open video as it's downloading in a background command

Question

The command below processes the output of youtube-dl with sed to get the filename of the video

youtube-dl "$URL" 2> /dev/null | \
sed -n 's/^\[download\] Destination: //p; s/^\[download\] \(.*\) has already been downloade.*/\1/p'

Immediately after youtube-dl finishes fetching and parsing the HTML and starts downloading the video, which is a couple/few seconds after it starts executing, it outputs what the filename of the video will be once it's done downloading (postfixed with a .part while it's downloading)

So the crux of the issue and what I'm stuck on is, How do I get the command above to the background (so it keeps downloading) and get the video filename from its standard output, so I can use it to open the video file before it's done downloading.

Answer 1

You could use inotifywait:

$ youtube-dl "$URL" &
$ inotifywait  --event create --format '"%f"' . | xargs vlc

There is a race condition there, if your connection is very fast (mine right now is) the file could be downloaded, and renamed, before your player opens it. Also, if youtube-dl does something like download audio and video separately, you might end up opening the wrong file.

Answer 2

@fra-san's comment reminded me of the while read loop which you can use to process output line-by-line/procedurally (as opposed to functionally, with pipes), you can use the < operator to pipe the output of a file to that loop with Bash Process Substitution of the youtube-dl command, and you can also send it to the background by adding an & and the while loop still can read the output of it (it's just reading a file, which BPS made).

through testing I found that I can keep playing the video normally even after it finishes downloading and gets renamed, though this might be a feature of mpv.

#!/bin/bash
# ytdl-stream - use youtube-dl to stream videos i.e watch videos as they download
# usage: ytdl-stream [YOUTUBE_DL_OPTIONS] URL 
# you can pipe into it a command to open you video player, e.g:
# echo mpv --mute=yes | ytdl-stream -f 'best[width<=1920,height<=1080]' --write-auto-sub [URL]

test ! -t 0 && player_cmd="$(cat /dev/stdin)" || player_cmd="mpv"

while IFS="" read -r line; do
  filename=$(echo "$line" | sed -n 's/^\[download\] Destination: //p')
  if [[ -z "$filename" ]]; then
    filename=$(echo "$line" | sed -n 's/^\[download\] \(.*\) has already been downloade.*/\1/p')
    [[ -z "$filename" ]] && continue || notify-send "file already downloaded, opening.."
  else
    notify-send "downloading.."
  fi
  withoutExtensions="${filename%.*}"
  withoutExtensions="${withoutExtensions%.*}"
  if [[ -e "$filename" ]]; then
    sleep 0.5 && ($player_cmd "$filename")&
  elif [[ -e "$filename".part ]]; then
    sleep 2
    if [[ -e "$filename".part ]]; then
      notify-send "found .part after sleep again"
      ($player_cmd "$filename".part)&
    else
      sleep 0.5 && ($player_cmd "$withoutExtensions"*)&
    fi
  fi
done < <(youtube-dl "$@" 2> /dev/null)&