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Chapter 8.3 - Advanced Cron and systemd Timers

Learning Objectives​

By the end of this chapter, you will be able to:

  • Use system cron (/etc/crontab, /etc/cron.d)
  • Understand anacron
  • Create systemd timers
  • Compare cron vs systemd timers
  • Manage cron logs
  • Implement advanced scheduled tasks

System Cron​

/etc/crontab​

Format with user:

# /etc/crontab
# Format: MIN HOUR DAY MONTH DAY_OF_WEEK USER COMMAND

# Example
0 2 * * * root /usr/local/bin/backup.sh
*/15 * * * * www-data /usr/local/bin/check.sh

Edit:

sudo nano /etc/crontab

/etc/cron.d/​

Directory for system cron files:

# Create file
sudo nano /etc/cron.d/my-task

# Format (with user)
0 2 * * * user1 /usr/local/bin/script.sh

# Important permissions
sudo chmod 644 /etc/cron.d/my-task

Advantages:

  • Organization by file
  • Easy to manage
  • No need for crontab -e

Special Directories​

/etc/cron.hourly/   # Executed every hour (00:00)
/etc/cron.daily/    # Executed every day (06:25)
/etc/cron.weekly/   # Executed every week (Sunday 06:47)
/etc/cron.monthly/  # Executed every month (1st at 06:52)

Place executable scripts (without .sh extension)

Anacron​

For Non 24/7 Systems​

Anacron executes tasks even if the system was off.

# Configuration
/etc/anacrontab

# Format
# period  delay  identifier  command
1         5      backup      /usr/local/bin/backup.sh
7         10     weekly-clean  /usr/local/bin/cleanup.sh

Functioning:

  • Checks if task executed within period
  • If not, executes after delay
  • Ideal for laptops, intermittent systems

systemd Timers​

Modern Alternative to Cron​

Advantages:

  • systemd integration
  • Logs with journalctl
  • Dependencies between units
  • More flexible

Create a Timer​

Service:

# /etc/systemd/system/backup.service
[Unit]
Description=Daily Backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh
User=backup
StandardOutput=journal
StandardError=journal

Timer:

# /etc/systemd/system/backup.timer
[Unit]
Description=Timer for Daily Backup
Requires=backup.service

[Timer]
OnCalendar=daily
OnCalendar=02:00
Persistent=true

[Install]
WantedBy=timers.target

Activate:

sudo systemctl daemon-reload
sudo systemctl enable backup.timer
sudo systemctl start backup.timer

OnCalendar Syntax​

# Format: DayOfWeek Year-Month-Day Hour:Minute:Second

# Every day at 2 AM
OnCalendar=daily
OnCalendar=02:00

# Every Monday at 9 AM
OnCalendar=Mon *-*-* 09:00:00

# The 1st of every month
OnCalendar=monthly
OnCalendar=*-*-01 00:00:00

# Every hour
OnCalendar=hourly

# Every 15 minutes
OnCalendar=*:0/15

Timer Options​

[Timer]
# Scheduling
OnCalendar=daily
OnBootSec=10min
OnUnitActiveSec=1h

# Persistence (execute if missed)
Persistent=true

# Accuracy (approximate execution)
AccuracySec=1min

# Randomization
RandomizedDelaySec=5min

Cron vs Timers Comparison​

FeatureCronsystemd Timers
LogsSeparate filesjournalctl
DependenciesNoYes
PersistenceNoYes (option)
FlexibilityLimitedHigh
SimplicitySimpleMore complex
AvailabilityEverywheresystemd required

Log Management​

Cron Logs​

# System logs
/var/log/syslog  # Ubuntu/Debian
/var/log/cron    # Red Hat/CentOS

# View cron logs
sudo grep CRON /var/log/syslog
sudo tail -f /var/log/syslog | grep CRON

systemd Timers Logs​

# Timer logs
sudo journalctl -u backup.timer

# Service logs
sudo journalctl -u backup.service

# Follow
sudo journalctl -u backup.timer -f

# Since date
sudo journalctl -u backup.timer --since "2024-01-01"

Advanced Examples​

Backup with Timer​

Service:

[Unit]
Description=Automatic Backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh
User=backup
StandardOutput=journal
StandardError=journal

Timer:

[Unit]
Description=Daily Backup Timer

[Timer]
OnCalendar=daily
OnCalendar=02:00
Persistent=true
RandomizedDelaySec=30min

[Install]
WantedBy=timers.target

Cleanup with Cron​

# /etc/cron.daily/cleanup
#!/bin/bash
# Daily cleanup

find /var/log -name "*.log" -mtime +30 -delete
find /tmp -type f -mtime +7 -delete

Monitoring with Timer​

Service:

[Unit]
Description=Check Disk Space

[Service]
Type=oneshot
ExecStart=/usr/local/bin/check-disk.sh

Timer:

[Unit]
Description=Disk Check Timer

[Timer]
OnCalendar=*:0/15
OnBootSec=5min

[Install]
WantedBy=timers.target

Best Practices​

Cron​

  • Use absolute paths
  • Redirect output/errors
  • Test scripts manually
  • Document in comments
  • Check logs regularly

systemd Timers​

  • Create separate service
  • Use Persistent=true if needed
  • Configure RandomizationDelay
  • Test with OnBootSec first
  • Monitor with journalctl

Summary​

In this chapter, you learned:

System cron: /etc/crontab, /etc/cron.d/, special directories
Anacron: For intermittent systems
systemd timers: Modern alternative with dependencies
Syntax: OnCalendar for flexible scheduling
Logs: journalctl for timers, /var/log for cron
Comparison: Simple cron, more powerful timers
Practice: Scheduled backup, cleanup, monitoring

Next Steps​

Module 9: System Security
Module 10: Package Management

Chapter created on: December 2024