Zsh script to generate YouTube Shorts (1080×1920) from a static image with optional audio; supports custom output path, duration, CRF/preset, audio bitrate, overwrite and dry-run.

create-youtube-short.zsh

#!/usr/bin/env zsh
# create-youtube-short.zsh
# Create a YouTube Short (1080x1920) from a static image
# Audio is OPTIONAL: if missing, the video will be muted.

set -euo pipefail

SCRIPT_NAME="${0:t}"
VERSION="1.1.0"

CRF_DEFAULT=18
PRESET_DEFAULT="slow"
AUDIO_BITRATE_DEFAULT="192k"
DURATION_DEFAULT="30"   # Default duration in seconds when no audio is provided

print_help() {
  cat <<'EOF'
Create a YouTube Short (vertical 9:16) from a static image.
Audio is optional.

Usage:
  create-youtube-short.zsh [options] <image> [audio]
  create-youtube-short.zsh [options] <file1> <file2> [...]

If multiple files are provided (e.g. from Finder):
  - the first image found is used
  - the first audio found is used (optional)

Options:
  -o, --output <path>     Output file path
  -d, --duration <sec>    Force duration in seconds (default: 30)
  -q, --crf <int>         H.264 quality (default: 18)
  -p, --preset <name>     x264 preset (default: slow)
  -b, --audio-bitrate <k> Audio bitrate (default: 192k)
  -y, --overwrite         Overwrite output if it exists
  -n, --dry-run           Print ffmpeg command only
  -h, --help              Show this help
  -v, --version           Print version

Notes:
  - If NO audio is provided, duration defaults to 30 seconds unless overridden with --duration.
  - Output format is YouTube Shorts compatible (1080x1920, H.264 + AAC).

Dependencies:
  ffmpeg (brew install ffmpeg)

EOF
}

die() { print -r -- "Error: $*" >&2; exit 1; }

need_cmd() {
  command -v "$1" >/dev/null 2>&1 || die "Missing dependency: $1 (brew install ffmpeg)"
}

is_image() {
  local ext="${1:e:l}"
  [[ "$ext" == jpg || "$ext" == jpeg || "$ext" == png || "$ext" == webp || "$ext" == heic || "$ext" == tiff ]]
}

is_audio() {
  local ext="${1:e:l}"
  [[ "$ext" == mp3 || "$ext" == wav || "$ext" == m4a || "$ext" == aac || "$ext" == flac || "$ext" == ogg || "$ext" == opus ]]
}

sanitize_filename() {
  print -r -- "${1//[^A-Za-z0-9._-]/_}"
}

# -------------------------
# Parse args
# -------------------------
OVERWRITE=0
DRY_RUN=0
CRF="$CRF_DEFAULT"
PRESET="$PRESET_DEFAULT"
AUDIO_BR="$AUDIO_BITRATE_DEFAULT"
OUT_PATH=""
FORCE_DURATION="$DURATION_DEFAULT"

ARGS=()

while (( $# > 0 )); do
  case "$1" in
    -h|--help) print_help; exit 0 ;;
    -v|--version) print -r -- "$SCRIPT_NAME v$VERSION"; exit 0 ;;
    -y|--overwrite) OVERWRITE=1; shift ;;
    -n|--dry-run) DRY_RUN=1; shift ;;
    -q|--crf) shift; CRF="$1"; shift ;;
    -p|--preset) shift; PRESET="$1"; shift ;;
    -b|--audio-bitrate) shift; AUDIO_BR="$1"; shift ;;
    -d|--duration) shift; FORCE_DURATION="$1"; shift ;;
    -o|--output) shift; OUT_PATH="$1"; shift ;;
    --) shift; ARGS+=("$@"); break ;;
    -*) die "Unknown option: $1" ;;
    *) ARGS+=("$1"); shift ;;
  esac
done

(( ${#ARGS[@]} >= 1 )) || die "You must provide at least one image file."

# -------------------------
# Dependencies
# -------------------------
need_cmd ffmpeg

# -------------------------
# Pick image + optional audio
# -------------------------
IMAGE=""
AUDIO=""

for f in "${ARGS[@]}"; do
  [[ -e "$f" ]] || continue

  if [[ -z "$IMAGE" ]] && is_image "$f"; then
    IMAGE="$f"
    continue
  fi

  if [[ -z "$AUDIO" ]] && is_audio "$f"; then
    AUDIO="$f"
    continue
  fi
done

[[ -n "$IMAGE" ]] || die "No image file found."

# -------------------------
# Output path
# -------------------------
IMAGE_DIR="${IMAGE:h}"
BASE="$(sanitize_filename "${IMAGE:t:r}")"

# If no output provided, default to image dir with _short suffix
if [[ -z "$OUT_PATH" ]]; then
  OUT_PATH="${IMAGE_DIR}/${BASE}_short.mp4"
else
  # If OUT_PATH is an existing directory, place file inside it
  if [[ -d "$OUT_PATH" ]]; then
    OUT_PATH="${OUT_PATH%/}/${BASE}_short.mp4"
  fi

  # If OUT_PATH has no extension, append .mp4
  if [[ "${OUT_PATH##*/}" != *.* ]]; then
    OUT_PATH="${OUT_PATH}.mp4"
  fi
fi

# If target exists and not overwriting, add timestamp before extension
if [[ -e "$OUT_PATH" && $OVERWRITE -eq 0 ]]; then
  OUT_PATH="${OUT_PATH:r}_$(date +%Y%m%d_%H%M%S).mp4"
fi

# -------------------------
# Duration logic
# -------------------------
if [[ -z "$AUDIO" && -z "$FORCE_DURATION" ]]; then
  die "No audio provided. You MUST specify --duration <seconds>."
fi

FILTER='scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2'

FFMPEG_Y=()
(( OVERWRITE == 1 )) && FFMPEG_Y=(-y)

FFMPEG_T=()
[[ -n "$FORCE_DURATION" ]] && FFMPEG_T=(-t "$FORCE_DURATION")

# -------------------------
# Build command
# -------------------------
CMD=(ffmpeg "${FFMPEG_Y[@]}" -loop 1 -i "$IMAGE")

if [[ -n "$AUDIO" ]]; then
  CMD+=(-i "$AUDIO" -c:a aac -b:a "$AUDIO_BR" -shortest)
else
  CMD+=(-an)
fi

CMD+=(
  -c:v libx264
  -preset "$PRESET"
  -crf "$CRF"
  -tune stillimage
  -pix_fmt yuv420p
  -profile:v high
  -level 4.2
  "${FFMPEG_T[@]}"
  -movflags +faststart
  -vf "$FILTER"
  "$OUT_PATH"
)

# -------------------------
# Execute
# -------------------------
if (( DRY_RUN == 1 )); then
  print -r -- "Dry run:"
  print -r -- "${(q)CMD[@]}"
  exit 0
fi

print -r -- "Image : $IMAGE"
[[ -n "$AUDIO" ]] && print -r -- "Audio : $AUDIO" || print -r -- "Audio : (none, muted)"
print -r -- "Output: $OUT_PATH"

"${CMD[@]}"

print -r -- "Done ✔"