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Systemd Timers

Chapter 82: systemd Timers

Section titled “Chapter 82: systemd Timers”

Comprehensive Guide to Scheduled Tasks with systemd

Section titled “Comprehensive Guide to Scheduled Tasks with systemd”

Why This Matters in DevOps/SRE

Section titled “Why This Matters in DevOps/SRE”

systemd timers are the modern replacement for cron in Linux systems:

  • Modern Systems: New Linux systems use systemd timers instead of cron
  • Reliability: Timers can persist missed jobs, handle system sleep
  • Logging: Better integration with journald for log management
  • On-Call: You’ll manage scheduled jobs and investigate missed executions
  • Certification: RHCSA and RHCE cover systemd timers

Most production servers now use systemd timers for scheduled tasks.

82.1 Understanding systemd Timers

Section titled “82.1 Understanding systemd Timers”

Timer Overview

Section titled “Timer Overview”
                 systemd Timers
+------------------------------------------------------------------+
|                                                                   |
|    What are timers?                                                |
|    +----------------------------------------------------------+  |
|    | • Units that trigger service activation at specific times |  |
|    | • Modern replacement for cron                           |  |
|    | • More precise and flexible                             |  |
|    | • Survive reboots with Persistent=true                 |  |
|    +----------------------------------------------------------+  |
|                                                                   |
|    Components:                                                     |
|    +----------------------------------------------------------+  |
|    | Timer unit (.timer) - defines schedule                 |  |
|    | Service unit (.service) - defines what to run           |  |
|    +----------------------------------------------------------+  |
|                                                                   |
|    Advantages over cron:                                           |
|    +----------------------------------------------------------+  |
|    | • Dependencies on other services                        |  |
|    | • Can be triggered by events                           |  |
|    | • Better logging via journal                           |  |
|    | • Can run missed jobs on boot                         |  |
|    +----------------------------------------------------------+  |
|                                                                   |
+------------------------------------------------------------------+

82.2 Timer Configuration

Section titled “82.2 Timer Configuration”

Timer Unit File

Section titled “Timer Unit File”
/etc/systemd/system/mytimer.timer
[Unit]
Description=My Scheduled Timer
Requires=myservice.service


[Timer]
# Time-based triggers
OnCalendar=daily                  # Every day at midnight
OnCalendar=Mon *-*-* 02:00:00    # Every Monday at 2 AM
OnCalendar=*:0/15                # Every 15 minutes
OnCalendar=*-*-01 00:00:00       # First day of month


# Time from events
OnBootSec=5min                  # 5 minutes after boot
OnActiveSec=1h                   # 1 hour after activation
OnUnitActiveSec=1h              # 1 hour after service last ran


# Options
Persistent=true                  # Run missed jobs on boot
RandomizedDelaySec=1h           # Random delay to prevent storms
WakeSystem=true                 # Wake system from suspend
AccuracySec=1us                # Accuracy (default: 1min)
Unit=myservice.service         # Default service to trigger


[Install]
WantedBy=timers.target

Calendar Time Format

Section titled “Calendar Time Format”
                 Calendar Format
+------------------------------------------------------------------+
|                                                                   |
|    Examples:                                                      |
|    +----------------------------------------------------------+  |
|    | daily            | Every day at midnight                 |  |
|    | hourly           | Every hour                           |  |
|    | weekly           | Every Monday at midnight             |  |
|    | monthly          | First day of month                  |  |
|    | *-*-01 00:00:00| 1st of month at midnight            |  |
|    | Mon *-*-* 02:00 | Every Monday at 2 AM                 |  |
|    | *-*-* *:0/15    | Every 15 minutes                     |  |
|    | *-*-* 12:00:00  | Every day at noon                    |  |
|    | 2024-*-* 00:00   | Daily after Jan 1, 2024            |  |
|    | *:0/5:0          | Every 5 seconds                      |  |
|    +----------------------------------------------------------+  |
|                                                                   |
+------------------------------------------------------------------+

82.3 Service Unit

Section titled “82.3 Service Unit”
/etc/systemd/system/myservice.service
[Unit]
Description=My Scheduled Service


[Service]
Type=oneshot                     # Run once and exit
ExecStart=/usr/local/bin/myscript.sh
User=root
StandardOutput=journal
StandardError=journal


# Or for long-running services:
# Type=simple
# RemainAfterExit=no

82.4 Management Commands

Section titled “82.4 Management Commands”
Terminal window
# List all timers
systemctl list-timers
systemctl list-timers --all


# Start/stop timer
sudo systemctl start mytimer.timer
sudo systemctl stop mytimer.timer


# Enable/disable timer
sudo systemctl enable mytimer.timer
sudo systemctl disable mytimer.timer


# Check timer status
systemctl status mytimer.timer


# View next run times
systemctl list-timers | grep mytimer


# Manual trigger (test)
sudo systemctl start myservice.service


# View related logs
journalctl -u myservice.service
journalctl -u mytimer.timer -f

82.5 Examples

Section titled “82.5 Examples”

Daily Backup Timer

Section titled “Daily Backup Timer”
/etc/systemd/system/daily-backup.timer
[Unit]
Description=Daily Backup Timer
Requires=daily-backup.service


[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=30m


[Install]
WantedBy=timers.target

Weekly Cleanup Timer

Section titled “Weekly Cleanup Timer”
/etc/systemd/system/weekly-cleanup.timer
[Unit]
Description=Weekly Cleanup
Requires=weekly-cleanup.service


[Timer]
OnCalendar=Sun 3:00
Persistent=true


[Install]
WantedBy=timers.target

Hourly Health Check

Section titled “Hourly Health Check”
/etc/systemd/system/health-check.timer
[Unit]
Description=Hourly Health Check
Requires=health-check.service


[Timer]
OnCalendar=*:0/60             # Every hour
WakeSystem=true


[Install]
WantedBy=timers.target

82.6 Interview Questions

Section titled “82.6 Interview Questions”

Basic Questions

Section titled “Basic Questions”

  1. What are systemd timers?

    • Units for scheduling tasks, modern replacement for cron

  2. What does Persistent=true do?

    • Runs missed jobs after reboot

  3. What’s the difference between cron and systemd timers?

    • Timers have better logging, dependencies, and persist across reboots

  4. What is OnCalendar?

    • Calendar-based scheduling syntax

Summary

Section titled “Summary”
                 Quick Reference
+------------------------------------------------------------------+
|                                                                   |
|    Commands:                                                      |
|    +----------------------------------------------------------+  |
|    | systemctl list-timers         | List timers             |  |
|    | systemctl start timer         | Start timer            |  |
|    | systemctl enable timer        | Enable at boot         |  |
|    +----------------------------------------------------------+  |
|                                                                   |
|    Key Options:                                                   |
|    +----------------------------------------------------------+  |
|    | OnCalendar                    | Calendar schedule       |  |
|    | Persistent=true               | Run missed jobs        |  |
|    | RandomizedDelaySec            | Random delay           |  |
|    +----------------------------------------------------------+  |
|                                                                   |
+------------------------------------------------------------------+

Common Mistakes & Anti-Patterns

Section titled “Common Mistakes & Anti-Patterns”

1. Not Enabling Timers

Section titled “1. Not Enabling Timers”

WRONG:

Terminal window
# Only start, not enable
sudo systemctl start backup.timer
# Timer won't start after reboot

CORRECT:

Terminal window
# Enable to start at boot
sudo systemctl enable --now backup.timer

Why: Without enable, timer doesn’t survive reboot.

2. Wrong OnCalendar Syntax

Section titled “2. Wrong OnCalendar Syntax”

WRONG:

OnCalendar=daily
# Too vague - runs at midnight every day

CORRECT:

OnCalendar=*-*-02 04:30:00
# Runs at 4:30 AM on 2nd of each month

Why: Specific schedules prevent confusion and conflicts.

Interview Questions

Section titled “Interview Questions”

Conceptual Questions

Section titled “Conceptual Questions”

  1. Q: What’s the advantage of systemd timers over cron?

    • A: Timers persist across system sleep/hibernate, better logging via journald, dependencies can control when jobs run, can delay randomly to prevent thundering herd.

  2. Q: How do you debug a timer that isn’t firing?

    • A: Check systemctl status timer, check journalctl -u service, verify timer is enabled, check next activation time with systemctl list-timers.

End of Chapter 82: systemd Timers