FFmpeg does not write to a specific log file, but rather sends its output to standard error. To capture that, you need to either
- capture and parse it as it is generated
- redirect standard error to a file and read that afterward the process is finished
Example for std error redirection:
ffmpeg -i myinput.avi {a-bunch-of-important-params} out.flv 2> /path/to/out.txt
Once the process is done, you can inspect out.txt.
It’s a bit trickier to do the first option, but it is possible. (I’ve done it myself. So have others. Have a look around SO and the net for details.)