Automatically tagging audio files (using systemd and inotify)

I follow several podcasts and, if you do the same, you may have noticed that podcast creators are terrible at consistently tagging their files. For example, is the artist the name of the podcast, the names of the presenters, just one of the presenters, some abbreviation, or the names of the presenters in a different order? Probably all of those, and more, get used inconsistently across the lifetime of a multi-year-old podcast.

Inconsistent tagging makes it a pain to use tools which use that information, which is pretty much every audio player.

For some years, my solution was a script which retagged all my podcasts. This can be done because I use a standard directory and file naming convention. But the downside is that it retagged every file of every podcast when ran, even though I’d only be adding one new file at a time.

systemd path units

I recently discovered systemd path units, which seemed like the solution to this problem: I could have a script which was triggered by a file being created, tagged it, and moved it to the right place. Path units turned out not to be the solution to this problem, but they were a solution to a slightly different one.

My first attempt was to add a subdirectory to every podcast directory called in, and to write this path unit:

Description=Automatically tag new podcast files


And this service file:



And this bash script:

#!/usr/bin/env bash

for mp3file in */in/*.mp3; do
  dir="$(echo "$mp3file" | sed 's:/in/.*::')"
  f="$(basename "$mp3file")"

  artist="$(echo "$dir" | sed 's: - .*::')"
  album="$(echo "$dir" | sed 's:.* - ::')"

  if [[ -z "$album" ]]; then

  n="$(echo "$f" | sed 's:\..*::')"
  track="$(echo "$f" | sed 's:^[0-9]*\. \(.*\)\.mp3:\1:')"

  echo "===== $mp3file" >&2
  echo $artist >&2
  echo $album >&2
  echo $n >&2
  echo $track >&2
  echo "$(echo "$mp3file" | sed 's:/in/:/:')" >&2
  echo >&2

  id3v2 -D "$mp3file"
  id3v2 -2 --song   "$track"  "$mp3file"
  id3v2 -2 --track  "$n"      "$mp3file"
  id3v2 -2 --artist "$artist" "$mp3file"
  id3v2 -2 --album  "$album"  "$mp3file"
  mv "$mp3file" "$(echo "$mp3file" | sed 's:/in/:/:')"

This turned out not to work. The unit just didn’t pick up any file changes. Any one podcast would work1, but I didn’t really want to have to make a unit for each of my podcasts… I’d need to update my system configuration if I started following a new podcast; that feels like too much mixing of global configuration and how I (admittedly the single user of the system) use it.

So I had to give up on path units for tagging my podcasts.

Tagging podcasts

I was too invested at this point to give up entirely, I wanted automatic tagging.

So I turned to inotifywatch, and stuck this at the end of my script:

# this can't be done as a systemd path unit because it doesn't seem to
# support multiple *s in a pattern
inotifywait --recursive --timeout 3600 --include '/mnt/nas/music/Podcasts/.*/in/.*\.mp3' $(pwd) >&2

# this script is run in a loop by systemd.

The next step was to make a systemd unit which just runs that script in a loop. Which is defined in my NixOS config as: = {
  enable = true;
  description = "Automatically tag new podcast files";
  wantedBy = [""];
  path = with pkgs; [ inotifyTools id3v2 ];
  unitConfig.RequiresMountsFor = "/mnt/nas";
  serviceConfig = {
    WorkingDirectory = "/mnt/nas/music/Podcasts/";
    ExecStart = pkgs.writeShellScript "" (fileContents ./;
    User = "barrucadu";
    Group = "users";
    Restart = "always";

And now I’ve got a script which, once an hour (or on detecting a file change, whichever is sooner) tags all new podcast files and moves them to the correct directories. No more SSHing in and running my tagging script, I can just save a file as, eg, How We Roll - Masks of Nyarlathotep/in/{number}. {title}.mp3, over Samba or NFS, and within a few seconds it gets picked up, tagged, and organised. Nice.

Tagging albums

I couldn’t use a single path unit to trigger my script for tagging podcasts, but that’s not the only time I want to tag some audio files. I have a collection of CDs, which I very infrequently add to, and I have those CDs ripped and stored as FLAC files. Appropriately tagged, of course.

I use Exact Audio Copy (EAC) to rip my CDs to WAV, which uses a predictable directory layout and file naming convention. I already had a script to take an EAC directory and produce a tagged and organised FLAC directory, I just needed to make it automatic.

First, here’s my systemd configuration:

systemd.paths.flac-and-tag-album = {
  enable = true;
  description = "Automatically flac and tag new albums";
  wantedBy = [""];
  unitConfig.RequiresMountsFor = "/mnt/nas";
  pathConfig.PathExistsGlob = "/mnt/nas/music/to_convert/in/*";
}; = {
  path = with pkgs; [ flac ];
  serviceConfig = {
    WorkingDirectory = "/mnt/nas/music/to_convert/in/";
    ExecStart = pkgs.writeShellScript "" (fileContents ./;
    User = "barrucadu";
    Group = "users";

An album consists of multiple files, but I don’t want to try to convert an album where some of the files are still part-way through being copied to the NAS; that sounds like an easy way to end up with incomplete FLACs. So I came up with this workflow:

  1. A CD is ripped to WAV with EAC on my desktop
  2. The EAC directory is copied over to /mnt/nas/music/to_convert over Samba (which will take a few seconds)
  3. Then the directory moved to /mnt/nas/music/to_convert/in (which will be instantaneous)
  4. The path unit notices the new subdirectory, and triggers the script.

And here’s the script:

#!/usr/bin/env bash

set -e

for artist in *; do
  if [[ -d $artist ]]; then
    pushd $artist
    for album in *; do
      if [[ -d $album ]]; then
        echo "===== $artist - $album" >&2
        pushd $album
        if [[ ! -e "$artist - $album.log" ]]; then
          echo "(missing log file)" >&2
        if [[ ! -e "cover.jpg" ]] && [[ ! -e "cover.png" ]] && [[ ! -e "cover.gif" ]]; then
          echo "(missing cover file)" >&2
        flac *.wav
        rm *.wav
        for flacfile in *.flac; do
          n="$(echo "$flacfile" | sed 's:\..*::')"
          track="$(echo "$flacfile" | sed 's:^[0-9]*\. \(.*\)\.flac:\1:')"
          metaflac --set-tag="tracknumber=$n" "$flacfile"
          metaflac --set-tag="title=$track"   "$flacfile"
          metaflac --set-tag="artist=$artist" "$flacfile"
          metaflac --set-tag="album=$album"   "$flacfile"
        mv $album "../../out/$artist - $album"
    rmdir $artist

Nice and straightforward. I’ve not ripped any new CDs since setting this up earlier this week, but I converted some FLACs back to WAVs, shuffled the directory layout around, and tested that they got picked up and re-converted properly.

  1. For example, if I changed the PathExistsGlob to something like /mnt/nas/music/Podcasts/The H. P. Lovecraft Literary Podcast/in/*.mp3↩︎