This script adds a subtitle file srt in a mkv file. In addition, it converts the encoding of the srt file to UTF-8, for some TVs (such as the Sony Bravia KDL-40W605) use this as the default encoding.

mkvsrtadd.sh

#!/bin/bash
################################################################
# This script adds a subtitle file srt in a mkv file.          #
# In addition, it converts the encoding of the srt file        #
# to UTF-8, for some TVs (such as the Sony Bravia KDL-40W605)  #
# use this as the default encoding.                            #
#                                                              #
# TIP: Combine with script "mkvsrtaddbatch" (available in      #
# https://gist.github.com/samirfor/3e9fe8aa2a2feb89f8e7to) to  #
# process a batch of files at once.                            #
################################################################
usage="[i] Usage: $(basename "$0") video.mkv"

# Checking parameter
if [ -z "$1" ]; then
	echo ""; echo "$usage"; echo ""; exit
fi

# Setting up vars
mkvfile=$1
newsrt="/tmp/tmpsrt.srt" # place to put converted subtitle

newmkvfolder=$(dirname "${mkvfile}")
filename="${mkvfile##*/}"
filename="${filename%.*}"
if [ "${filename: -6}" == ".nosub" ]; then
	filename=${filename:0:${#filename}-6} # remove '.nosub' suffix
	echo "INFO: '.nosub' suffix removed."
fi
if [ "${filename: -3}" == ".br" ]; then
	echo "INFO: Skipping... this file has subtitle already."
	exit 0
fi
# Adding suffix in new filename
suffix="br" # to distinguish the old mkv from the new
newmkvfilename="${filename}.${suffix}.mkv" # gets the filename without the path
subtitle="${newmkvfolder}/${filename}.srt" # full path filename
newmkv=$newmkvfilename # do the work on current directory

# Checking available free space
freeSpace=$(df -k . | tail -n 1 | awk -F' ' '{ print $4 }') # k = 1024 bytes
fileSize=$(du -k "$1" | awk -F' ' '{ print $1 }') # k = 1024 bytes
diffSize=$((fileSize-freeSpace))
#echo -e "   Size: $fileSize\nF.Space: $freeSpace\n   Diff: $diffSize"
if [ "$diffSize" -gt 0 ]; then
        echo "ERROR: Insufficient disk space. The minimum required free space is at least the size of the input file."
        echo
        exit 1
fi

# Checking if subtitle file exist
if [ ! -f "$subtitle" ]; then
	echo "SKIP: Subtitle cannot be found. Be sure it is a srt file and name \
match with video's name."
	echo
	exit 2
fi

# Checking srt encode type
strencode=$(file "$subtitle")
if [[ "$strencode" =~ ISO-8859 ]]; then
    # Converting srt subtitle from ISO-8859-1 to UTF-8
    iconv -f ISO-8859-1 -t UTF-8 "$subtitle" > "$newsrt"
    echo "INFO: Subtitle encode converted to UTF-8."
elif [[ "$strencode" =~ UTF-8 ]]; then
	# Simply copying the srt file
    cp "$subtitle" "$newsrt"
    echo "INFO: Subtitle encode UTF-8 ok."
else
	echo "ERROR: Subtitle encode unknown."
	read -p "Convert to UTF-8 anyway? [Y/n] " -n 1 -r
	echo    # (optional) move to a new line
	if [[ $REPLY =~ ^[Nn]$ ]]; then
		echo "INFO: Aborted. You have to convert subtitle encode manually."
    	exit 1
    else
    	iconv -f ISO-8859-1 -t UTF-8 "$subtitle" > "$newsrt"
    	echo "INFO: Subtitle encode converted to UTF-8."
	fi
fi

# Add the new srt file into mkv container without video reencode (about 20 seconds)
mkvmerge -o "$newmkv" "$mkvfile" --language 0:por --track-name 0:Portuguese "$newsrt"
success=$?

# Cleaning original files
if [ $success == 0 ]; then
	echo "SUCCESS: Subtitle added"
	rm "$mkvfile" "$subtitle"
	echo "INFO: The original files were removed."
else
	echo "ERROR: A error ocurred. Be aware."
fi
rm "$newsrt" # Cleaning temp subtitle file
exit 0

Fix MacOS compatibility