Ubuntu – Good PPA for ffmpeg

The maintainer hasn't built the .deb packages for 13.04 yet. And I don't think you'll find them anywhere else.

You could wait who knows how long till he gets around building them, or downgrade to 12.10. Or you could do what I and many others do: Compile it from source yourself. It's pretty straightforward, and this approach comes with many advantages such as giving us the ability to make optional customizations, such as adding support for more codecs for example.

FFmpeg provides a step-by-step tutorial, which I myself have followed many times without incidents so far:

http://ffmpeg.org/trac/ffmpeg/wiki/UbuntuCompilationGuide

You can uninstall your custom build and go back to the repositories versions any time you want(but honestly why would you).

This will get you the duration of your file in seconds:

let A=$(avprobe input.webm 2>&1 | grep "^ *Duration: " | sed "s/^ *Duration: \\([0-9:]*\\).*/(((\\1)/" | sed "s/0\\([0-9]\\)/\\1/g" | sed "s/:/)*60+/g")

Although avprobe does return the fractions of the seconds, bash doesn't deal with fractions. So i'm just dropping everythinging that is less than 1 second.

After that you can easily do something like this:

let A=$A-5
avconv -i input.webm -t $A -c:v copy -c:a copy output.webm

Sorry for not using ffmpeg. I've switched to avconv a while ago, i believe that the options should be the same, and if not, it should be trivial to rewrite them as needed.

After this the question is only in wrapping the whole thing in the for loop and doing all the files at once.