A little script to help with extracting subtitles from .mkv files.

subextract.sh

#!/bin/sh

if ! command -v mkvmerge mkvextract >/dev/null 2>&1; then
    echo >&2 "Tools mkvmerge & mkvextract are required. Install package mkvtoolnix."
    exit 1
fi

filename=$1

if [ ! -f $filename ]; then
    echo "File not found."
    exit 1
fi

filename_no_ext=$(echo $1 | sed -r 's/.mkv//')
sed_exp='s/.*([0-9]+):.*language:([a-z]+).*/\1:'$filename_no_ext'.\2.\1.srt/'

mkvmerge -I $filename | grep subtitles | sed -r $sed_exp | xargs mkvextract tracks $filename

And in order to make this work with filenames containing the path, you need to do this:

filename_no_ext=$(basename $1 .mkv)