Bash script to scan input video for commentary tracks and output `transcode-video` options to add those tracks.

commentary-tracks.sh

#!/bin/bash
#
# commentary-tracks.sh
#
# Copyright (c) 2013-2019 Don Melton
#

die() {
    echo "$(basename "$0"): $1" >&2
    exit ${2:-1}
}

readonly input="$1"

if [ ! "$input" ]; then
    die 'missing argument'
fi

if [ ! -f "$input" ]; then
    die "no such file: $input"
fi

last_track_name=''

echo "$(HandBrakeCLI --scan --previews 2:0 --input "$input" 2>&1)" |
    sed -n '/^    Stream #0[.:][0-9]\{1,\}[^ ]* Audio: /,$p' |
    sed '/^[^ ]\{1,\}.*$/,$d' |
    sed '/^    Stream #0[.:][0-9]\{1,\}[^ ]* Subtitle: /,$d' |
    sed '/^    Metadata:$/d' |
    sed -e :a -e '$!N;s/\n     / /;ta' -e 'P;D' |
    sed -n '/^    Stream #[^(]\{1,\}(eng): Audio: .* Commentary/p' |
    sed 's/^    Stream #[0-9]\{1,\}[.:]\([0-9]\{1,\}\).* \{1,\}title \{1,\}: \(.*Commentary\).*$/\1=\2/' |

while read line; do
    track_name="$(echo "$line" | sed 's/^[0-9]\{1,\}=//')"

    if [ "$track_name" == "$last_track_name" ]; then
        continue
    fi

    last_track_name="$track_name"
    echo "$line"
done