Scheduling with at and cron on RHEL

Scheduling tasks on Red Hat Enterprise Linux is an essential skill for system administrators who need to automate maintenance activities, scripts, and recurring operations. RHEL provides two powerful tools for this purpose: at for one-time scheduled jobs and cron for recurring tasks.

In this guide, we will explore how to use at and cron on RHEL, including daemon configuration, job management commands, and practical scheduling examples. These tools allow administrators to automate processes without requiring manual execution.

When you need to automate tasks on Red Hat Enterprise Linux, you have two go-to tools:

• at for one-off, one-time jobs.
• cron (via crontab) for recurring schedules.

The “at” command

at queues single-run jobs to be executed once, at a specified future time.

Key Concepts:

• Jobs are stored in /var/spool/at (job files) and /var/spool/at/jobs.
• The atd daemon must be running (systemctl status atd).
• Each job gets an ID you can list, remove, or view.

Common Subcommands

Schedule a command or script:

echo "/path/to/script.sh" | at 02:30 pm tomorrow

Interactive scheduling:

at now + 1 hour
at> /usr/bin/backup.sh
at> <EOT>     # press Ctrl+D to end

List pending jobs:

atq

Remove a job by its ID:

atrm 5

Time Specifications

You can use a variety of formats:

• HH:MM (24-hour) or h:mm am/pm
• Relative: now + 45 minutes, now + 2 days
• Named: noon, midnight, teatime (4 pm)

The “cron” Daemon and the “crontab”

cron runs jobs on a recurring schedule defined by fields in a crontab file.

Anatomy of a Crontab Entry

Managing Crontabs

Edit your user’s crontab:

crontab -e

List your scheduled jobs:

crontab -l

Edit a crontab for a specific user – Considering, for instance, that the username is “thor”:

crontab -e -u thor

Remove all your jobs:

crontab -r

List and remove jobs for a particular username, respectively:

crontab -l -u thor
crontab -r -u thor

System-wide schedules live in:

• /etc/crontab
• /etc/cron.d/ (drop-in files)
• /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly

Special Strings to use on crontab

Instead of five fields, you can use shortcuts:

Shortcut	Equivalent
@reboot	Run once at system boot
@yearly	0 0 1 1 *
@monthly	0 0 1 * *
@weekly	0 0 * * 0
@daily	0 0 * * *
@hourly	0 * * * *

Comparison: at vs. cron

Here is a basic comparison between at and cron:

Feature	at	cron
Frequency	Once	Repeating
Configuration file	atq queues; /var/spool/at job files	crontab -e; /etc/cron.*
Use case	Ad hoc, immediate scheduling	Regular, periodic maintenance and jobs
Removal	atrm <jobID>	crontab -r (user) or edit crontab

Scheduling Jobs with “at”

Firstly, check if the daemon “atd” is running:

systemctl status atd

If the “atd” daemon is not running, enable and start it:

systemctl enable --now atd

Let’s provide an example of using “at” to schedule a one-time job:

echo "hello world" > greetings.txt | at 07:36 am

As we can see, the command “echo” was scheduled to run at a specific time:

As a result, the file “greetings.txt” was created correctly:

To see the “at” job queue (jobs pending to be executed):

atq

In this case, for instance, the job ID “3” is pending execution:

To remove a pending job:

atrm <JOB_ID>

In this example, the job ID “4” was removed successfully:

Scheduling Jobs with “cron”

Before starting, check if the “crond” daemon is running:

systemctl status crond

If the daemon is not running, enable and start it first:

systemctl enable --now crond

Let’s create a crontab entry considering that the current user is “thor”:

crontab -e

# Add the following entry:
*/2 * * * * /home/thor/jobs/scripts/hello_world.sh >> /home/thor/jobs/logs/hello_world.log 2>&1

Where:
*/2 –> Runs every 2 minutes
/home/thor/jobs/scripts/hello_world.sh –> Script to be executed
/home/thor/jobs/logs/hello_world.log –> Log file, in case of errors or something that is not expected

To inspect the user’s crontab:

crontab -l

To remove the user’s crontab:

crontab -r

Common Cron Examples

1- Run a script every day at midnight:

0 0 * * * /home/user/backup.sh

2- Run a command every 15 minutes:

*/15 * * * * /usr/bin/php /var/www/html/cron.php

3- Run a job every Monday at 8 AM:

0 8 * * 1 /home/user/monday_report.sh

4- Run a job on the 1st day of every month at 5 AM:

0 5 1 * * /home/user/monthly_cleanup.sh

5- Run a job only in December at 10 AM:

0 10 * 12 * /home/user/holiday_script.sh

6- How do you schedule a cron job to run every Sunday at 3 AM?

0 3 * * 0 /path/to/your/script.sh

0 3: At 3:00 AM
* *: Every day and every month
0: Sunday (can also use 7)

7- What does */10 * * * * mean?

*/10 * * * * /path/to/your/script.sh

This runs the job every 10 minutes.

• */10: Every 10 minutes
• The remaining * * * * means every hour, every day, every month, and every day of the week.

8- How do you redirect the output of a cron job to a log file?

0 0 * * * /home/user/script.sh >> /home/user/script.log 2>&1

• >>: Appends standard output to the log file
• 2>&1: Redirects standard error to the same log file

This ensures both output and errors are logged.

9- How do you ensure a cron job runs only on weekdays?

0 9 * * 1-5 /path/to/your/script.sh

• 1-5: Monday to Friday
• 0 9: At 9:00 AM

This skips weekends entirely.

10- How do you check if your cron job ran?

grep CRON /var/log/syslog       # Debian/Ubuntu
grep CRON /var/log/cron         # CentOS/RHEL

That’s it for now 🙂