MPG/MPEG files:

sudo apt-get install mpgtx
mpgjoin vid1.mpg vid2.mpg vid3.mpg -o combinedvideo.mpg

WMV/MP4 files (same codec/properties):

ls -la > list.txt
#edit list and sort/format lines as
# file 'video1.wmv'
# file 'video2.wmv'
# file 'video3.wmv'

#or

ls -1 *.wmv | sort -n | sed "s/\(.*\)/file '\1'/" > list.txt

ffmpeg -f concat -safe 0 -i list.txt -c copy outputfilename.wmv

#or

ffmpeg -f concat -safe 0 -i video1.wmv video2.wmv video3.wmv -c copy outputfilename.wmv

https://stackoverflow.com/questions/49371422/how-to-merge-two-videos-without-re-encoding/49373401#49373401

Script for multiple conversions based on filename pattern. Converts all related files that have a number at the end and start with 1:

#!/bin/bash
SAVEIFS=$IFS
IFS=$(echo -en "\n\b")
for f in *1.wmv
do
  shortfile=${f::-5}
  echo "file '"$shortfile"1.wmv" > list.txt
  echo "file '"$shortfile"2.wmv" >> list.txt
  echo "file '"$shortfile"3.wmv" >> list.txt
  ffmpeg -f concat -safe 0 -i list.txt -c copy $shortfile.wmv
  rm list.txt
done
IFS=$SAVEIFS

Script to merge files:

video_merge.sh

#!/bin/bash
 
# Check if a parameter is provided
if [ -z "$1" ]; then
    echo "Usage: "$(basename $0)" <files>"
    exit 1
fi
 
TMPFILE=$(mktemp)
 
for input in "$@";
do
    if [ -f "$input" ]; then
        if [ "$container" == "" ]; then
            container="${input##*.}"
        fi
        if [ "$container" != "${input##*.}" ]; then
            echo "Files need to be in same format to merge!"
            echo "Abandoning merge"
            if [ -f "$TMPFILE" ]; then
                rm $TMPFILE
            fi
            echo;read -rsn1 -p "Press any key to continue . . .";echo
            exit
        fi
        echo "file '$input'" >> $TMPFILE
        container="${input##*.}"
        filename=${input%.*}
    fi
done
 
output="$filename.mergedprogress.$container"
sort -n -o $TMPFILE"_sort" $TMPFILE
 
echo "The following files will be merged in this order:"
cat $TMPFILE"_sort"
echo;read -rsn1 -p "Press any key to continue . . .";echo
 
cmd='ffmpeg -f concat -safe 0 -i "'"$TMPFILE"'_sort" -c copy "'"$output"'"'
eval $cmd
if [ $? -eq 0 ];
then
    actualheight=$(ffprobe -v error -select_streams v -show_entries stream=height -of csv=p=0:s=x "$output")
    #output2="${output%.*}."$actualheight"p.$container"
    output2="${output//.mergedprogress./.$actualheight""p.merged.}"
    mv "$output" "$output2"
    echo "Output file: $output2"
    rm $TMPFILE
    rm $TMPFILE"_sort"
else
    echo "An error occured. File not converted properly!"
    echo "Full ffmpeg command:"
    echo "$cmd"
fi
 
echo;read -rsn1 -p "Press any key to continue . . .";echo

Nemo action to call this merge script (to be put in ~/.local/share/nemo/actions:

video_merge.nemo_action

[Nemo Action]
Name=Video Merge
Comment=Video Merge
Exec=video_merge.sh %F
Icon-Name=stock_down
Selection=m
Extensions=mp4;wmv;avi;mkv;mov
Quote=double
EscapeSpaces=true
Terminal=true

alternative merge script with chapters, inspired by https://gist.github.com/bdurrow/b51470869dd72b2333407dbfcb947801 has some issues with single quotes in filenames or paths.

video_mergechapters.sh

#!/bin/bash
#http://redsymbol.net/articles/unofficial-bash-strict-mode/
set -euo pipefail
IFS=$'\n\t'
 
cd "$1"
echo "$1"
## Script to merge all mp4 videos in current directory (recursively 2 levels)
## And update chapter marks to retain the folder/filename
 
## Script for merging videos
 
temp_dir=$(mktemp -d)
function finish {
  rc=$?
  if [[ $rc != 0 ]]; then
    echo
    echo "FAILED!"
  fi
  echo -n "Cleaning up "
  rm -rf "${temp_dir}"
  echo "..........[ DONE ]"
  exit $rc
}
trap finish SIGHUP EXIT
 
current_dir=$(pwd)
bname=$(basename ${current_dir})
final_mp4=${bname}.mp4
input_list=${temp_dir}/${bname}-input_list.txt
file_list=${temp_dir}/${bname}-file_list
meta_file=${temp_dir}/${bname}-metadata.txt
 
#Hopefully this will work for either BSD or GNU sed
extended_match="-r"
echo "" | sed ${extended_match} 's|foo|bar|' 2>/dev/null || extended_match="-E"
 
if [ -e "${final_mp4}" ]; then
    echo "${final_mp4} already exists, please remove it."
    echo;read -rsn1 -p "Press any key to continue . . .";echo
    exit 1
fi
 
echo -n "Generating file lists "
find . -maxdepth 2 -type f -iname '*.mp4' | sort -V |
    sed -e 's|^./||' | sed -e "s|'|'\"'\"'|" |
    tee "${file_list}" |
    awk "{printf \"file '${current_dir}/%s'\n\", \$0}" > "${input_list}"
echo "..........[ DONE ]"  
 
cat "${input_list}"
echo;read -rsn1 -p "Press any key to continue . . .";echo
 
## chapter marks
#Do this first so we fail early
#TODO: Test (‘=’, ‘;’, ‘#’, ‘\’) are escaped
ts=0
echo -n "Generating chapter marks "
ffmpeg -i "$(head -1 "${file_list}")" -f ffmetadata "${meta_file}" -v quiet
 
cat "${file_list}" | while read file
do
    ds=$(ffprobe -v quiet -of csv=p=0 -show_entries format=duration "${file}")
    # echo "$ds"
    escaped_title=$(echo ${file} | sed ${extended_match} -e 's|([=;#\])|\\\1|g' -e 's|.[Mm][Pp]4$||' )
    echo "[CHAPTER]" >> "${meta_file}"
    echo "TIMEBASE=1/1" >> "${meta_file}"
    echo "START=${ts}" >> "${meta_file}"
    ts=$(awk "BEGIN {print ${ts}+${ds}; exit}")
    echo "END=${ts}" >> "${meta_file}"
    echo "TITLE=${escaped_title}" >> "${meta_file}"
done
echo "..........[ DONE ]"
 
echo -n "Merging the files "
ffmpeg -f concat -safe 0 -i "${input_list}" -i "${meta_file}" -map_metadata 1 -codec copy "${final_mp4}" -v quiet
echo "..........[ DONE ]"
 
echo "Job Completed."
 
echo;read -rsn1 -p "Press any key to continue . . .";echo

Notes re subtitles:
note: mp4 format does not support ass or srt subtitles, but mov_text subtitles. ffmpeg can convert them. formatting from ass subtitles will be lost.

Embed srt subtitle file into mp4 as additional user-controllable track:

ffmpeg -i infile.mp4 -i infile.srt -c copy -c:s mov_text outfile.mp4

set metadata of first subtitle stream to english:

ffmpeg -i infile.mp4 -i infile.srt -c copy -c:s mov_text -metadata:s:s:0 language=eng outfile.mp4
ffmpeg -i infile.mp4 -i infile.ass -c copy -c:s mov_text -metadata:s:s:0 language=eng outfile.mp4

extract subtitle from video:

ffmpeg -i Movie.mkv -map 0:s:0 subs.srt

Multiple subs require mapping, otherwise first sub will be overwritten
First add another input: -i input2.srt. Second, map that as 2nd stream: -map 2:0. Finally, select encoder for 2nd subtitle stream (the same as the first one): -c:s srt. The complete example\

ffmpeg -i input.mp4 -f srt -i input.srt -i input2.srt -map 0:0 -map 0:1 -map 1:0 -map 2:0 -c:v copy -c:a copy -c:s srt -c:s srt output.mkv 

convert srt to ass or ass to srt:

ffmpeg -i input.srt input.ass
ffmpeg -i input.ass input.srt