Open Terminal and find your devices
ls /dev/video*
arecord -l
When you have found the right devices, make a script like
#! /bin/bash
time=$(date +”%Y-%m-%d-%S”)
mencoder tv:// -tv driver=v4l2:width=800:height=600:device=/dev/video0:fps=30:outfmt=yuy2:forceaudio:alsa:adevice=hw.0,0 -noskip -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1800 -ffourcc xvid -oac mp3lame -lameopts cbr=128 -o video$time.avi
Make the script executable
chmod a+x VHS-capture.sh
Open the script in Terminal and end the script via keyboard command Ctrl c
See also Terratec Grappy VHS capturer