Add a downmixed stereo track to a video file using ffmpeg

add_stereo_track.sh

#!/usr/bin/env bash

# USAGE: 
#    add_stereo_track *.mkv
#
# Will downmix a 5.1 track and add a new stereo audio track to the file.
# Will create a new file (same format as original, with other tracks copied, including subs)
# and rename the original with a .bak extension.

set -o nounset
set -o errexit
trap 'echo "Aborting due to errexit on line $LINENO. Exit code: $?" >&2' ERR
set -o errtrace
set -o pipefail
IFS=$'\n\t'

shopt -s nullglob

###############################################################################
# Program Functions
###############################################################################

_add_track_for_all_files() {
  FILES=("$@")
  for f in "${FILES[@]}"
  do
    echo "Processing $f..."
    BACKUP_FILENAME="$f.bak"
    # create backup copy
    cp "$f" "$BACKUP_FILENAME"

    TEMP_FILENAME="stereo.$f"
    ffmpeg -i "$f" -y -ignore_unknown -map 0:v -c:v copy \
      -map "0:a:0?" -c:a:0 copy \
      -map "0:a:0?" -c:a:1 aac -b:a:1 192k -ac 2 -metadata:s:a:1 title="2.0 Stereo" \
      -map "0:a:1?" -c:a:2 copy \
      -map "0:a:2?" -c:a:3 copy \
      -map "0:a:3?" -c:a:4 copy \
      -map "0:a:4?" -c:a:5 copy \
      -map "0:a:5?" -map "0:s?" -c:s copy \
      "$TEMP_FILENAME"

    echo "Deleting original file and replacing it with processed version..."
    rm "$f"
    mv "$TEMP_FILENAME" "$f"
    echo "====================================================="
    echo "Finished processing $f"
    echo "====================================================="
  done
}

###############################################################################
# Main
###############################################################################

# Description:
#   Entry point for the program, handling basic option parsing and dispatching.
_main() {
  _add_track_for_all_files "$@"
}

# Call `_main` after everything has been defined.
_main "$@"