Mass .flac --> .mp3 transcoding: How to write a shell script that preserves ID3 tag information?

Question

In recent weeks I've gone from a fairly 'hands-on' approach to .flac --> .mp3 transcoding, to one that's far more 'set & forget'.

The first step was to stop using a GUI front end (Audacity with a LAME plug-in) and instead use the method I outlined here.

The second step was to find a bash shell script that would tell that command loop to work recursively, allowing directories with many subdirectories containing .flac files to be transcoded in one simple step. That answer was provided by a user at askubuntu.com.

Now I wish to learn how to further refine things so that ID3 tag information is preserved. The methods linked to above strip ID3 tag data, leaving the bare minimum (i.e. only the title field remains).

Can anyone teach me how to write such a shell script?

---

The shell script has been updated thus:

#!/bin/bash
file="$1"
flac -cd "$file" | lame --preset fast extreme - "${file%.flac}.mp3"
id3cp "$file" "${file%.flac}.mp3"

Doing find . -name '*.flac' -exec ~/bin/flac2mp3 '{}' \; in ~/Desktop/stack gives the following output:

01 - Amon Tobin - Chomp Samba.flac: done         
LAME 3.98.4 64bits (http://www.mp3dev.org/)
Using polyphase lowpass filter, transition band: 19383 Hz - 19916 Hz
Encoding <stdin> to ./01 - Amon Tobin - Chomp Samba.mp3
Encoding as 44.1 kHz j-stereo MPEG-1 Layer III VBR(q=0)
Parsing ./01 - Amon Tobin - Chomp Samba.flac: done.  Copying to ./01 - Amon Tobin - Chomp Samba.mp3: done

id3info for the original .flac and resultant .mp3 gives, respectively:

*** Tag information for 01 - Amon Tobin - Chomp Samba.flac

(i.e. nothing);

*** Tag information for 01 - Amon Tobin - Chomp Samba.mp3
*** mp3 info
MPEG1/layer III
Bitrate: 128KBps
Frequency: 44KHz

The .flac definitely has tag information. I can verify this by opening up EasyTAG. EasyTAG refers to this as 'FLAC Vorbis Tag' but 'ID3 Tag' for the .mp3. Is this the problem?

Answer 1

#!/bin/sh

file="$1"
outfile=${file%.flac}.mp3

eval $(metaflac --export-tags-to - "$file" | sed "s/=\(.*\)/='\1'/")

flac -cd "$file" | lame --preset fast extreme \
        --add-id3v2 --tt "$TITLE" --ta "$ARTIST" --tl "$ALBUM" \
        --ty "$DATE" --tn "$TRACKNUMBER" --tg "$GENRE" \
        - "$outfile"

Answer 2

How to use mp3fs to mass encode flac to mp3

Instead of using a program to convert flac files to mp3, you can use mp3fs to present them as virtual mp3's, and a program that can queue copy jobs. It works wonderfully well for me.

Download and install mp3fs and ultracopier:

sudo apt-get install mp3fs ultracopier

Create a pair of flac and mp3 directories for each mp3fs you want to run. I have a dual core cpu so I run two. If you keep the flac directories on the partition where your flac files are, you can quickly move those. That partition is called "Storage" on my system.

cd /media/Storage
mkdir mp3fs mp3fs/flac_1 mp3fs/mp3_1 mp3fs/flac_2 mp3fs/mp3_2

You can mount the directories from the command line or by adding them to /etc/fstab. You can also configure the bitrate and encoding algorithm of the virtual mp3's at mount time. I want mp3's with a constant bitrate of 320kbps, and lame may take all the time it needs to produce the best mp3 it possibly can.

On the commandline:

mp3fs -b 320 -oquality=0 /media/Storage/mp3fs/flac_1 /media/Storage/mp3fs/mp3_1 -o allow_other,ro
mp3fs -b 320 -oquality=0 /media/Storage/mp3fs/flac_2 /media/Storage/mp3fs/mp3_2 -o allow_other,ro

If fuse complains, like

fusermount: failed to open /etc/fuse.conf: Permission denied
fusermount: option allow_other only allowed if 'user_allow_other' is set in etc/fuse.conf

then uncomment the user_allow_other option in /etc/fuse.conf, and/or make it readable with

sudo chmod 644 /etc/fuse.conf

Or from /etc/fstab:

mp3fs#/media/Storage/mp3fs/flac_1 /media/Storage/mp3fs/mp3_1 fuse user,ro,allow_other,bitrate=320,quality=0 0 0
mp3fs#/media/Storage/mp3fs/flac_2 /media/Storage/mp3fs/mp3_2 fuse user,ro,allow_other,bitrate=320,quality=0 0 0

After adding the directories to /etc/fstab, you can mount them by using

sudo mount -a

Done! Start ultracopier. It'll place an indicator icon in your gnome panel or whatever it is that you use. Click it, select add copy. I did that twice to use two cores. Use your file browser to move some flacs into flac_1. Navigate to the mp3_1 directory, and drag your mp3's to one of the ultracopier windows to start a copy queue. Ultracopier will ask for the destination of the files. You now have one core busy encoding a batch of flacs to mp3.

To use the other core, move flac files to flac_2, and drag the mp3's from mp3_2 to the second ultracopier window.

Update: Instead of ultracopier I'm now using MiniCopier, it's a little easier to use. See comments below.

Answer 3

id3cp can not work since FLAC does not use ID3; it uses Vorbis tags instead.

ffmpeg has an option to maintain tags through conversion. It's kind of fickle, its value depending on the combination of codecs, but it works like this in your case (version 3.4.8):

ffmpeg -i in.flac -map_metadata 0:g:0 out.mp3

Applying this all FLAC files in a folder (recursively) is as easy as this: (using bash)

for f in */**/*.flac; do
  ffmpeg -i "${f}" -map_metadata 0:g:0 "${f%.flac}.mp3";
done

Note that ffmpeg uses lame automatically; further options exist to control quality and whatnot. Check man ffmpeg for detail.

Answer 4

Raphael's answer helped me a lot. However to make things work with my set of FLAC files, I had to manually adjust the streams, so that the tags were preserved correctly and audio was the first stream in the mp3 file. I ended up with this line:

$nohup bash -c 'for f in */**/*.flac; do ffmpeg -i "${f}" -map 0:a -map 0:v:0? -map 0:v:1? -c:v copy -codec:a libmp3lame -qscale:a 2 -y "${f%.flac}.mp3"; done' 2>&1 > log.txt &

The question marks allow the related streams to be missing. The qscale parameter defines the mp3 encoding quality of the lame encoder library used within ffmepg.