We will merge the files using cat and mencoder. For that purpose we will first install the appropriate software to use mencoder, then we will concatenate our files and last we will repair some out-of-sync due to the concatenation.
We need to do the following:
1. sudo apt-get install mencoder mplayer
install the appropriate software
2. cat file1.avi file2.avi ...filen.avi > file.avi
we are concatenating files 1 to n
3. mencoder -forceidx -oac copy -ovc copy file.avi -o file_final.avi
to repair some possible break in sync between video and sound due to cat command.