Michael Smith
6209a087d7
Shell script to recombine individual story m4a files back into per-cassette files using ffmpeg concat demuxer (stream copy, no re-encoding). Generates chapter markers from input filenames and preserves album art. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
108 lines
2.4 KiB
Bash
Executable File
108 lines
2.4 KiB
Bash
Executable File
#!/usr/bin/env bash
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#
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# concat_cassette.sh — Losslessly concatenate m4a files into a single file
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# with chapter markers derived from input filenames.
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#
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# Usage:
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# ./tools/concat_cassette.sh -o "Cassette 1.m4a" story1.m4a story2.m4a story3.m4a
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#
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# Requires: ffmpeg, ffprobe
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set -euo pipefail
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usage() {
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echo "Usage: $0 -o OUTPUT INPUT1 INPUT2 [INPUT3 ...]"
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echo ""
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echo "Losslessly concatenate m4a files into a single file with chapter markers."
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echo "Album art is preserved from the first input file."
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echo ""
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echo "Options:"
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echo " -o OUTPUT Output filename (must not already exist)"
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exit 1
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}
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die() {
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echo "Error: $1" >&2
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exit 1
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}
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# --- Parse arguments ---
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output=""
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while getopts "o:h" opt; do
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case "$opt" in
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o) output="$OPTARG" ;;
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h) usage ;;
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*) usage ;;
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esac
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done
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shift $((OPTIND - 1))
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[ -z "$output" ] && die "Missing -o OUTPUT"
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[ $# -lt 2 ] && die "Need at least 2 input files"
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[ -e "$output" ] && die "Output file already exists: $output"
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for cmd in ffmpeg ffprobe; do
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command -v "$cmd" >/dev/null 2>&1 || die "$cmd not found"
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done
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inputs=("$@")
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for f in "${inputs[@]}"; do
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[ -f "$f" ] || die "Input file not found: $f"
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done
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# --- Set up temp dir ---
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tmpdir=$(mktemp -d)
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trap 'rm -rf "$tmpdir"' EXIT
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concat_list="$tmpdir/concat.txt"
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metadata="$tmpdir/metadata.txt"
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# --- Build concat list and probe durations ---
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echo ";FFMETADATA1" > "$metadata"
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cumulative_ms=0
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for f in "${inputs[@]}"; do
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# Concat list entry (absolute path for -safe 0)
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abs=$(cd "$(dirname "$f")" && pwd)/$(basename "$f")
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printf "file '%s'\n" "$abs" >> "$concat_list"
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# Probe duration
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dur=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$f")
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[ -z "$dur" ] && die "Could not read duration of: $f"
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dur_ms=$(awk "BEGIN { printf \"%d\", $dur * 1000 }")
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# Chapter title from filename (strip path and extension)
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title=$(basename "$f")
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title="${title%.*}"
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end_ms=$((cumulative_ms + dur_ms))
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cat >> "$metadata" <<EOF
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[CHAPTER]
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TIMEBASE=1/1000
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START=$cumulative_ms
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END=$end_ms
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title=$title
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EOF
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cumulative_ms=$end_ms
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done
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# --- Concatenate ---
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ffmpeg -y -f concat -safe 0 -i "$concat_list" -i "$metadata" \
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-map_metadata 1 -c copy -movflags +faststart "$output" 2>&1
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# --- Summary ---
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total_secs=$((cumulative_ms / 1000))
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mins=$((total_secs / 60))
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secs=$((total_secs % 60))
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echo ""
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echo "Done: $output"
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echo " Chapters: ${#inputs[@]}"
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echo " Duration: ${mins}m ${secs}s"
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