How can I download just thumbnails using youtube-dl?

Building on the other answers, you can also specify the -a or --batch-file flag to provide a list of files to import from, which simplifies the code a bit:

youtube-dl -a urls.txt --write-thumbnail --skip-download

You can simply add --skip-download to your code and it will work fine. Like so:

with open('urls.txt') as f:
for line in f:
    os.system("youtube-dl "+"--write-thumbnail "+"--skip-download "+line)