subtitle-add

#!/bin/sh

# add subtitles to a video

# script usage
usage()
{
# if argument passed to function echo it
[ -z "${1}" ] || echo "! ${1}"
# display help
echo "\
# add subtitles to a video

$(basename "$0") -i infile.(mp4|mkv|mov|m4v) -s subtitle.srt -o outfile.mp4
-i infile.(mp4|mkv|mov|m4v)
-s subtitle.srt
-o outfile.mp4 :optional agument # if option not provided defaults to infile-name-subs-date-time"
exit 2
}

# error messages
NOTFILE_ERR='not a file'
INVALID_OPT_ERR='Invalid option:'
REQ_ARG_ERR='requires an argument'
WRONG_ARGS_ERR='wrong number of arguments passed to script'
FILE_EXT_ERR='has the wrong file extension'
NOT_MEDIA_FILE_ERR='is not a media file'

# if script is run arguments pass and check the options with getopts,
# else display script usage and exit
[ $# -gt 0 ] || usage "${WRONG_ARGS_ERR}"

# getopts check and validate options
while getopts ':i:s:o:h' opt
do
  case ${opt} in
     i) video="${OPTARG}"
	[ -f "${video}" ] || usage "${video} ${NOTFILE_ERR}";;
     s) subs="${OPTARG}"
	[ -f "${subs}" ] || usage "${subs} ${NOTFILE_ERR}";;
     o) outfile="${OPTARG}";;
     h) usage;;
     \?) usage "${INVALID_OPT_ERR} ${OPTARG}" 1>&2;;
     :) usage "${INVALID_OPT_ERR} ${OPTARG} ${REQ_ARG_ERR}" 1>&2;;
  esac
done
shift $((OPTIND-1))

# infile, infile name and extension
subs_ext="${subs##*.}"
video_nopath="${video##*/}"
video_name="${video_nopath%.*}"

# run the ffmpeg function based on the audio file extension
case "$subs_ext" in
    srt);;
    *) usage "${subs} ${FILE_EXT_ERR}";;
esac

# file command check input file mime type
video_filetype="$(file --mime-type -b "${video}")"
subs_filetype="$(file --mime-type -b "${subs}")"

# video mimetypes
mov_mime='video/quicktime'
mkv_mime='video/x-matroska'
mp4_mime='video/mp4'
m4v_mime='video/x-m4v'

# srt subtitles mime type
srt_mime='text/plain'

# check the subtitles mime type
case "${subs_filetype}" in
    ${srt_mime});;
    *) usage "${subs} ${NOT_MEDIA_FILE_ERR}";;
esac

# defaults for variables if not defined
outfile_default="${video_name}-subs-$(date +"%Y-%m-%d-%H-%M-%S").mp4"

# audio is aac, copy audio stream
addsubs () {
    ffmpeg \
    -hide_banner \
    -stats -v panic \
    -i "${video}" \
    -f srt \
    -i "${subs}" \
    -c:a copy \
    -c:v copy \
    -c:s mov_text -metadata:s:s:0 \
    language=eng \
    -movflags +faststart \
    -f mp4 \
    "${outfile:=${outfile_default}}"
}

# check the video mime type
case "${video_filetype}" in
    ${mov_mime}|${mkv_mime}|${mp4_mime}|${m4v_mime}) addsubs "$video" "$subs";;
    *) usage "${video} ${NOT_MEDIA_FILE_ERR}";;
esac