1. 우분투 

cd /etc/rsyslog.d/

sudo nano 50-default.conf

Uncoment line:

#cron.*                         /var/log/cron.log

Save file and restart rsyslog

sudo service rsyslog restart 

Restart your cron daemon for get it's messages from new file

sudo service cron restart

2. 라즈베리파이 

$ vi /etc/rsyslog.conf

and uncomment the line

# cron.*                          /var/log/cron.log

After that, you need to restart rsyslog via

$ /etc/init.d/rsyslog restart 

and you will find the cron logs in /var/log/cron.log