My goal is simple , I have several webm files need to be concated, but first I need to determine their durations.
It seems webm file are played as streams, so there
I ran into your question while trying to solve mine: Re-encoding vlc-created mpeg2 .ts file results in 20 second file
If you want to find the duration, use something like this:
ffmpeg -probesize 90M -analyzeduration 90M -i my_mpeg2_file.webm
I chose 90M since my file was just over 70mb, choose the appropriate size according to your file.
EDIT:
Not sure why it is not working for others, I have now tested with multiple .webm files, and am having success getting the duration for all of them. If it fails for you, can you please provide a link to the webm file so I can test? In my case, it worked for the following:
https://thepaciellogroup.github.io/AT-browser-tests/video/ElephantsDream.webm
http://dl3.webmfiles.org/big-buck-bunny_trailer.webm