Cron & Crontab : Linux Scheduler Made Simple

Cron & Crontab : Linux Scheduler Made Simple

Getting Started

Cron or also known as cron job; is a time-based job scheduler in Unix-like operating systems. It is very useful for cases like doing system backup, recovery, integration, updates, and many more. Crontab (Cron Table) on the other hand is a configuration file that specifies what and when a certain job will be executed.

Here are lists of paths where the crontab configuration is located in your system :

  • Mac OS X : /usr/lib/cron/tabs/
  • FreeBSD/OpenBSD/NetBSD : /var/cron/tabs/
  • CentOS/Red Hat/RHEL/Fedora/Scientific Linux : /var/spool/cron/
  • Debian / Ubuntu Linux : /var/spool/cron/crontabs/
  • HP-UX Unix : /var/spool/cron/crontabs/
  • IBM AIX Unix : /var/spool/cron/

Installation

Although for most cases, cron has already been pre-installed in your system if it isn’t, here are steps by step on how to install it :

  1. Do system update & upgrade
    sudo apt-get update && apt-get upgrade
  2. Install cron package
    sudo apt-get install cron
  3. Verify cron service is running
    systemctl status cron

Crontab Syntax

Crontab comes with predefined syntax; This syntax is used to define two cron parameters which are Time and Command to execute.

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed

For instance, you can do system update & upgrade daily at 00:00 by defining this syntax in your crontab config :

0 0 * * * apt-get update && apt-get upgrade

Crontab Guru

One of the most popular tools that are being used by developers to set up a cron syntax is by using a tool called Crontab.guru , this tool will help you to generate the correct time syntax for your cron job. There are also samples on frequently used crontab syntax e.g every 15 minutes, every hour, daily, weekly, monthly, and many more.

How to add a cron job to crontab (cron table)

Cron comes with a crontab utility tool command that helps us dealing with cron tables e.g (Add, Remove, Lists). To add a jo to the table you can simply execute the command :

crontab -e

Then you will need to put the job syntax (Time and Command) at the very end of your crontab file.

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed

0 0 * * * apt-get update && apt-get upgrade

Notes :

Crontab also comes with some other useful tags, which are :

  • -e :  Edit current crontab
  • -l : Lists all your cron jobs
  • -r : Delete the current cron jobs

Conclusion

Cron can be a very powerful tool to set up an automated tasks/jobs in your Unix environment, though there is still a lack of support for multiple command execution, logging, integrations, etc; it is still one of the easiest and simplest ways to set up automated task. For some cases just like mentioned before, there is some more powerful tool that we recommend you to check which are Apache Airflow, which uses DAGs (Direct Acyclic Graphs) to manage a set of processes within a job, it also supports logging and many more.

Reference

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