One suggestion - http://stackoverflow.com/questions/3002601/converting-avi-frames-to-jpgs-on-linux .  Try using google witht he search string "processing avi files on linux"

You have to export each frame as an image and then make a movie out of them. It is better to use the PNG format for the images and MP4 for the movie. You can make the movie using ImageMagick's convert program. It can probably also make AVI movies if you want to stay with that particular format.

AVI is a Microsoft format, which is the likely reason why it is missing in Linux COMSOL.

Thanks Jerry and Kimmo for your inputs

I have got another workaround. COMSOL in Linux gives GIF and SWF animation files. As GIF format is not that good, I have converted the SWF to AVI using a video converter. It works good.

There is a direct way for that, at least for the Windows version of COMSOL.

In the Model Builder, find Export node, right click on it and select Animation. There you should select "Subject", which is one of your plots. And from Format select GIF. For filename it is better to Browse first and select the folder and the filename. Make sure that your solution has at least two frames. If you have a time dependent solution Sequence Type should be the "Stored solutions". If you need to see some sort of modal analysis then you need to select the "Dynamic Data Extension". Finally in order to finish up click Export at the very top of the same window.