você pode usar o mkvtoolnix.
sudo apt-get install mkvtoolnix
Outra dica agora, porque os arquivos mkv podem conter muitas legendas, então a dica é esse script que você pode procurar pelo idioma desejado, por exemplo, se você quiser inglês, ele baixará apenas o inglês.
Script:
#!/bin/bash
# Extract subtitles from each MKV file in the given directory
# If no directory is given, work in local dir
if [ "$1" = "" ]; then
DIR="."
else
DIR="$1"
fi
# Get all the MKV files in this dir and its subdirs
find "$DIR" -type f -name '*.mkv' | while read filename
do
# Find out which tracks contain the subtitles
mkvmerge -i "$filename" | grep 'subtitles' | while read subline
do
# Grep the number of the subtitle track
tracknumber='echo $subline | egrep -o "[0-9]{1,2}" | head -1'
# Get base name for subtitle
subtitlename=${filename%.*}
# Extract the track to a .tmp file
'mkvextract tracks "$filename" $tracknumber:"$subtitlename.srt.tmp" > /dev/null 2>&1'
'chmod g+rw "$subtitlename.srt.tmp"'
# Do a super-primitive language guess: ENGLISH
langtest='egrep -ic ' you | to | the ' "$subtitlename".srt.tmp'
trimregex=""
# Check if subtitle passes our language filter (10 or more matches)
if [ $langtest -ge 10 ]; then
# Regex to remove credits at the end of subtitles (read my reason why!)
'sed 's/\r//g' < "$subtitlename.srt.tmp" \
| sed 's/%/%%/g' \
| awk '{if (a){printf("\t")};printf $0; a=1; } /^$/{print ""; a=0;}' \
| grep -iv "$trimregex" \
| sed 's/\t/\r\n/g' > "$subtitlename.srt"'
'rm "$subtitlename.srt.tmp"'
'chmod g+rw "$subtitlename.srt"'
else
# Not our desired language: add a number to the filename and keep anyway, just in case
'mv "$subtitlename.srt.tmp" "$subtitlename.$tracknumber.srt" > /dev/null 2>&1'
fi
done
done
Salve este script nameyouwant.sh e torne-o executável
Agora no diretório de alteração de terminal para a pasta de script e escreva
./nameyouwant.sh /pathtosave