mp3-convert

This is a script I made to convert my music library to MP3.

usage: ./convert.py [--dry-run] [--no-remove] [--keep-threshold <keep>] [--default-keep <keep>] [--prometheus-metrics] [--metrics-path <dir>] <source> <destination>

The idea is that we have a bunch of audio files organised in a directory, for example like this :

source_directory
 +- Artist1
 |   +- Album1
 |   |   +- music1.flac
 |   |   +- music2.flac
 |   |   +- album.png
 |   +- Album2
 |   |   +- music1.flac
 |   |   +- music2.flac
 |   |   +- album.jpg
 +- Artist2
 |   +- Album1
 |   |   +- music1.m4a
 |   |   +- music2.m4a
 |   |   +- music3.m4a
 +- random_song.mp3

Note: the folder depth doesn't really matter, audio files can be found in any directory under the source directory, as long as they have a valid extension.

This script can then be used to convert all the files to MP3, while retaining the folder structure :

destination_directory
 +- Artist1
 |   +- Album1
 |   |   +- music1.mp3
 |   |   +- music2.mp3
 |   +- Album2
 |   |   +- music1.mp3
 |   |   +- music2.mp3
 +- Artist2
 |   +- Album1
 |   |   +- music1.mp3
 |   |   +- music2.mp3
 |   |   +- music3.mp3
 +- random_song.mp3

This conversion typically takes a substantial amount of time for big libraries. In order to avoid doing unnecessary work, the musics will only be converted if there is no equivalent in the destination directory with a higher modification date. Also, the musics that are already in MP3 are directly copied. Hence, when changes are made to the source directory, one simply has to call the script again so that these changes are transposed to the MP3 library. If a music file in the destination directory has no counterpart in the source directory, it will be removed by default (we consider that it was there before but has been removed since the last conversion). This can be prevented with the --no-remove argument.

Any file that is not recognised as being an audio file will not be taken into account at all.

For the conversion, the script uses ffmpeg, which must be installed on the system. So far, the script has only be tested on a Linux-based OS.

Edge cases that are currently not taken into account :

• node is a folder in the source tree but a file in the destination tree (or the other way around)
• filesystem naming errors in the destination folder (e.g. source FS is EXT4, destination FS is NTFS)

Thresholds

Music hoarders can be faced with a dilemma : on one hand, we would like to keep all the tracks from the original album, but on the other hand we would also like to actually use the music library in our daily lives. The problem is that sometimes albums include tracks that we are not interested in listening to, such as instrumentals, short versions, remixes, ... We don't really have an interest in keeping a copy of those and they can be quite annoying when we just want to play a random music from the library. The threshold concept is an attempt to reconcile both these worlds.

The idea is that we are going to add a custom Convert-Keep tag to the audio files in the origin directory. The possible values are :

• always/keep: kept all the time
• bonus: good to have
• never/skip: do not keep

We can then pass the desired level via the --keep-threshold command-line option. Any file whose level is strictly under the desired level will be considered as inexisting by the script. This is to say that it will not be converted/copied and it will be removed from the destination directory (if it is allowed). For files that do not have the tag, the default value is always but it can be changed via the --default-keep command-line option.

Warning: The code used for parsing MP3/M4A/FLAC tags was only tested with metadata written via kid3, there might still be edge cases that are not taken into account.

The main downside with this method is that it requires modifying the source files, which is incompatible with torrent seeding (as an example).

Prometheus metrics

In order to monitor what the script does when it is executed periodically in a crontab, Prometheus metrics can be written to a file. This is intended to be used with the textfile collector from the Prometheus Node Exporter. This is disabled by default and must be enabled with --prometheus-metrics. By default, the file is written in /var/lib/node_exporter/textfile_collector. This can be changed using the --metrics-path command-line option.