This is a script to convert all flac files in a given directory to ogg/opus. I wrote this because mp3 kinda sucks and opus is a free format that is pretty much taking over the internet.

flac2opus

#!/bin/bash

# Copyright 2017 Keefer Rourke <mail@krourke.org>
#
# Permission to use, copy, modify, and/or distribute this software for any
# purpose with or without fee is hereby granted, provided that the above
# copyright notice and this permission notice appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCNUMBERLAIMS ALL WARRANTIES WITH
# REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
# AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
# INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
# LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
# OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
# PERFORMANCE OF THIS SOFTWARE.

# usage: flac2opus PATH [-b BITRATE] [ -p <path to cover image>]

which metaflac >> /dev/null
if [ $? -ne 0 ]; then
    echo "flac tools are not installed. Run \`sudo dnf install flac\` and try again."
    exit 1
fi
which opusenc >> /dev/null
if [ $? -ne 0 ]; then
    echo "opus-tools is not installed. Run \`sudo dnf install opus-tools\` and try again"
    exit 1
fi

# check script usage
if [ -z "$1" ]; then
    echo "File path is required." && exit 2;
else
    [ ! -d "$1" ] && echo "File path is required." && exit 2;
fi

# "global" variables
wd="$PWD"
br=256
artfile="$1/.tempart.img"
cleanup=0
hasart=0
coverspecified=0

POSITIONAL=()
while [[ $# -gt 0 ]]
do
key="$1"

case $key in
    -b|--bitrate)
    br="$(sed 's/[^0-9]//g' <<< $2)"
    shift # past argument
    shift # past value
    ;;
    -p|--picture)
    coverspecified=1
    hasart=1
    cover="$2"
    shift # past argument
    shift # past value
    ;;
    *)    # unknown option
    POSITIONAL+=("$1") # save it in an array for later
    shift # past argument
    ;;
esac
done
set -- "${POSITIONAL[@]}" # restore positional parameters
# cleanup actions
cleanup() {
    if [ -f "$artfile" ]; then
        rm "$artfile"
    fi
}
# parse each flac file in the specified directory and output to opus format
for f in "$1"/*.flac; do
    # give output correct extension
    basefilename=${f##*/}
    # export metadata; I don't necessarily trust or want every single tag in
    # the exported file, so let's grab the relevant tags and make opus ignore
    # extraneous data
    TITLE=$(metaflac "$f" --show-tag=TITLE | sed s/.*=//g)
    ARTIST=$(metaflac "$f" --show-tag=ARTIST | sed s/.*=//g)
    ALBUMARTIST=$(metaflac "$f" --show-tag=ALBUMARTIST | sed s/.*=//g)
    ALBUM=$(metaflac "$f" --show-tag=ALBUM | sed s/.*=//g)
    DATE=$(metaflac  "$f" --show-tag=DATE | sed s/.*=//g)
    GENRE=$(metaflac "$f" --show-tag=GENRE | sed s/.*=//g)
    DISCNUMBER=$(metaflac "$f" --show-tag=DISCNUMBER | sed s/.*=//g)
    TRACKNUMBER=$(metaflac "$f" --show-tag=TRACKNUMBER | sed s/.*=//g)
    TRACKTOTAL=$(metaflac "$f" --show-tag=TRACKTOTAL | sed s/.*=//g)
    LYRICS=$(metaflac "$f" --show-tag=LYRICS | sed s/.*=//g)
    
    # for the curious I suppose
    #metaflac "$f" --export-tags-to=-
    # if we don't already have an art file, let's try to extract one from the
    # current flac file
    if [ $hasart -eq 0 ]; then
        metaflac "$f" --export-picture-to="$artfile"
        hasart=1
        cleanup=1
    fi
    # convert flac via opusenc and tag the new file; pictures should be copied
    # automagically if $coverspecified isn't set.
    if [ $coverspecified -eq 1 ]; then
        opusenc --vbr --bitrate "$br" \
        --discard-comments --date "$DATE" \
        --title "$TITLE" --artist "$ARTIST" --album "$ALBUM" --genre "$GENRE" \
        --comment "ALBUMARTIST=$ALBUMARTIST" --comment "DISCNUMBER=$DISCNUMBER" \
        --comment "TRACKNUMBER=$TRACKNUMBER" --comment "TRACKTOTAL=$TRACKTOTAL" \
        --comment "LYRICS=$LYRICS" \
        --picture "$cover" \
        "$f" "$wd/${basefilename/%flac/ogg}"

    else
        # opusenc --vbr --bitrate "$br" --date "$DATE" \
        # --title "$TITLE" --artist "$ARTIST" --album "$ALBUM" --genre "$GENRE" \
        # --comment "ALBUMARTIST=$ALBUMARTIST" --comment "DISCNUMBER=$DISCNUMBER" \
        # --comment "TRACKNUMBER=$TRACKNUMBER" --comment "TRACKTOTAL=$TRACKTOTAL" \
        # --comment "LYRICS=$LYRICS" \
        # "$f" "$wd/${basefilename/%flac/opus}"
        opusenc --vbr --bitrate "$br" \
        "$f" "$wd/${basefilename/%flac/opus}"
    fi
    # cleanup and exit if failure
    if [ $? -ne 0 ]; then
        cleanup
        exit 1
    fi
done
# move the temp artfile to destination with meaningful extension
if [ -f "$artfile" ]; then
    mime=$(file -b --mime-type "$artfile" | sed 's/.*\///g')
    cp "$artfile" "$wd/front.$mime"
fi
cleanup
exit 0

Found another issue with some FLAC files.
FLAC files are not supposed to contain ID3 tags. This breaks compatibility with opusenc. (Opusenc won't be able to parse the file)

If the FLAC file that have ID3 tags can be found out if the output is "ID3" after running this:

    dd if=test.flac of=/dev/stdout bs=1 count=3 2>/dev/null

Source

I need to try adding it to the script to prevent errors.

Experimental script to convert:

for input in *.flac; do
        temp=$(basename "$input".flac)
        metaflac --export-tags-to="$temp".metaflac "$input"
        id3v2 -D "$input"
        metaflac --import-tags-from="$temp".metaflac "$input"
        rm -v "$temp".metaflac
done

Update 18/11/17 :

If -p is specified, set hasart=1 so that unwanted artwork won't get copied

Awesome. Happy to see this fork! Thanks for fixing some of things I was too lazy to do :)

Hello, I wanted to share my version with you.
This version is multi-threaded and can strip ID3 tags while preserving metadata of flac files.
Though, in this one, metadata is copied "AS IS" because I like to keep all Music Brainz tags.
It can also delete opus files if present before encoding.
And the output folder is passed as an argument to the script (and only if required by the chosen actions).
-h option explains everything.

flac2opus.sh

I am not a very good bash script writer, I think that it can be simplified or better written but, at least, for me, everything works great.

@berturion Thanks!