To list available crontab:
crontab -l

To delete:
crontab -r

Privileged users can access others crontab:
crontab -u

Run cron jobs as a different user:

su - <user> -c <command or script>

Depending on your default editor crontab will either open in vi or nano. Crontab line format should look like:

# m h dom mon dow user command

Under which you will have to specify when your cron job should run:

m: minute of the day, from 0 to 59
h: hour of the day, from 0 to 23
dom: day of month, from 1 to 31
dow: day of week, from 1 to 7
user: user your cron job should run as
command: command or script that should run

Root has access to two crontabs. One is accessible through crontab and the other can be opened in your editor: vi /etc/crontab or nano /etc/crontab. Systemwide cron jobs should be added to /etc/crontab which take precedence over your regular cron.

A sample cron job should look like the following line. Here is a line I have in my crontab to sync time with external time servers:

# m h dom mon dow user command
01 0 * * * root ntpdate us.pool.ntp.org

This cron job will sync time at 12:01 AM every night with pool.ntp.org

Tip: to create a cron job to run every time your system reboots (game servers, etc.) start your line with @reboot:

@reboot /home/user/script

Notice that you don’t need to specify anything but @reboot and path to executable. Regular and privileged users can both use this command.

Other commands:

@reboot = run at boot and reboot only
@yearly = run at midnight Jan 1 (0 0 1 1 *)
@annually = run at midnight Jan 1(0 0 1 1 *)
@monthly = run at midnight on the first day of every month (0 0 1 * *)
@weekly = run at midnight every Sunday (0 0 * * 0)
@daily = run at midnight every day (0 0 * * *)
@midnight = run at midnight (0 0 * * *)
@hourly = run on the first second of every hour (0 * * * *)

Related posts:

1. Linux: Share a directory between local users
2. Debian: Sync time and date with external servers