#!/bin/bash
for f in *.mkv
do

##  Detect what audio codec is being used:
audio=$(ffprobe "$f" 2>&1 | sed -n '/Audio:/s/.*: \([a-zA-Z0-9]*\).*/\1/p' | sed 1q)
##  Detect video codec:
video=$(ffprobe "$f" 2>&1 | sed -n '/Video:/s/.*: \([a-zA-Z0-9]*\).*/\1/p' | sed 1q)
##  Set default audio settings (you may need to use a different encoder,
##  since libfdk_aac is not re-distributable)
aopts="-c:a libfdk_aac -vbr 3"
##  Set default video settings:
vopts="-c:v libx264 -crf 22 -preset veryfast"

    case "$audio" in
        aac|alac|mp3|mp2|ac3 )
##  If the audio is one of the MP4-supported codecs,
##  copy the stream instead of transcoding
            aopts="-c:a copy"
            ;;
        "" )
##  If there is no audio stream, don't bother with audio
            aopts="-an"
            ;;
        * )
            ;;
    esac

    case "$video" in
##  If the video stream is one of the MP4-compatible formats,
##  copy the stream
        h264|mpeg4|mpeg2video|mpeg1video )
            vopts="-c:v copy"
            ;;
        "" )
## If no video stream is detected, don't bother with video
            vopts="-vn"
            ;;
        * )
            ;;
    esac

##  This will tell you what is going on; that is,
##  it will echo a line that will make the terminal output
##  easier to follow:
echo -e "\n    \E[1;30mffmpeg -i $f -map 0 $vopts $aopts ${f/%mkv/mp4}\E[0m"

##  And now, to the meat of the thing.
##  Normally you should ALWAYS quote your variables
##  so that spaces in filenames etc will be preserved.
##  But in this case, doing so for $vopts and $aopts
##  would break the ffmpeg command
##  because we WANT the spaces to break up the strings:
ffmpeg -y -i "$f" -map 0 $vopts $aopts "${f/%mkv/mp4}"
done

exit 0