FFmpeg Script Safety Rules

This rule applies to files matching the patterns above.

# FFmpeg Script Safety Rules

## Rule
All FFmpeg scripts MUST validate inputs, protect against output overwrite, handle errors, check disk space, and test on samples before full processing.

## Input Validation
```bash
# Good — validate input exists and is a media file
if [ ! -f "$input" ]; then
  echo "Error: Input file not found: $input"
  exit 1
fi

# Verify it's a valid media file
if ! ffprobe -v error "$input" 2>/dev/null; then
  echo "Error: Not a valid media file: $input"
  exit 1
fi
```

## Output Protection
```bash
# Good — check before overwriting
if [ -f "$output" ]; then
  echo "Warning: Output exists: $output"
  read -p "Overwrite? (y/N) " confirm
  [ "$confirm" = "y" ] || exit 0
fi

# In batch scripts, use a separate output directory
# NEVER process files in place
OUTPUT_DIR="./processed"
mkdir -p "$OUTPUT_DIR"
```

## Error Handling
```bash
# Good — check FFmpeg exit code
ffmpeg -i "$input" [options] "$output" 2>>"$LOG_FILE"
if [ $? -ne 0 ]; then
  echo "Error: Failed to process $input" >> "$LOG_FILE"
  rm -f "$output"  # Clean up partial output
  continue  # In loops, skip to next file
fi
```

## Disk Space Check
```bash
# Check available space before batch processing
input_size=$(du -sm "$INPUT_DIR" | cut -f1)
available=$(df -m "$OUTPUT_DIR" | awk 'NR==2{print $4}')

if [ "$available" -lt "$((input_size * 2))" ]; then
  echo "Error: Insufficient disk space. Need ${input_size}MB+, have ${available}MB"
  exit 1
fi
```

## Test Before Full Run
```bash
# ALWAYS test on a sample first
# Process only first 10 seconds
ffmpeg -i "$input" -t 10 [options] test_output.mp4

# Process first 3 files in batch
ls "$INPUT_DIR"/*.mp4 | head -3 | while read file; do
  process_file "$file"
done
echo "Test complete. Review output before running full batch."
```

## Good Script Structure
```bash
#!/bin/bash
set -euo pipefail  # Exit on error, undefined vars, pipe failures

INPUT_DIR="${1:?Usage: $0 <input_dir>}"
OUTPUT_DIR="./processed"
LOG_FILE="./processing.log"

# Validate
[ -d "$INPUT_DIR" ] || { echo "Input directory not found"; exit 1; }
mkdir -p "$OUTPUT_DIR"

# Process
for file in "$INPUT_DIR"/*.mp4; do
  [ -f "$file" ] || continue
  output="$OUTPUT_DIR/$(basename "$file")"

  ffmpeg -i "$file" [options] -y "$output" 2>>"$LOG_FILE" || {
    echo "Failed: $file" >> "$LOG_FILE"
    rm -f "$output"
    continue
  }
done
```

## Anti-Patterns
- Processing in place (overwriting original files)
- No error handling (partial files left on failure)
- No disk space check (fills disk, corrupts output)
- Running full batch without testing on samples
- Using `-y` without output directory separation (overwrites originals)
- No logging (can't debug failures after batch completes)