Schedule tasks using cron, at, and systemd timer units

7. Deploy, Configure, and Maintain Systems

📘Red Hat Certified System Administrator (RHCSA – EX200)

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In Linux, automating tasks is essential for system administration. You rarely want to do everything manually, so Linux provides tools to schedule jobs to run automatically. The main tools for this are:

1. cron – for recurring jobs
2. at – for one-time jobs
3. systemd timer units – newer method, works with systemd, both recurring and one-time jobs

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1. Using cron

cron is a service that runs in the background (daemon) and executes commands at scheduled intervals.

How it works

• cron reads crontab files which list commands to run and when to run them.
• Each user can have their own crontab, including root.

Crontab File Format

Each line in a crontab file follows this pattern:

* * * * * command
- - - - -
| | | | |
| | | | +---- Day of week (0-7) (Sunday=0 or 7)
| | | +------ Month (1-12)
| | +-------- Day of month (1-31)
| +---------- Hour (0-23)
+------------ Minute (0-59)

Example: Run a backup script every day at 2:30 AM:

30 2 * * * /usr/local/bin/backup.sh

Commands to Manage crontab

• crontab -e → edit your own crontab
• crontab -l → list your crontab
• crontab -r → remove your crontab

Special Strings in crontab: These make scheduling easier:

• @reboot → run once at boot
• @daily → run once per day
• @weekly → run once per week
• @monthly → run once per month
• @hourly → run once per hour

Example:

@daily /usr/local/bin/cleanup_logs.sh

System-wide crontab

• Located in /etc/crontab
• Also, scripts can be placed in /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly for automatic execution.

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2. Using at

at is for scheduling one-time jobs, unlike cron which is recurring.

How to use at

1. Install at if not installed:

sudo yum install at

2. Start the service (if not running):

sudo systemctl enable --now atd

3. Schedule a command:

echo "/usr/local/bin/script.sh" | at 14:30

or interactively:

at 14:30
at> /usr/local/bin/script.sh
at> <Ctrl+D>

4. List pending jobs:

atq

5. Remove a scheduled job:

atrm <job_number>

Time formats supported:

• HH:MM (24-hour format)
• now + 10 minutes
• tomorrow
• next week

Example: Run a patch script at 3 PM today:

at 15:00
at> /usr/local/bin/apply_patches.sh
at> <Ctrl+D>

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3. Using systemd Timer Units

Modern Linux uses systemd, which can replace cron and at for both recurring and one-time jobs. This is becoming preferred in Red Hat systems.

Components

1. Service unit (.service) – defines what to run
2. Timer unit (.timer) – defines when to run it

Example Setup

1. Create a service file: /etc/systemd/system/backup.service

[Unit]
Description=Daily Backup Service[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

2. Create a timer file: /etc/systemd/system/backup.timer

[Unit]
Description=Run backup daily at 2:30 AM[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true[Install]
WantedBy=timers.target

3. Enable and start the timer:

sudo systemctl enable --now backup.timer

4. Check timer status:

systemctl list-timers

OnCalendar values:

• *-*-* 02:30:00 → daily at 2:30 AM
• Mon *-*-* 12:00:00 → every Monday at noon

Advantages over cron/at:

• Integrated with systemd logs (journalctl)
• Can trigger based on events, not just time
• Better error handling

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4. Practical IT Examples

• Cron: Schedule log rotation every night at 1 AM:

0 1 * * * /usr/sbin/logrotate /etc/logrotate.conf

• At: Schedule a server restart at 11 PM for maintenance:

echo "shutdown -r now" | at 23:00

• Systemd Timer: Automatically run a backup script every day at 2 AM and log output:

/etc/systemd/system/backup.service
/etc/systemd/system/backup.timer

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5. Key Exam Points

1. Understand differences:
   • cron → recurring tasks
   • at → one-time tasks
   • systemd timer → modern replacement, more flexible
2. Know commands:
   • crontab -e/-l/-r, at, atq, atrm, systemctl enable/start/status timer.service
3. Crontab syntax – must know the 5 fields and special strings.
4. Systemd timer units – create .service + .timer, enable/start, check timers.
5. Always ensure services are running:

systemctl status crond
systemctl status atd
systemctl status backup.timer

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✅ This covers everything for RHCSA 7.1 – scheduling tasks. If you master these commands, understand the syntax, and can create cron/at/systemd timers, you’ll pass this part of the exam.