i need to convert my video files with VP8 / Opus in Matroska(mkv) container using
avconv in terminal
Can anyone give example usage of it?
Actually you can't because the Matroska container doesn't support the Opus audio codec yet.
source: Codec Specs | Matroska
Apart from that the most simple way to transcode a video with VP8 is to use transmageddon
Opus support for transmageddon was added in November 2012 (just after 0.25 was released), so you'll have to wait for 0.26. (source: Gnome Transmageddon Repository)
Alternatively you can compile ffmpeg from source (otherwise you won't have VP8 or Opus) following the instructions on the ffmeg site.