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µTunes

µTunes (utunes; pronounced "micro-tunes") is a music library manager and music player which makes aggressive design decisions in order to simplify its implementation. Users are expected to perform some scripting to access advanced functionality. Supported use cases include:

  • Import an existing music library, including metadata embedded in the files.
  • Search and sort your music library by song, album, artist, or other metadata fields.
  • Create, edit, and delete playlists.
  • Play and pause music from an "up next" queue.
  • Organize music files on disk, and update embedded metadata.
  • Maintain a directory of album artwork.

Architecture

Conceptually, µTunes is cleanly separated into three parts:

  • Music library. Your media files on disk, a JSON file describing their metadata and playlists, and a JSON file describing saved playback state (used to restart playback when the µTunes playback server is stopped).
  • Command-line tool. Python program which provides a minimal, UNIX-style command-line interface to manipulate the music library and a transparently managed background playback server.
  • Emacs frontend. Emacs Lisp package which provides minimal user commands that use the command-line tool to implement helpful abstraction for music library management (e.g. "edit an album").

These three components are described in the next three sections.

Music library

µTunes identifies a directory as a music library by the presence of a file called utunes.json, in the same manner that Git identifies a repository by the presence of a file or directory called .git.

Instead of relying on µTunes' automatic library detection, you can export the environment variable UTUNES_LIBRARY to a directory to force it to be recognized as a music library. This is necessary in order to initialize a music library for the first time, since utunes.json does not yet exist.

The filesystem layout of a music library is as follows:

.
|
|-- utunes.json
|
|-- music
|   |
|   |-- <album>
|   |   |
|   |   |-- <song>.mp3
|   |   |
|   |   |-- ...
|   |
|   |-- ...
|
|-- playback.json

The file utunes.json is structured as follows:

{
  "version": 1,
  "songs": {
    "60c223fb": {
      "title": "Speed of Sound",
      "album": "X&Y",
      "artist": "Coldplay",
      "track": "6",
      "disc": "1",
      "artwork": "x&y.png",
      ...
    },
    ...
  },
  "playlists": [
    {
      "Up Next": [
        "60c223fb",
        ...
      ],
      ...
    }
  ]
}

The version key will be incremented every time breaking changes are made to the music library format.

Songs may have arbitrary keys and values, as long as both are non-empty, but the id and filename keys are required. The id key is a eight-character hexadecimal string guaranteed to be unique within the music library. The filename key is the path to the media file, relative to the music directory. Some of the keys, if present, are used in automatically generated filenames for song files: album, title, disc, track, and id.

After some normalization is performed on special characters to make them play nicely with the filesystem, the format for song filenames is essentially music/{album}/{disc:02d}-{track:03d}-{id} {title}.mp3.

The file playback.json has the following format:

{
  "playlist": "Up Next",
  "index": 5,
  "seek": 140
}

The playlist index is one-based, and the approximate seek position is in seconds.

Command-line tool

The µTunes command-line interface exposes several subcommands, which are described in the following sub-sections.

µTunes recognizes one environment variable, UTUNES_LIBRARY. If it is set, then µTunes will use that directory as its music library. Otherwise, µTunes will look at the working directory and its parents, and it will identify the music library directory by the presence of the file utunes.json.

read

$ utunes read FORMAT
    [-f, --filter   FIELD=REGEX]...
    [-s, --sort   Q:FIELD      ]...
    [-i, --illegal-chars  REGEX]

This subcommand writes to stdout. By default, all songs are printed. For each song, the provided format string is passed to Python's str.format with the song's key/value pairs available as keyword arguments (e.g. {id} expands to 60c223fb). Generally, the format string is expected to end with a newline.

The --filter argument can be used to limit output. The field can be any key; songs which are missing that key or whose value for that key does not match the provided regex (with a match spanning the full value) are skipped. For example, to list a particular album, consider --filter album="X&Y". Any number of filters are allowed, and song objects must match all of them to be listed.

Aside from the keys present on the song objects themselves, one additional field is allowed in filters: playlist. If you filter by playlist, then in order for a song to be listed, it must be in a playlist at an index such that the provided regex matches the string PLAYLIST:INDEX. (You cannot use playlist in the format string.)

The --sort argument can be used to order the list. The qualifier Q may be either s (sort normally; S for numeric), r (sort in reverse; R for numeric), or x (sort randomly). Sorts are applied stably and from right to left.

The --illegal-chars argument can be used to ensure that the output is machine-parseable. If any field substituted into the format string contains any of the characters specified, then an error is reported and the command terminates.

write

$ utunes write FORMAT [PLAYLIST]

This subcommand reads from stdin. The input must be a sequence of strings which match the provided Python-style regular expression, with no delimiter. (You will probably want to include a trailing newline in the regex.) The regex must include named capture groups whose names correspond to keys in a song object (e.g. (?P<album>...)). If there is a format specifier for id (e.g. (?P<id>)), then the other fields will be updated in the library database as directed, if needed, and song files will be renamed accordingly. Otherwise, there must be a format specifier for filename, and the song will be imported into the library database, with the file renamed appropriately from the given filename. If the value for a capture group is empty, then the key is removed from the song. This is not allowed for the id field. If the special field delete is non-empty, then songs are removed from the library database and their files are moved to the trash subdirectory of the library directory (next to music, with the same subdirectory structure).

If you provide a playlist name, then the given playlist is overwritten with the provided songs in the given order.

playback

$ utunes playback

This subcommand reads from stdin and writes to stdout, both in JSON format. Both the input and output JSON look like this, except that for the input JSON any or all of the keys may be omitted, and the seek_end key must be omitted:

{
  "playlist": "Up Next",
  "index": 1,
  "seek": 140,
  "seek_end": 288,
  "playing": true
}

If the playback server is not running, µTunes starts it first. Then, µTunes makes all of the changes described in the input JSON, including: switching playlist, seeking to a particular (one-based) index in the playlist, seeking to a particular offset within the song (in seconds), and playing or pausing. The new state of the playback server is returned as the output JSON. Thus, querying the playback server can be done by passing an empty map as the input JSON.

The response may have a null value for the playlist, index, seek, and/or seek_end keys if these parameters have not yet been set, or if data is unavailable.

Emacs frontend

Interactive commands provided by the Emacs frontend are:

  • M-x utunes-import-dir
  • M-x utunes-read
    • M-x utunes-read-album
    • M-x utunes-read-playlist
  • [C-u] M-x utunes-write
  • M-x utunes-playback
    • M-x utunes-playback-toggle
    • [C-u] M-x utunes-seek-previous
    • [C-u] M-x utunes-seek-current
    • [C-u] M-x utunes-seek-next

Installation

Playback requires installing MPV.

The backend may be installed as a standard Python package from this GitHub repository. It is recommended to use pipx:

$ pipx install utunes --spec git+https://github.com/raxod502/utunes.git

The frontend may be installed as a standard Emacs Lisp package from this GitHub repository. It is recommended to use straight.el:

(straight-use-package '(utunes :host github :repo "raxod502/utunes"))

To make sure the packages remain in sync, I recommend installing the Emacs package first and then running:

$ pipx install utunes -e --spec ~/.emacs.d/straight/repos/utunes

(This last command currently suffers from a bug in Pip.)

Todo

In order of priority.

  • Emacs frontend.
  • Generic importer.
  • Removing songs.
  • Play statistics.
  • Better documentation.

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