A simple bash script to extract video sections between chapters using ffmpeg

ffmpeg_extract_video_chapters.sh

#!/bin/bash

# Script to extract video section between chapters using ffmpeg
# Usage: ./extract_chapters.sh <input_file> <start_chapter_number> <end_chapter_number>

set -e  # Exit on any error

# Function to display usage
usage() {
    echo "Usage: $0 <input_file> <start_chapter_number> <end_chapter_number>"
    echo "Example: $0 movie.mkv 3 6"
    echo "This will extract from chapter 3 to chapter 6 (inclusive)"
    echo "Note: Chapter numbers start from 1 (first chapter is chapter 1)"
    exit 1
}

# Check if correct number of arguments provided
if [ $# -ne 3 ]; then
    echo "Error: Incorrect number of arguments"
    usage
fi

INPUT_FILE="$1"
START_CHAPTER="$2"
END_CHAPTER="$3"

# Validate input file exists
if [ ! -f "$INPUT_FILE" ]; then
    echo "Error: Input file '$INPUT_FILE' does not exist"
    exit 1
fi

# Validate chapter numbers are positive integers
if ! [[ "$START_CHAPTER" =~ ^[0-9]+$ ]] || ! [[ "$END_CHAPTER" =~ ^[0-9]+$ ]]; then
    echo "Error: Chapter numbers must be positive integers"
    exit 1
fi

# Validate chapter numbers are at least 1
if [ "$START_CHAPTER" -lt 1 ] || [ "$END_CHAPTER" -lt 1 ]; then
    echo "Error: Chapter numbers must be 1 or greater (chapters start from 1)"
    exit 1
fi

# Validate start chapter is not greater than end chapter
if [ "$START_CHAPTER" -gt "$END_CHAPTER" ]; then
    echo "Error: Start chapter number cannot be greater than end chapter number"
    exit 1
fi

echo "Extracting chapters $START_CHAPTER to $END_CHAPTER from '$INPUT_FILE'"

# Get chapter information from the video file
echo "Reading chapter information..."
CHAPTER_INFO=$(ffprobe -v quiet -print_format json -show_chapters "$INPUT_FILE" 2>/dev/null)

if [ -z "$CHAPTER_INFO" ] || [ "$CHAPTER_INFO" = '{"chapters":[]}' ]; then
    echo "Error: No chapters found in the input file"
    exit 1
fi

# Extract chapter count
CHAPTER_COUNT=$(echo "$CHAPTER_INFO" | jq '.chapters | length')

echo "Found $CHAPTER_COUNT chapters in the file"

# Convert user's 1-based chapter numbers to 0-based indexes for ffmpeg
START_INDEX=$((START_CHAPTER - 1))
END_INDEX=$((END_CHAPTER - 1))

# Validate chapter numbers are within range
if [ "$START_INDEX" -ge "$CHAPTER_COUNT" ] || [ "$END_INDEX" -ge "$CHAPTER_COUNT" ]; then
    echo "Error: Chapter number out of range. Available chapters: 1 to $CHAPTER_COUNT"
    exit 1
fi

# Get start time of the start chapter (using 0-based index)
START_TIME=$(echo "$CHAPTER_INFO" | jq -r ".chapters[$START_INDEX].start_time")

# Get end time of the end chapter (using 0-based index)
END_TIME=$(echo "$CHAPTER_INFO" | jq -r ".chapters[$END_INDEX].end_time")

echo "Start time: ${START_TIME}s"
echo "End time: ${END_TIME}s"

# Generate output filename (using original 1-based chapter numbers)
INPUT_DIR=$(dirname "$INPUT_FILE")
INPUT_BASENAME=$(basename "$INPUT_FILE")
INPUT_NAME="${INPUT_BASENAME%.*}"
INPUT_EXT="${INPUT_BASENAME##*.}"

OUTPUT_FILE="${INPUT_DIR}/${INPUT_NAME}_ch${START_CHAPTER}-${END_CHAPTER}.${INPUT_EXT}"

echo "Output file: $OUTPUT_FILE"

# Check if output file already exists
if [ -f "$OUTPUT_FILE" ]; then
    read -p "Output file already exists. Overwrite? (y/N): " -n 1 -r
    echo
    if [[ ! $REPLY =~ ^[Yy]$ ]]; then
        echo "Operation cancelled"
        exit 0
    fi
fi

# Run ffmpeg command to extract the section
echo "Extracting video section..."
ffmpeg -i "$INPUT_FILE" \
    -ss "$START_TIME" \
    -to "$END_TIME" \
    -c copy \
    -map 0 \
    -avoid_negative_ts make_zero \
    "$OUTPUT_FILE"

if [ $? -eq 0 ]; then
    echo "Successfully extracted chapters $START_CHAPTER-$END_CHAPTER to: $OUTPUT_FILE"
    
    # Display file size information
    INPUT_SIZE=$(stat -c%s "$INPUT_FILE" 2>/dev/null || echo "unknown")
    OUTPUT_SIZE=$(stat -c%s "$OUTPUT_FILE" 2>/dev/null || echo "unknown")
    
    if [ "$INPUT_SIZE" != "unknown" ] && [ "$OUTPUT_SIZE" != "unknown" ]; then
        INPUT_MB=$((INPUT_SIZE / 1024 / 1024))
        OUTPUT_MB=$((OUTPUT_SIZE / 1024 / 1024))
        echo "Original file size: ${INPUT_MB}MB"
        echo "Extracted file size: ${OUTPUT_MB}MB"
    fi
else
    echo "Error: ffmpeg extraction failed"
    exit 1
fi

FFmpeg Chapter Extractor

A simple bash script to extract video sections between chapters using ffmpeg.

What it does

Extracts a portion of a video file from one chapter to another, preserving the original quality by copying streams without re-encoding.

Requirements

• ffmpeg - for video processing
• ffprobe - for reading chapter information (usually comes with ffmpeg)
• jq - for parsing JSON output from ffprobe

Usage

./ffmpeg_extract_video_chapters.sh <input_file> <start_chapter> <end_chapter>

• input_file: Path to your video file
• start_chapter: Chapter number to start from (starting at 1)
• end_chapter: Chapter number to end at (inclusive)

Examples

# Extract chapters 2 through 4
./ffmpeg_extract_video_chapters.sh video.mkv 2 4

# Extract just chapter 1
./ffmpeg_extract_video_chapters.sh video.mp4 1 1

Output

The script creates a new file in the same directory as the input file, with the naming pattern:

original_filename_ch<start>-<end>.extension

For example: video_ch2-4.mkv

Notes

• Chapter numbering starts from 1 (like in handbrake, not 0 like in actual ffmpeg)
• The script will show you how many chapters are available if you specify invalid ranges
• File sizes are displayed after successful extraction
• The script will ask before overwriting existing output files