ffmpeg command: remove unwanted audio stream from video

konga@ubuntu:~/work$ ffprobe  gf.mkv 
Input #0, matroska,webm, from 'gf.mkv':
  Metadata:
    encoder         : libebml v1.3.0 + libmatroska v1.4.0
    creation_time   : 2014-11-22 12:29:18
  Duration: 01:39:26.02, start: 0.000000, bitrate: 2347 kb/s
    Stream #0:0: Video: h264 (High), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1(chi): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
    Metadata:
      title           : 粤语
    Stream #0:2(chi): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s
    Metadata:
      title           : 国语

konga@ubuntu:~/work$ ffmpeg  -i gf.mkv  -map 0:0 -map 0:2  -vcodec copy -acodec copy out.mkv

Note: -map specifies which streams are used as input, 0:0 means the 0th stream of the 0th input file

konga@ubuntu:~/work$ ffprobe out.mkv 
Input #0, matroska,webm, from 'out.mkv':
  Metadata:
    ENCODER         : Lavf57.25.100
  Duration: 01:39:26.02, start: 0.000000, bitrate: 1900 kb/s
    Stream #0:0: Video: h264 (High), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Metadata:
      DURATION        : 01:39:25.918000000
    Stream #0:1(chi): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s
    Metadata:
      title           : 国语
      DURATION        : 01:39:26.016000000