Day 06 · Linux

Create a Cron Job

By Published Linux~1h to complete

Technical Overview

Automating routine administrative tasks is a core practice in DevOps and system administration. In Unix-like operating systems, the cron daemon (crond) serves as a time-based job scheduler. It enables users to run shell commands, scripts, or system tasks automatically at specific times, dates, or intervals.

Individual user schedules are managed using crontabs (cron tables). These are configuration files containing lists of commands and their execution schedules. The system daemon constantly runs in the background and checks the system's crontab files once per minute to determine if any jobs are scheduled for execution.

This guide outlines the complete syntax of the cron expression format, details the steps to install and enable the cron daemon on Nautilus application servers, sets up a cron job for the root user that writes to a temporary file every 5 minutes, and verifies its execution.

Understanding the Cron Format

A cron job is defined by a single line containing five time and date fields, followed by the command to execute:

.---------------- 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
|  |  |  |  |
*  *  *  *  *  command to be executed

Cron Operators

To schedule complex intervals, you can use specialized operator characters:

Operator Name Description Example
* Wildcard Represents all possible values in that field. * in the hour field means "every hour".
, Value List Specifies a list of discrete values for execution. 1,3,5 in the hour field means "at 1 AM, 3 AM, and 5 AM".
- Range Defines a range of consecutive values. 1-5 in the day-of-week field means "Monday to Friday".
/ Step / Interval Specifies step values or repeating intervals. */5 in the minute field means "every 5 minutes".

Common Cron Schedule Examples

  • Every minute: * * * * *
  • Every 5 minutes: */5 * * * *
  • At the top of every hour: 0 * * * *
  • Daily at midnight: 0 0 * * *
  • Every Sunday at 4:30 AM: 30 4 * * 0
  • First day of every month at 2:00 AM: 0 2 1 * *
  • Weekday working hours (9 AM to 5 PM, Mon-Fri): 0 9-17 * * 1-5

Infrastructure & Configuration Requirements

  • Target Hosts: Nautilus Application Servers (e.g., stapp01, stapp02, stapp03)
  • SSH Users: Standard administrative users (e.g., tony, steve, banner)
  • Cron Execution Identity: root
  • Cron Schedule: Every 5 minutes (*/5 * * * *)
  • Target Command: echo hello > /tmp/cron_text

Step-by-Step Implementation

Apply the following setup instructions to each of your application servers:

Step 1: Connect to the Server

SSH into the target application server from the Jump Host:

# Example for App Server 1
ssh tony@stapp01

Step 2: Install the Cron Utility

By default, some minimal server environments might not have the cron package pre-installed. Install cronie (the standard cron daemon for RHEL/CentOS):

sudo yum install -y cronie

Step 3: Start and Enable the Cron Service

Ensure the cron daemon is configured to start automatically on system boot and start the service in the current session:

# Start and enable the service
sudo systemctl enable --now crond

Verify that the daemon is active and running:

sudo systemctl status crond

Step 4: Configure the Cron Job for the Root User

Open the crontab editor for the root user. Using the crontab tool ensures that your configuration is syntax-validated before saving:

sudo crontab -e

Add the following line to the crontab configuration:

*/5 * * * * echo hello > /tmp/cron_text

Save and close the editor. If you are using the default vi editor:

  1. Press Esc.
  2. Type :wq and press Enter.
  3. The terminal should display: crontab: installing new crontab.

Post-Deployment Verification

1. Inspect the Active Crontab

Verify that the cron job was successfully written and is active for the root user:

sudo crontab -l

Expected output:

*/5 * * * * echo hello > /tmp/cron_text

2. Monitor Log Execution

Check the cron utility logs under /var/log/cron to verify that the cron daemon is executing the scheduled job:

sudo tail -f /var/log/cron

Expected execution trace snippet:

Jun 27 12:00:01 stapp01 CROND[10245]: (root) CMD (echo hello > /tmp/cron_text)

3. Check the Output File

Wait at least 5 minutes for the job to fire, and verify that the output file /tmp/cron_text has been created with the expected content:

cat /tmp/cron_text

Expected output:

hello

Log out of the Application Server to return to the Jump Host:

exit

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