038 Schedule Tasks Using at and Cron

Tech Tutorial: Deploy, Configure, and Maintain Systems #

Introduction #

In the realm of system administration, particularly when preparing for the Red Hat Certified System Administrator (RHCSA) exam, understanding how to schedule tasks efficiently is crucial. In Linux systems like Red Hat Enterprise Linux (RHEL), two primary tools are used for task scheduling: at and cron. This tutorial aims to provide a comprehensive understanding of how to use these tools to automate tasks on a schedule, ensuring your systems run smoothly and efficiently without manual intervention.

Prerequisites #

Before diving into the tutorial, ensure you have:

  • Access to a RHEL system (version 7 or higher recommended).
  • Sudo or root access to manage scheduling tasks.
  • Basic familiarity with command-line interface (CLI) operations.

Step-by-Step Guide #

Part 1: Using the at Command #

The at command is used for scheduling a job that needs to be executed once at a specified time.

1. Installing the at package #

Before using at, you need to ensure it is installed on your system. You can install it using the following command:

sudo yum install at

2. Starting the atd service #

To use at, the atd daemon must be running. You can start and enable it on boot using:

sudo systemctl start atd
sudo systemctl enable atd

3. Scheduling a Job with at #

To schedule a job, use the at command followed by the time the job should run. For example, to schedule a job at 5 PM today:

echo "echo 'Task executed' > /tmp/task.log" | at 5pm

4. Listing Scheduled Jobs #

To see a list of scheduled jobs, use:

atq

5. Removing a Scheduled Job #

If you need to cancel a job, you can remove it using its job number:

atrm <job number>

Part 2: Using the cron Command #

The cron daemon allows you to schedule recurring tasks using a crontab file.

1. Understanding Crontab Syntax #

A crontab file consists of lines of six fields each. The fields are separated by spaces or tabs. The first five are integer patterns that specify:

  • minute (0-59),
  • hour (0-23),
  • day of the month (1-31),
  • month (1-12),
  • day of the week (0-7, where both 0 and 7 mean Sun).

The sixth field is a command to be run at the specified times.

2. Editing Crontab Entries #

To edit the crontab file for the current user, use:

crontab -e

3. Scheduling a Cron Job #

For instance, to back up a directory every day at 3 AM, you might add the following line to the crontab:

0 3 * * * /usr/bin/rsync -a /home/user/data /backup/data

4. Listing Cron Jobs #

To display the current user’s crontab entries:

crontab -l

5. Removing Cron Jobs #

To remove all cron jobs for the current user:

crontab -r

Detailed Code Examples #

Let’s create a practical example using both at and cron.

Example: Using at to Run a Script #

Suppose we need to run a script (/path/to/script.sh) at 4:15 PM on the coming Monday. The script is intended to compress logs. Here’s how we’d set that up:

echo "/bin/bash /path/to/script.sh" | at 4:15 PM Mon

Example: Using cron for Daily Database Backup #

To back up a database at midnight every day, you could add the following line to your crontab:

0 0 * * * /usr/bin/mysqldump -u root -pPassword database > /backup/db_`date +\%Y\%m\%d`.sql

Conclusion #

Understanding and utilizing the at and cron commands are fundamental skills for any system administrator, especially when preparing for the RHCSA exam. These tools help in automating tasks, ensuring that essential operations like backups, updates, and custom scripts are performed consistently and without direct intervention. Practice setting up various jobs using these commands to gain confidence in managing and automating your Red Hat system effectively.