Recently I needed to show some people how to do something, rather than writing a load of descriptions and capturing lots of screen captures, it was easier to record my session. For this you will need a recent version of ffmpeg, im using this one

[jtang@duo ~]$ ffmpeg -version
FFmpeg version SVN-r15261, Copyright (c) 2000-2008 Fabrice Bellard, et al.
  configuration: --prefix=/usr --incdir=/usr/include/ffmpeg
  --libdir=/usr/lib64 --shlibdir=/usr/lib64
  --mandir=/usr/share/man --arch=x86_64 --extra-cflags=-O2
  -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
  -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic
  -D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112 -fasm -std=c99
  -fno-math-errno --enable-libdc1394 --enable-libfaac
  --enable-libfaad --enable-libgsm --enable-libmp3lame
  --enable-libtheora --enable-libvorbis --enable-libx264
  --enable-libxvid --enable-x11grab --enable-avfilter
  --enable-avfilter-lavf --enable-postproc --enable-swscale
  --enable-pthreads --disable-static --enable-shared
  --enable-gpl --enable-x11grab --enable-libdirac --disable-debug
  --disable-optimizations --disable-stripping
  libavutil     49.10. 0 / 49.10. 0
  libavcodec    51.71. 0 / 51.71. 0
  libavformat   52.22. 1 / 52.22. 1
  libavdevice   52. 1. 0 / 52. 1. 0
  libavfilter    0. 1. 0 /  0. 1. 0
  libswscale     0. 6. 1 /  0. 6. 1
  libpostproc   51. 2. 0 / 51. 2. 0
  built on Jan 10 2009 12:53:49, gcc: 4.1.2 20071124 (Red Hat 4.1.2-42)
FFmpeg SVN-r15261
libavutil     49.10. 0 / 49.10. 0
libavcodec    51.71. 0 / 51.71. 0
libavformat   52.22. 1 / 52.22. 1
libavdevice   52. 1. 0 / 52. 1. 0
libavfilter    0. 1. 0 /  0. 1. 0
libswscale     0. 6. 1 /  0. 6. 1
libpostproc   51. 2. 0 / 51. 2. 0

Note --enable-x11grab is needed for all this to work. First you need to pick the window you want to record by using xwininfo

[jtang@duo ~]$ xwininfo -frame

xwininfo: Please select the window about which you
      would like information by clicking the
      mouse in that window.

xwininfo: Window id: 0x800529 "urxvt"

  Absolute upper-left X:  2229
  Absolute upper-left Y:  231
  Relative upper-left X:  2229
  Relative upper-left Y:  231
  Width: 478
  Height: 303
  Depth: 32
  Visual Class: TrueColor
  Border width: 1
  Class: InputOutput
  Colormap: 0x800522 (not installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +2229+231  -491+231  -491-664  +2229-664
  -geometry 79x22+2229+231

It will ask you to select a window, this will give you the window location and dimensions including the window decor. You will need from the xwininfo output is the size of the window, and the co-ordinates of the window (the co-ordinate of the upper left hand window) I my case I needed the absolute X and absolute Y co-ordinates and the Height and Width.

Now to run ffmpeg to capture your session

ffmpeg -f x11grab -s 478x302 -b 10000  -i :0.0+2229,231 out.avi

According to the docs at ffmpeg, in my case

• 0.0 is display.screen number of your X11 server, same as the DISPLAY environment variable.
• 2229 is the x-offset and 231 the y-offset for the grabbing.
• 478x302 is the Height and Width of the window I want to capture, noticed I rounded it down to 302, this is because some (if not all) video formats require a number divisible by 2 (or even 4) for the input/output sizes
• 10000 is the bitrate in bits per second (need to check this), the bigger the value the better the quality.
• out.avi is of course the target output file.
• you can also specify the frame rate with the -r option