Run this from a local folder containing whatever you want to go on the Shokz OpenSwim

format-openswim.sh

#!/bin/bash

# Shokz OpenSwim headphones (formerly AfterShokz Xtrainerz) have a bug where
# tracks are not played in alphabetical or track order. They are played in the
# order that they were copied to the device. Read more about the issue here:
# 
# - https://en.help.shokz.com/s/article/How-to-list-the-track-order-on-OpenSwim-formerly-Xtrainerz-OPENSWIM1408
# - https://en.help.shokz.com/s/get-article?urlName=how-to-list-tracks-order-EN
#
# After extensive testing, it seems that specifying transmission order alone is
# not enough to get tracks playing in order. Otherwise, the following command
# would work:
#
# rsync -avr --delete --exclude=".*" --exclude="System Volume Information" --exclude="*.DS_Store" ./ /Volumes/XTRAINERZ
#
# It seems that adding a delay between each file allows the device to process
# file order correctly. Filenames also seem to matter, so files are renamed to
# sequential numbers before copying.
#
# This script also removes all track metadata to save space, and adds audible
# folder titles to assist with navigation.
#
# Other things I tried that didn’t work:
#
# - Adding --bwlimit=1024 to rsync to slow down the transfer.
# - Adding track number metadata tags. Source: `add-track-numbers.sh`:
#     id3v2 -T "$((i + 1))/${#FILES[@]}" "${FILES[i]}"
# - Editing file modification and creation dates to match the track order:
#     # Set the modification date.
#     gtouch -d "+$i minutes" "/Volumes/XTRAINERZ/$d${FILES[i]}"
#     # Copy the modification date to the creation date.
#     SetFile -d "$(GetFileInfo -m "/Volumes/XTRAINERZ/$d${FILES[i]}")" "/Volumes/XTRAINERZ/$d${FILES[i]}"
# - Adding a random suffix to the end of each file name.
#     for f in *.mp3; do
#       mv "$f" "$(mktemp "${f%.mp3} - XXXX").mp3"
#     done
#     # mktemp creates some 0 byte files that we need to clean up.
#     find . -type f ! -name '*.mp3' -delete

rsync -Rr . _tmp
cd _tmp

for d in */; do
  if [ "$d" = "System Volume Information/" ]; then
    continue
  fi

  cd "$d"

  # Create title tracks in each directory. Source: `mp3titles.sh`.
  say "$d" -o "00. Title.aiff"
  lame -S -m m "00. Title.aiff" "00. Title.mp3"
  rm "00. Title.aiff"

  # Remove all track metadata.
  id3v2 -D *.mp3

  # Rename all files to sequential numbers.
  ls -1p | grep -v "/$" | cat -n | while read n f; do mv -n "${f}" "$(printf "%03d" $n).mp3"; done

  FILES=(*.mp3)
  for i in "${!FILES[@]}"; do
    echo -e "Copying from $d [$(($i + 1))/${#FILES[@]}]: \t${FILES[i]}"
    # Copy the single file to the USB drive.
    rsync -a --delete "${FILES[i]}" "/Volumes/XTRAINERZ/$d"
    sleep 0.5
  done

  cd ..
done

cd ..
rm -rf _tmp

Turns out that songs play in the order they were loaded onto the device: https://en.help.shokz.com/s/article/How-to-list-the-track-order-on-OpenSwim-formerly-Xtrainerz-OPENSWIM1408

More info: https://en.help.shokz.com/s/article/How-to-list-the-track-order-on-OpenSwim-formerly-Xtrainerz-OPENSWIM1408

Note: controlling load order doesn’t necessarily ensure proper song order. The way to be 100% sure is to use FATSort.