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

Photo by Ono Kosuki on pexels.com

This post was originally published on my personal blog mariadcampbell.com.

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: <[email protected]>
X-Original-To: maria
Delivered-To: [email protected]
Received: by maria-VirtualBox.mynetworksettings.com (Postfix, from userid 1000) id 405E52EA36A; Thu, 25 Jul 2024 22:45:01 -0400 (EDT)
From: [email protected] (Cron Daemon)
To: [email protected]
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?

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