Change .jpg quality of terminal screenshots
Simply add the command: 

sips -s formatOptions [low|normal|high|best|&lt;percent&gt;] ~/Desktop/AutoScreenCap/$vardate.jpg;


after the screencapture line.

Finally it looks like this:

while :; do 
    vardate=$(date +%d\-%m\-%Y\_%H.%M.%S)
    screencapture -t jpg -x ~/Desktop/AutoScreenCap/$vardate.jpg 
    sips -s formatOptions [low|normal|high|best|&lt;percent&gt;] ~/Desktop/AutoScreenCap/$vardate.jpg 
    sleep 600
done


Use only one of the sips formatOptions but without square brackets. The square brackets, the single angle quotes and the vertical lines are meta instructions which mustn't be  entered literally. They mean: choose one of the listed and  the last one is a freely selectable percentage (which should work with or without the %-symbol).

Using low will save about 55% of disk space needed, 10% about 65% and 1% about 70%. This depends on your screen content though.