What is a Cron?

A Cron is a time-based job scheduler in Unix Operating Systems.

It is driven by Crontab (cron table) - a configuration file that specifies shell commands to run periodically on a given schedule.

Crontab files are usually stored system-wide in /etc and only sudo users can edit.

Basic Cron Syntax
* * * * * command_to_execute

e.g:
30 2 * * * rm /log..txt

This cron job will run everyday at 2:30am and remove the file log.txt from server root.

* * * * *
| | | | |___________ day of week (0 - 6) (Saturday to Sunday) (7 is also Sunday on some systems)
| | | |____________ month (1 -12)
| | |____________ day of month (1 -31)
| |____________ hour (0 - 23)
|____________minute (0 - 59)

Looking at another example: 59 23 31 12 * rm /log..txt
This cron will be executed once a year on 31st December at 11:59 pm. One minute to every new year.

Standard Characters Allowed in Cron Syntax:

  • Commas (,): Used to separate items of a list. e.g: Mon,Tue,Wed. Notice the absence of space in the list.
  • Hyphen (-): Defines ranges. e.g 2017-2025 is every year from 2017 to 2025. Both years inclusive.
  • Percentage (%): Are changed into newline characters unless escaped by backslash (). Then all the data after the first % aare sent to the command as standard inputs.

Non Standard Characters in Cron Syntax:

  • Hash (#): Only allowed in the day of the month field. Must be followed by a number between 1 and 5. Specifies scenarios such as 2nd Wednesday of the month.
    e.g: 5#3 implies 3rd Friday of every month.
  • Question Mark (?): Used instead of * for blank values. Some Crons substitute this with the startup time of the cron daemon. e.g: ? ? * * * would be updated to 30 7 * * * if the cron daemon started at 7.30am. And would continue to run at this specific time until restarted.
  • Slash (/): Used to specify step values or intervals. e.g: */5 in the minutes fields indicates every 5 minuts. shorthand for 5,10,15,20,25,30,35,40,45,50,55,00.

For more interactive samples of crontabs, checkout:
- https://crontab.guru
- https://help.ubuntu.com/community/CronHowto