title: How to determine webm duration using ffprobe
body: <p>My goal is simple , I have several webm files need to be concated, but first I need to determine their durations.</p>
  
  <p>It seems webm file are played as streams, so there is no way to tell the length of each file.</p>
  
  <p>I have been using ffprobe to do the job ,but the duration returned is N/A.The command I use was:</p>
  
  <pre><code>ffprobe -i input.file -show_format | grep duration
  </code></pre>
  
  <p>thanks.</p>
  
  <p>The complete output of ffprobe list below:</p>
  
  <pre><code> ffprobe version 2.6.2 Copyright (c) 2007-2015 the FFmpeg developers
    built with Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
    configuration: --prefix=/usr/local/Cellar/ffmpeg/2.6.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libtheora --enable-libvorbis --enable-libvpx --enable-vda
    libavutil      54. 20.100 / 54. 20.100
    libavcodec     56. 26.100 / 56. 26.100
    libavformat    56. 25.101 / 56. 25.101
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 11.102 /  5. 11.102
    libavresample   2.  1.  0 /  2.  1.  0
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  1.100 /  1.  1.100
    libpostproc    53.  3.100 / 53.  3.100
  Input #0, matroska,webm, from '231':
    Metadata:
      encoder         : GStreamer matroskamux version 1.5.91
      creation_time   : 2015-12-05 07:59:29
    Duration: N/A, start: 0.000000, bitrate: N/A
      Stream #0:0(eng): Video: vp8, yuv420p, 640x480, SAR 1:1 DAR 4:3, 14.99 fps, 14.99 tbr, 1k tbn, 1k tbc (default)
      Metadata:
        title           : Video
      Stream #0:1(eng): Audio: vorbis, 48000 Hz, stereo, fltp (default)
      Metadata:
        title           : Audio
  duration=N/A
  </code></pre>