bin/bash
clear

# Install this 1st : 
# sudo apt install subtitleripper

cd ~/tmp/test1005

export rawTotalSrt="rawTotal.srt"
export totalSrt="total.srt"
rm -f ${rawTotalSrt} ${totalSrt}

EPOCH='jan 1 1970'
sum=0
first_srt=`ls $1/*.srt | grep "1_"`
prevVid=`ls $1/*.mp4 | grep "1_"`
prevVidDuration=`ffmpeg -i ${prevVid} |& grep -i "Duration" | awk -F " " '{ print $2 }' | sed 's/,//g' | sed 's/\./,/g'`


cp $first_srt .
previous_srt=$first_srt

echo "${prevVid} ${prevVidDuration}"
echo "${first_srt}"
sum="$(date -u -d "$EPOCH $prevVidDuration" +%s) + $sum"
sumBc=`echo ${sum}|bc`
currHostName=`hostname`
currFilename=$(basename "$0")
for currVideo in `ls $1/*.mp4 | grep -v "1_"`; do
  echo "Current total sum is ${sum}"
  echo "Current sumBc is ${sumBc}"	
  echo "------------------------"	
  #echo "${currVideo} ..."
  vidDuration=`ffmpeg -i ${currVideo} |& grep -i "Duration" | awk -F " " '{ print $2 }' | sed 's/,//g' | sed 's/\./,/g'`
  echo "${currVideo} ${vidDuration}"
  currSrt="${currVideo%.*}.srt"
  currStrFilename=$(basename "$currSrt")
  #cp $currSrt $currStrFilename
  srttool -d ${sumBc} -i ${currSrt}>${currStrFilename}
  #echo "prev ${previous_srt}..."
  #echo "curr ${currSrt}..."
  echo "${currSrt} should start at ${sumBc}"

  sum="$(date -u -d "$EPOCH $vidDuration" +%s) + $sum"
  sumBc=`echo ${sum}|bc`
  previous_srt=$currSrt
  prevVidDuration=$vidDuration
done
echo "------------------------"
#echo "Total sum is ${sum}"
#sumBc=`echo ${sum}|bc`
#echo "Total sumBc is ${sumBc}"
echo "Creating to full srt file ... " 
cat *.srt > ${rawTotalSrt}
srttool -r -i ${rawTotalSrt} -o ${totalSrt}

# Look at this for info, if something doesn't work for you : 
# https://unix.stackexchange.com/questions/2283/how-do-i-merge-two-srt-files