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 <[email protected]>
#
# 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 [-bBITRATE]

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

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

# cleanup actions
cleanup() {
    if [ -f "$artfile" ]; then
        rm "$artfile"
    fi
}

# check script usage
[ -z "$1" ] && echo "File path is required." && exit 2;

# parse args
if [ ! -z "$2" ] && [ "${2:0:2}" == "-b" ]; then
    br="$(sed 's/[^0-9]//g' <<< $2)"
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
    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=$LYRICS" \
    "$f" "$wd/${basefilename/%flac/opus}"

    # 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

Nice but why not use ffmpeg?