Systemd Timers

A systemd timer is a moderl scheduling tool that’s part of the systemd init system used by most of the Linux distributions. Unlike cron, systemd timers can trigger tasks based not just on time, but also on events(reboot,specific service starts, when a file change, etc.).

They require bot a service file(defines what to run) and a timer file (defines when/how to run). Systemd timers offer advanced features like dependency management, built-in logging, resource limits, and fine-grained control over the execution environment.

They also ensure that only one instance of a task runs at a time, preventing overlaps, and making them ideal for complex or critical automation scenarios.

Best for complex scheduling, dependency management, event-based triggers, and when you need robust logging and resource control.

Cron Jobs

A cron job is a simple, time-based scheduler built into nearly all Unix-like operating systems. It lets you run scripts or command at specific times or intervals by editing a configuration file (crontab -e).

Cron is easy to setup and widely used for basic automation tasks like backups or log rotation, and limited logging and environment control.

Each job runs independently, and multiple instance of the same job can overlap if ones takes too long.

Best for simple, time-based tasks where portability and simplicity are the key.

File Structure

.timer Filer

A .timer file defines when and how a service is triggered. It works in tandem with .service file, which defines what to run.

Key Sections and Directive:

  • [Unit]: Description and dependencies
  • [Timer]: Timing and scheduling directive.
  • [Install]: Where to enable the timer.

Important Directive

  • OnBootSec: Run the service after a specified time interval following the system boot. (OnBootSec=5m runs 5 minutes after boot.)
  • OnUnitActiveSec: Run the service after a specified time interval following the last activation of the unit. (OnUnitActiveSec=5s runs every hour after the last run).
  • OnCalendar: Schedule the service to run at specific calender times(like cron; OnCalendar=*-*-* 02:00:00) Runs everyday at 2AM.
  • AccuracySync: Allow a time window for the timer to fire (for efficiency; AccuracySync=10m timer can run anytime within 10 min window )
  • RandomizedDelaySec: Add a random delay to prevent many timers from running at the same time.
  • Persistent=: If true(bool), ensures the timer runs immediately at boot if it misssed its schedules time while the system was off.
  • Unit=: Specify which unit to activate if the service name does not match the timer name.

Defining Dependency

  • WantedBy= or Requires= in the [Install] or [Unit] section: Specifies dependencies or when the timer should be activated.
  • Dependencies between units (services/timer): You can make one unit depend on another by using After=, Requires=, or Wants= in the [Unit] section.

Example

[Unit]
Descriptio=My Systemd Timer

[Timer]
OnBootSec=5min
OnUnitActiveSec=1h
AccuracySec=1m
RandomizedDelay=30s

[Install]
WantedBy=timers.target

crontab File

Cron itself does not have directives for dependencies or advanced scheduling, but the format is very simple.

Important Crontab Fields

minute hour day-of-month month day-of-week command

Example:

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

Defining Dependency in Cron:

  • Cron has no built-in dependency management. Each cron job is independent. If you want ensure one job runs after another, you can use journalctl logs find out what happened.

Example

  1. Automate daily backups of an important directory using systemd timers, similar to what many organizations or users do for data safety. Script File Location: /usr/local/bin/backup-docs.sh
  • Systemd File Location: /etc/systemd/system/backup-docs.service
[Unit]
Description=Backups Docs Directory

[Service]
Type=oneshot
ExecStart=/usr/loca/bin/backup-docs.sh
User=username
  • Timer File Location: /usr/systemd/system/backup-docs.timer
[Unit]
Description=Daily Backup of Docs Directory

[Timer]
OnCalendar=*-*-* 02:00:00
RandomizedDelaySec=3m
Persistent=true

[Install]
WantedBy=timers.target
  • Enable and Start the timer
sudo systemctl daemon-reload
sudo systemctl enable backup-docs.timer
sudo systemctl start backup-docs.timer
  • Verify
systemctl status backup-docs.timer
systemctl list-timers