The Cron daemon in Linux and how to create a cron job

Thursday, July 25, 2024 at 12:58 PM | 3 min read

Last modified on Friday, July 26, 2024 at 4:33 AM

#linux, #linux mint, #cron daemon, #linux cron alternatives, #anacron, #anacron cron service, #crontab, #automation, #task automation

Task automation: Boring machine drilling hole in wooden plank

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Table of Contents

The Cron daemon

The cron daemon is a built-in Linux utility that reads a crontab (cron table) file and executes commands and scripts listed there at pre-defined times and intervals.

The cron daemon used to also be a built-in utility in macOS, but has now been deprecated in favor of launchd. It is still supported, however. To learn more about cron vs launchd, you can read the article entitled Scheduling Timed Jobs on apple.developer.com.

We set up cron jobs in the crontab file to automate routine maintenance activities, such as updating software, creating backups, or clearing caches.

Crontab Syntax

MIN HOUR DOM MON DOW Command

MIN: stands for minute.

HOUR: stands for hours.

DOM: stands for day of the month.

MON: stands for month.

DOW: stands for day of the week.

These fields determine when the cron job takes place.

CMD: stands for command.

One thing to note about cron jobs in Linux and other OS such as macOS (Unix). Scheduled cron jobs don't run when the cron daemon is not running. However, there are "vanilla" cron daemon alternatives that do run when the OS is either asleep or shut down. For example, one cron daemon alternative that does run when the OS is either asleep or shut down is called anacron, and it is available in Linux Mint by default. anacron is available in Ubuntu and Ubuntu derivatives. Linux Mint is an Ubuntu derivative. I will discuss anacron in a future post.

Creating a simple script for cron automation

Let's create a simple script to add to the crontab for automation to make sure that everything works.

First, I create a new folder in /home/maria (my Linux home directory) called cron-job-scripts:

mkdir cron-job-scripts
# psth to cron-job-scripts in Terminal
maria@maria-VirtualBox:~/cron-job-scripts$

And inside the cron-job-scripts directory, I create a file called current_date_time.sh:

# psth to cron-job-scripts in Terminal
maria@maria-VirtualBox:~/cron-job-scripts$ touch current_date_time.sh

Creating a new or editing an existing crontab file

Next, I run the following command that either creates a new crontab file for me or edits an existing one:

crontab -e
# which returns in my case:
no crontab for maria - creating an empty one
# If you have not already selected a default text editor to use in Terminal, something like the following may appear
Select an editor.   To change later, run 'select editor'.
1.  /bin/nano       <---- easiest
2.  /usr/bin/vim.tiny
3. /bin/ed

Choose 1-3 [1]: 1

Creating the cron job itself

Next, after I have gone into the crontab file in Vim (my editor of choice), I add the following line at the bottom of the file (below all the commented lines):

45 22 * * * /home/maria/cron-job-scripts/current_date_time.sh

Then I save my changes and exit out of Vim. I already have everything set up as far as email notification when the cron job is executed, so no problem there. At 10:45pm (represented by 45 22), I checked my local mbox, which is located in my home directory (/home/maria) by running the mailx command:

mailx

Then the new email appears, indicated by "N", and the ? prompt appears at the end. I type the number of the email after the ? and hit the Enter key. And then the contents of the email appears:

mailx
# returns:
"/home/maria/mbox": 3 messages 1 new
     1 Maria              Thu Jul 25 19:09  48/1795  Another attachment for yo
     2 Maria              Thu Jul 25 20:13 684/18504 Sendiing history log file
>N   3 Cron Daemon        Thu Jul 25 22:45  21/854   Cron <maria@maria-Virtual
? 3
Return-Path: <maria@maria-VirtualBox.mynetworksettings.com>
X-Original-To: maria
Delivered-To: maria@maria-VirtualBox.mynetworksettings.com
Received: by maria-VirtualBox.mynetworksettings.com (Postfix, from userid 1000)
	id 405E52EA36A; Thu, 25 Jul 2024 22:45:01 -0400 (EDT)
From: root@maria-VirtualBox.mynetworksettings.com (Cron Daemon)
To: maria@maria-VirtualBox.mynetworksettings.com
Subject: Cron <maria@maria-VirtualBox> /home/maria/cron-job-scripts/current_date_time.sh
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/home/maria>
X-Cron-Env: <LOGNAME=maria>
Message-Id: <20240726024501.405E52EA36A@maria-VirtualBox.mynetworksettings.com>
Date: Thu, 25 Jul 2024 22:45:01 -0400 (EDT)
Status: R
X-UID: 4

Current Date and Time: Thu Jul 25 10:45:01 PM EDT 2024

?

The result of the cron job is at the bottom, right above the ? prompt. Each time I run this cron job every day at 10:45pm IF the computer is powered on, meaning Linux Mint in VirtualBox is running, I will receive a local email stating the Current Date and Time. Cool, right?