It's in the FAQ: http://www.lavrsen.dk/foswiki/bin/view/Motion/FrequentlyAskedQuestions#How_can_I_get_a_big_movie_per_da
figure out a way to name your files, like YEAR-MONTH-DAY-EVENT (eg. 09-04-17-23.avi) and then put a command in crontab to run every night:
 mencoder `date +%y-%m-%d`-*.avi -noidx -o `date +%y-%m-%d`.avi -ovc copy -oac copy
this will output a file named 09-04-17.avi with all the smaller ones included. In theory at least, I haven't tested it myself but it should work. You must also add the whole path to the moviefolder (I guess).

Good luck!

2009/4/18 Grant <emailgrant@gmail.com>
It would be very useful for me to be able to string together a day's
avi's into a single avi.  Is anyone doing this?

- Grant

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