u20 crontab

https://crontab.guru/

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of the month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of the week (0 - 6) 
│ │ │ │ │
│ │ │ │ │
│ │ │ │ │

* * * * * command

install

apt install cron

enable service

systemctl enable --now cron

service cron status

edit config file

sudo ls /var/spool/cron/crontabs/

sudo vim /var/spool/cron/crontabs/

log

sudo apt-get install postfix

or add
>/dev/null 2>&1 to every job:

* * * * * yourCommand >/dev/null 2>&1

• modify rsyslog config: open
  /etc/rsyslog.d/50-default.conf
  remove # before cron.*

• restart rsyslog service:

sudo service rsyslog restart

• restart cron service:

sudo service cron restart

tail -f /var/log/cron.log

your_command 2>&1 | tee -a file.txt

https://serverfault.com/questions/136461/how-to-check-cron-logs-in-ubuntu