#Red Hat / CentOs:
service crond start
#or
/etc/init.d/crond start
#Ubuntu:
sudo service cron start
#or
sudo /etc/init.d/cron start#write out current crontab
crontab -l > mycron
#echo new cron into cron file
echo "00 09 * * 1-5 echo hello" >> mycron
#install new cron file
crontab mycron
rm mycron
# to list all crontab content (including scheduled jobs)
crontab -l
# to edit crontab file
crontab -e
# once you have finished adding tasks, save the file and exit
# the cron daemon will read and execute the instructions provided