FFmpeg Batch Transcode Audio

Recently I have been dealing with transcoding media files for my streaming site travisflix and performing ad-hoc media conversions is no longer realistic. I strongly recommend checking out my FFmpeg Examples to give you a head start if you’re not familiar with deep diving in FFmpeg details.

Before you choose to batch transcode the audio for your entire library, the following will stdout anything not using the AAC codec:

find . -mount -depth -maxdepth 2 -regextype posix-extended -regex '.*mkv$|.*mp4$' -type f -exec bash -c 'echo "{}" $(ffprobe -loglevel error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 "{}")' 2>/dev/null \; | grep -v 'aac$'

# Same command but multiple lines
find . -mount -depth -maxdepth 2 -regextype posix-extended -regex '.*mkv$|.*mp4$' -type f -exec bash -c \
'echo "{}" $(ffprobe -loglevel error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 "{}")' 2>/dev/null \; | grep -v 'aac$'

 

The following script will look in $TEMPDIR for mkv/mp4 files not using the AAC audio codec, and if found, transcodes them into $WORKDIR then moves the final output file back to the original source, overwriting the original file. Test functionality first before running on a large batch.

ffmpeg-transcode-acodec.sh

#!/usr/bin/env bash
#
## DESCRIPTION: Transcode with FFmpeg all (mkv, mp4) media files in specified directory,
##              overwriting the original source files.
##
## AUTHOR:  Travis Runyard
## Revised: 04/18/2020
## URL:     https://sysinfo.io/ffmpeg-batch-transcode-audio/

# Exit on first non-zero exit code
set -e
# Set shell options
shopt -s globstar
shopt -u nullglob
# Declare variables (optionally) are indexed arrays
# Only associative arrays require declaration, but standardizing is a good thing
declare -a FILENAME
declare -a LOG

# Modify TEMPDIR and WORKDIR to suit your needs
TEMPDIR="/mnt/pool0/p0ds0smb/temp/ffmpeg"
WORKDIR="$TEMPDIR/.working"
SCRIPT_NAME=$(basename "$0")

usage()
{
  cat 1>&2 <<EOF
Usage: "$SCRIPT_NAME" [options]

-h| --help         Help shows this message.
-d| --directory    Specify the directory to process.
-w| --workdir      Writable working directory for FFmpeg.
                   If you don't specify this option, the
                   subdirectory named "working" will be used
                   within the --directory path or \$TEMPDIR.
EOF
}

while [ "$1" != "" ]; do
    case $1 in
        -d | --directory )	shift
                                TEMPDIR="${1:-$TEMPDIR}"
                                WORKDIR="$WORKDIR"
                                ;;
        -w | --workdir )	shift
                                WORKDIR="${1:-$WORKDIR}"
                                ;;
        -h | --help )           usage
                                exit 0
                                ;;
        * )                     usage
                                exit 1
    esac
    shift
done

cd $TEMPDIR
echo -e "*** DEBUG ***\n\$TEMPDIR = $TEMPDIR\n\$WORKDIR = $WORKDIR"
[ -d "$WORKDIR" ] || mkdir "$WORKDIR"
# Set field separator to newline
OIFS="$IFS"
IFS=$'\n'
FILENAME=($(find . ! -path "*$(basename "$WORKDIR")/*" -regextype posix-extended -regex '.*\.(mkv|mp4)$' -type f -print))
if [ -n "$FILENAME" ]; then
  for f in "${FILENAME[@]}"; do
    LOG+=("$f")
    AUDIOFORMAT=($(ffprobe -loglevel error -select_streams a:0 -show_entries stream=codec_name,channels -of default=nw=1:nk=1 "$f"))
  	if ! [ "${AUDIOFORMAT[0]}" = "aac" ]; then
# Build ffmpeg array within the loop to populate variables
      args=(
      -nostdin
      -y
      -i "$f"
      -map 0:v:0
      -map 0:a
      -map 0:s?
      -ac "${AUDIOFORMAT[1]}"
      -ar 48000
      -metadata:s:a:0
      language=eng
      -c:v copy
      -c:a aac
      -c:s copy
      "$WORKDIR/$(basename "$f")"
      )
      echo -e "*** DEBUG: ffprobe detected '${AUDIOFORMAT[0]}' in the default audio stream with '${AUDIOFORMAT[1]}' channels\nPreparing to convert audio codec to AAC..." ; sleep 1
      echo -e "*** DEBUG: ffmpeg ${args[@]}"
      ffmpeg "${args[@]}" || break
      echo -e "*** DEBUG: Moving '$WORKDIR/$(basename "$f")' back to source directory name '$(dirname "$f")'"; sleep 1
      mv -ufv "$WORKDIR/$(basename "$f")" "$(dirname "$f")" || break
    fi
  done
  printf '*** DEBUG ***\nPROCESSED:%s\n'
  for i in "${LOG[@]}"; do echo "$i"; done
else
  echo -e "*** DEBUG: No files to process"
fi