Hi I need help on ffmpeg or similar ...... Pls help me out

Dear Helper,

I want to
1)Extract a portion of audio/video clip and combine into new clip,
2)adding audio track to a 'video only' track.
3)And For eg there are video clips say v1, v2 and we should merge them v1+v2 into a new one say v3.(similarly with audio clips)
all these, we must be able to do from a program/script.

may be using FFmpeg or any other...

pls help me
Thanks in advance.