broken attempt to convert timecode data into a vtt file

tc2vtt

#!/bin/bash

# these two functions cross-convert between frame number and timecode value
# borrowed from https://github.com/yamaq/timecode/blob/master/timecode
function convToFN() {
    # Convert to frame number(1800) from timecode(00:01:00:00)
    # $1 --> tc
    # $2 --> fpsHz
    # $3 --> fpsCalc
    local tc frames frameBase
    if [ $2 -eq 29 -o $2 -eq 59 ]; then  # DF
        if [ $2 -eq 29 ]; then
            if [[ "$1" =~ ([0-1][0-9]|[2][0-3]):[0-5][^0]:00:0[0-1] ]]; then
                tc="${1:0:10}2"  # 00:01:00:00 --> 00:01:00:02
            else
                tc="$1"
            fi
        else  # 59
            if [[ "$1" =~ ([0-1][0-9]|[2][0-3]):[0-5][^0]:00:0[0-3] ]]; then
                tc="${1:0:10}4"  # 00:01:00:00 --> 00:01:00:04
            else
                tc="$1"
            fi
        fi
        frameBase=$(( ($3 * 60 - $3 / 15) * 9 + $3 * 60 ))
        frames=$(( 10#${tc:0:2} * 6 * $frameBase + ${tc:3:1} * $frameBase ))
        if [ ${tc:4:1} -eq 0 ]; then
            frames=$(( $frames + 10#${tc:6:2} * $3 + 10#${tc:9:2} ))
        else
            frames=$(( $frames + ${tc:4:1} * ($3 * 60 - $3 / 15) ))
            frames=$(( $frames + 10#${tc:6:2} * $3 + 10#${tc:9:2} ))
        fi
    else  # NDF
        tc="$1"
        frames=$(( 10#${tc:0:2} * 60 * 60 * $3 ))      # hour
        frames=$(( $frames + 10#${tc:3:2} * 60 * $3 )) # minute
        frames=$(( $frames + 10#${tc:6:2} * $3 ))      # second
        frames=$(( $frames + 10#${tc:9:2} ))           # frame
    fi
    echo "$frames"
    return 0
}

function convToTC() {
    # Convert to timecode(00:01:00:00) from frame number(1800)
    # If fn is over 24H, will be returned 0H.
    # $1 --> fn
    # $2 --> fpsHz
    # $3 --> fpsCalc
    local hh mm m10 m1 ss ff tc fn dfSequence
    if [ $2 -eq 29 -o $2 -eq 59 ]; then
        fn=$(( $1 % (86400 * $3 - ($3 / 15 * 1296)) ))  # To fn is under 24H
        dfSequence=$(( $fn / (($3 * 60 - $3 / 15) * 9 + $3 * 60) ))
        hh=$(( $dfSequence / 6 ))
        m10=$(( $dfSequence % 6 ))
        fn=$(( $fn % (($3 * 60 - $3 / 15) * 9 + $3 * 60) ))
        if [ $fn -lt $(( $3 * 60 )) ]; then
            m1=0
            ss=$(( $fn / $3 ))
            ff=$(( $fn % $3 ))
        else
            fn=$(( $fn - $3 * 60 ))
            for i in $(seq 1 9); do
                if [ $fn -lt $(( $3 * 60 - $3 / 15 )) ]; then
                    m1=$i
                    fn=$(( $fn + $3 / 15 ))
                    ss=$(( $fn / $3 ))
                    ff=$(( $fn % $3 ))
                    break
                fi
                fn=$(( $fn - ($3 * 60 - $3 / 15) ))
            done
        fi
        printf "%02d:%01d%01d:%02d:%02d\n" $hh $m10 $m1 $ss $ff
    else
        hh=$(( ($1 / (3600 * $3)) % 24 ))
        mm=$(( $1 % (3600 * $3) / (60 * $3) ))
        ss=$(( ($1 % (3600 * $3)) % (60 * $3) / $3 ))
        ff=$(( ($1 % (3600 * $3)) % (60 * $3) % $3 ))
        printf "%02d:%02d:%02d:%02d\n" $hh $mm $ss $ff
    fi
    return 0
}

# dave rice created convToTS by modifying convToTC
function convToTS() {
    # Convert to timecode(00:01:00:00) from frame number(1800)
    # If fn is over 24H, will be returned 0H.
    # $1 --> fn
    # $2 --> fps
    local hh mm m10 m1 ss ff tc fn dfSequence
    echo "1 $1 2 $2"
    time=$(echo "scale=3; $1 / ($2)" | bc)
    hh=$(echo "$time / (60 * 60)" | bc)
    mm=$(echo "$time % (60 * 60) / 60" | bc)
    ss=$(echo "$time % 60" | bc)
    ff=$(echo "$time" | cut -d . -f2)
    echo
    echo "===="
    echo "time $time tc $hh $mm $ss $ff"
    printf "%02d:%02d:%02d.%04d\n" $hh $mm $ss $ff

    return 0
}

MEDIAFILE="${1}"

# trying to grab timecode using ffmpeg alone, resourcespace-style
TIMECODE=$(ffmpeg -i "${MEDIAFILE}" 2>&1 | grep timecode | head -n 1 | cut -d : -f2- | sed 's/ //g')

if [[ -n "${TIMECODE}" ]] ; then
    DUARTION_TC=$(ffmpeg -i "${MEDIAFILE}" 2>&1 | grep "Duration: " | head -n 1 | cut -d : -f2- | cut -d , -f1 | sed 's/ //g')
    DUARTION_FN=$(convToFN "${DUARTION_TC}" 100 30)
    echo "Timecode value is ${TIMECODE}"
    echo "Duration as time is ${DUARTION_TC}"
    echo "Duration as frame number is ${DUARTION_FN}"
    FRAME_START=$(convToFN "${TIMECODE}" 29 30)
    VTT_NAME="${MEDIAFILE%.*}.vtt"
    
    echo "WEBVTT" > "${VTT_NAME}"
    echo >> "${VTT_NAME}"
    COUNTER="0"

    while [ "$COUNTER" -lt "$DUARTION_FN" ] ; do
        FRAME_VALUE="$(echo ${FRAME_START} + $COUNTER | bc)"
        convToTC "$FRAME_VALUE" 29 30
        VTT_FRAME_START=$(convToTS "$COUNTER" "30000/1001")
        COUNTER=$[$COUNTER+1]
        VTT_FRAME_END=$(convToTS "$COUNTER" "30000/1001")
        echo "$VTT_FRAME_START -> $VTT_FRAME_END"
    done

else
    echo "sorry, no timecode"
fi