What's the best & easiest GUI software that can join a video that I have downloaded in several parts?
- there is no re-encoding involved, so
the merging process takes only a
couple of seconds
- it can merge other video formats, like avi, mp4, but the output is always mkv
- it can't merge videos with different apect ratios
For more info see the documentation here
Click "add", select the first video, then click "append", then select the other videos to append. You can select multiple videos as long as you select them in order or you can append one by one. Then click "Start muxing". You might want to change the output filename. The other options are not really necessary for basic merging. Nevertheless, you can also add multiples subtitles in various formats or languages and also multiple audio tracks.
AVI files - avimerge
avimerge is a versatile tool. It can contatenate several AVI files into one. It can also be used to fix an index of a broken file and can also replace audio tracks or muxes new ones. It can read raw AC3 and MP3 files for multplex- ing.
sudo apt-get install transcode transcode-utils
You can merge avi files with:
avimerge -i input1.avi input2.avi -o output.avi
It's encoding-free, so joining the parts usually doesn't take much longer than a minute.
Other available options are:
Usage: avimerge [options] -o file output file name -i file1 [file2 [...]] input file(s) -p file multiplex additional audio track from file -a num select audio track number from input file  -A num select audio track number in output file [next] -b n handle vbr audio [autodetect] -c drop video frames in case audio is missing [off] -f FILE read AVI comments from FILE [off] -x FILE read AVI index from FILE [off] (see aviindex(1))
You can use Mencoder (Mplayer's encoded), that I recently found as a very good alternative to avconv.
sudo apt-get install mencoder
To merge some AVI files to a single AVI file, just call mencoder with setting "copy" as both video and audio encoder, so that no encoding is done:
mencoder -ovc copy -oac copy video1.avi video2.avi -o mergedvideo.avi
With this you can merge not only AVI videos, but also every format supported by Mencoder (see Selecting codecs and container formats page).
Source: Ubuntu How Tos
openshot video editor The one and only. Import videos edit crop and what not.
and export them to any format you want
avi etc... etc... etc...
to install run these command in gnome-terminal
wget https://launchpad.net/openshot/1.4/1.4.2/+download/openshot_1.4.2-1_all.deb sudo dpkg -i openshot_1.4.2-1_all.deb