Ygtjki

I'm on Fedora 28 OS and have youtube-dl version 2019.03.18 and I wanted to download all the subs and only the subs of a Crashcourse playlist off of youtube, consisting of 45 videos.

I want all the subs to be written in a single *.txt file so that I can then edit it and print-it-to-pdf format (with evince), also applying the appropriate page numbering/header/footer.

I tried the following:

youtube-dl --skip-download --write-sub --playlist-items 6-10 <PLAYLIST-URL> >> all-subs.txt

This however only wrote the 'verbose' output to all-subs.txt and just downloaded the *.vtt format subs (I don't know why youtube-dl won't download in any other format, even if told to do so, a glitch still unresolved).

It seems that youtube-dl doesn't support such feature, but you can use cat to concatenate all the downloaded files:

$ youtube-dl --output '%(autonumber)s.%(ext)s' --skip-download --write-sub 'https://www.youtube.com/watch?v=-5wpm-gesOY' 'https://www.youtube.com/watch?v=w3_0x6oaDmI'
[youtube] -5wpm-gesOY: Downloading webpage
[youtube] -5wpm-gesOY: Downloading video info webpage
[info] Writing video subtitles to: 00001.en-GB.vtt
[youtube] w3_0x6oaDmI: Downloading webpage
[youtube] w3_0x6oaDmI: Downloading video info webpage
[info] Writing video subtitles to: 00002.en.vtt

$ cat ./* > all-subs.vtt

Note that %(autonumber)s is needed to concatenate the files in order. When downloading from playlists, you may use %(playlist_index)s instead.

I don't know why youtube-dl won't download in any other format, even if told to do so, a glitch still unresolved

While it's possible to request a specific format using the --sub-format option, please note that the availability of such format depends on the website. YouTube only offers ttml and vtt files, as far as I can tell (and I'm able to download subtitles in both formats):

$ youtube-dl --list-subs 'https://www.youtube.com/watch?v=-5wpm-gesOY'
...
Available subtitles for -5wpm-gesOY:
Language formats
en-GB vtt, ttml
ru vtt, ttml
fr vtt, ttml
...

$ youtube-dl --output '%(autonumber)s.%(ext)s' --skip-download --sub-format 'vtt' --write-sub 'https://www.youtube.com/watch?v=-5wpm-gesOY'
[youtube] -5wpm-gesOY: Downloading webpage
[youtube] -5wpm-gesOY: Downloading video info webpage
[info] Writing video subtitles to: 00001.en-GB.vtt

$ youtube-dl --output '%(autonumber)s.%(ext)s' --skip-download --sub-format 'ttml' --write-sub 'https://www.youtube.com/watch?v=-5wpm-gesOY'
[youtube] -5wpm-gesOY: Downloading webpage
[youtube] -5wpm-gesOY: Downloading video info webpage
[info] Writing video subtitles to: 00001.en-GB.ttml