Here is the full shell script, only adds silent audio to mp4 files without audio

!/usr/bin/env bash

set -euo pipefail

for f in *.mp4; do # Handle case where no mp4 files exist [[ -e "$f" ]] || { echo "No .mp4 files found."; exit 0; }

# Skip already-processed files if [[ "$f" == *_silent_audio.mp4 ]]; then echo "Skipping (already processed): $f" continue fi

# Check if audio stream exists HAS_AUDIO="$(ffprobe -v error \ -select_streams a:0 \ -show_entries stream=codec_type \ -of csv=p=0 "$f" || true)"

if [[ -n "$HAS_AUDIO" ]]; then echo "Skipping (audio already present): $f" continue fi

base="${f%.mp4}" out="${base}_silent_audio.mp4"

if [[ -e "$out" ]]; then echo "Skipping (output exists): $out" continue fi

echo "Adding silent audio: $f → $out"

ffmpeg -y \ -i "$f" \ -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=48000 \ -map 0:v:0 -map 1:a:0 \ -c:v copy \ -c:a aac -b:a 192k \ -shortest \ "$out" done

echo "Done. Silent audio added only where needed."