A script that encodes mp3 audiobook directory into a single audio file with lowest but usable quality using either mp3, opus or speex codec

reencode_audiobook.sh

#!/bin/bash

set -ex

# fail function to exit with error message
fail() {
    echo "$@" 1>&2
    exit 1
}

# check that input directory and output file are specified
test -d "$1" || fail "No valid input directory specified"
test -n "$2" || fail "No valid output file specified"

# set input directory and output file variables
inf="$1"
outf="$2"

# create temporary file list
tmpf=$(mktemp)

# find all MP3 files in input directory and add them to temporary file list
find "$inf" -type f -regex '.*\.mp3$' | sort | xargs -I{} echo file \'$(pwd)/{}\' > "$tmpf"

# check if temporary file list is empty
if [ -s "$tmpf" ]; then
    # check output file extension
    ext="${outf##*.}"
    case "$ext" in
        mp3)
            # re-encode MP3 files and combine them into single file using ffmpeg
            outopt="-ac 1 -ar 8000 -c:a libmp3lame -b:a 8k -q:a 1"
            ;;
        opus)
            # re-encode MP3 files and combine them into single file using ffmpeg, converting to opus format
            outopt="-ac 1 -ar 8000 -c:a libopus -b:a 8k"
            ;;
        spx)
            # re-encode MP3 files and combine them into single file using ffmpeg, converting to speex format
            outopt="-ac 1 -ar 8000 -c:a libspeex -q:a 1"
            ;;
        *)
            fail "Unsupported output file extension"
            ;;
    esac
    ffmpeg -f concat -safe 0 -i "$tmpf" $outopt "$outf"
else
    fail "No MP3 files found in input directory"
fi

# remove temporary file list
rm "$tmpf"