If you wish to keep first (top) N lines of a log file and delete the older lines, you can use this command with Cron:
echo "$(head -100 /home/user/public_html/log.txt)" > /home/user/public_html/log.txt
If you wish to keep first (top) N lines of a log file and delete the older lines, you can use this command with Cron:
echo "$(head -100 /home/user/public_html/log.txt)" > /home/user/public_html/log.txt