MKVToolNix is a set of tools to create, alter and inspect Matroska(.mkv) files under Linux, Unix and Windows. MKVToolNix can be used to get information about (mkvinfo) Matroska files, extract tracks/data from (mkvextract) Matroska files and create (mkvmerge) Matroska files from other media files.
MKVToolNix provides command-line tools only. If you want to work with graphical tools, you should install MKVToolNix-gui. MKVToolNix-gui interface is simple, plain and easy to use. You can add the MKV file(s) using file browser or simply drag and drop them into MKVToolNix. It allows you to edit the track names and tags, also it is used to change the video files aspect ratio, display width and height etc.
Install MKVToolNix On Ubuntu 13.10/13.04
MKVToolNix is available in Ubuntu default repositories, but it might be a bit old. So add the MKVToolNix repository to /etc/apt/sources.list file to get the latest version.
First you have to import the GPG key.
wget -q -O - http://www.bunkus.org/gpg-pub-moritzbunkus.txt | sudo apt-key add -
Open /etc/apt/sources.list file,
sudo nano /etc/apt/sources.list
Add the following lines.
deb http://www.bunkus.org/ubuntu/raring/ ./ deb-src http://www.bunkus.org/ubuntu/raring/ ./
Now update the sources list and install MKVToolNix.
sudo apt-get update sudo apt-get install mkvtoolnix mkvtoolnix-gui
For other distributions, please look here.
Open it either from Dash or Menu. It will look like below.
Click on the Add button and select the video files that you want to convert to mkv format.
I suggest you to read the official documentation page to know more about MKVToolNix.