Ubuntu Crontab (Cron job)

#1. list all current cron jobs
> sudo crontab -l

#2. edit crontab
> sudo crontab -e

#3. cronjob format
* * * * * [command/task]
mm hh d m y 
mm: minute (0-59)
hh: hour (0-23)
d: day (0-31)
m: month (0-12)
w: weekday (1 - Mon, 2- Tue, ... 5- Fri, 6 - Sat, 7- Sun)

Example:
30 8 * * * open https://vnexpress.net
=> open website vnexpress.net everyday at 8.30 AM