Conditionally copy or encode video stream into avc (x264) mp4 with ffmpeg and ffprobe

ensure_avc_mp4.sh

#!/usr/bin/env bash

set -euxo pipefail

video_in="video_in.mkv"
video_out="video_out.mp4"

# full metadata payload: $(ffprobe -loglevel error -print_format json -show_format -show_streams "${video_in}")
is_avc=$(ffprobe -loglevel error -select_streams v:0 -show_entries stream=is_avc -of default=noprint_wrappers=1:nokey=1 "${video_in}")

if [ "${is_avc}" = "true" ]; then
    # copy avc video stream (no re-encode), discard audio (-an), add faststart ref https://trac.ffmpeg.org/wiki/Encode/H.264#faststartforwebvideo
    ffmpeg \
    -y \
    -hide_banner \
    -loglevel \
    error \
    -i \
    "${video_in}" \
    -an \
    -c:v \
    copy \
    -movflags \
    +faststart \
    "${video_out}"
else
    # re-encode to avc (x264), discard audio (-an), add faststart ref https://trac.ffmpeg.org/wiki/Encode/H.264#faststartforwebvideo
    ffmpeg \
    -y \
    -hide_banner \
    -loglevel \
    error \
    -i \
    "${video_in}" \
    -an \
    -c:v \
    libx264 \
    -crf \
    18 \
    -preset \
    veryfast \
    -movflags \
    +faststart \
    "${video_out}"
fi