ffmpeg-progress.sh

#!/bin/bash
#"simple" ffmpeg progress indicator, though because evil ffmepg doesn't use Carriage Returns or Line Feeds like any normal person, it is actually a right pain in the bum ;)
#by Rupert Plumridge
#for updates visit www.prupert.co.uk
#Creative Commons Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales Licence
#
# File name: fs.sh - saved in the "nautilus-scripts" directory.
# http://handybashscripts.blogspot.com/2011/01/ffmpeg-with-progress-bar-re-work.html
# 
# Author: robz
# Version: 310111

# Vars.

LOG=$HOME/ffmpeg.log

display () # Calculate/collect progress 
{
START=$(date +%s); FR_CNT=0; ETA=0; ELAPSED=0
while [ -e /proc/$PID ]; do                         # Is FFmpeg running?
    sleep 2
    VSTATS=$(awk '{gsub(/frame=/, "")}/./{line=$1-1} END{print line}' \
    /tmp/vstats)                                  # Parse vstats file.
    if [ $VSTATS -gt $FR_CNT ]; then                # Parsed sane or no?
        FR_CNT=$VSTATS
        PERCENTAGE=$(( 100 * FR_CNT / TOT_FR ))     # Progbar calc.
        ELAPSED=$(( $(date +%s) - START )); echo $ELAPSED > /tmp/elapsed.value
        ETA=$(date -d @$(awk 'BEGIN{print int(('$ELAPSED' / '$FR_CNT') *\
        ('$TOT_FR' - '$FR_CNT'))}') -u +%H:%M:%S)   # ETA calc.
    fi
    echo -ne "\rFrame:$FR_CNT of $TOT_FR Time: $(date -d @$ELAPSED -u +%H:%M:%S) ETA: $ETA Percent: $PERCENTAGE"                # Text for stats. output.

done
}

trap "killall ffmpeg fs.sh; rm -f "$RM/vstats*"; exit" \
INT TERM EXIT                                       # Kill & clean if stopped.

# Get duration and PAL/NTSC fps then calculate total frames.
    FPS=$(ffprobe "$1" 2>&1 | sed -n "s/.*, \(.*\) tbr.*/\1/p")
    DUR=$(ffprobe "$1" 2>&1 | sed -n "s/.* Duration: \([^,]*\), .*/\1/p")
    HRS=$(echo $DUR | cut -d":" -f1)
    MIN=$(echo $DUR | cut -d":" -f2)
    SEC=$(echo $DUR | cut -d":" -f3)
    TOT_FR=$(echo "($HRS*3600+$MIN*60+$SEC)*$FPS" | bc | cut -d"." -f1)
    if [ ! "$TOT_FR" -gt "0" ]; then echo error; exit; fi

    # Re-code with it.

	nice -n 15 ffmpeg -deinterlace -vstats_file /tmp/vstats -y -i "$1" -vcodec libx264 -level 41 -vpre main -vpre medium -crf 24 -threads 0 -sn -acodec libfaac -ab 128k -ac 2 -ar 48000 -vsync 1 -async 1000 -map 0.0:0.0 -map 0.1:0.1 "$1".mkv 2>/dev/null &                       # Background FFmpeg.
        PID=$! && 
	echo "ffmpeg PID = $PID"
	echo "Length: $DUR - Frames: $TOT_FR  "
	display                               # GUI stats. func.
        rm -f "$RM"/vstats*                             # Clean up tmp files.

    # Statistics for logfile entry.
    ((BATCH+=$(cat /tmp/elapsed.value)))                # Batch time totaling.
    ELAPSED=$(cat /tmp/elapsed.value)                   # Per file time.
    echo -e $COUNT\. ${NFILE%.*}\
    "\nDuration: $DUR - Total frames: $TOT_FR" >> $LOG
    # Elapsed time for 1 pass, average for 2 passes, fps average for both.
    AV_RATE=$(( TOT_FR / ELAPSED ))
    echo -e "Re-coding time taken: $(date -d @$ELAPSED -u +%H:%M:%S)"\
    "at an average rate of $AV_RATE""fps.\n" >> $LOG
done

# Notify finished batch ding-dong gong and message, log it, all done bye ;¬)
TEXT="Total re-coding time: $(date -d @$BATCH -u +%H:%M:%S)."
echo -e "$TEXT\n\
__________________________________________________________________" >> $LOG
echo -e "$TEXT\n\
__________________________________________________________________"

exit

I ended up using your algorithm for ffmkv: https://github.com/Fmstrat/ffmkv which is a bash wrapper for FFMPEG with an Inquirer menu interface and progress monitoring. During that I did a rewrite after noticing a few division by 0 issues and some inefficiencies with things like the repeated parsing of ffmpeg1.log and calculating variables. Much of this may just be because it's now 2019 and this code is 10 years old. If you'd like to pull code from there, feel free!

This is my personal use Here.

Pretty much same algo as yours, i just did a few optimizations.