systemd timers in Linux CLI - Step-by-Step Execution

Concept Flow - systemd timers

Create .service file

↓

Create .timer file

↓

Enable timer with systemctl

↓

Timer triggers at set time

↓

systemd runs .service file

↓

Service executes task

↓

Timer waits for next trigger

This flow shows how systemd timers work: you create a service and a timer file, enable the timer, and systemd runs the service at scheduled times.

Execution Sample

Linux CLI

[Unit]
Description=Run my script

[Service]
Type=oneshot
ExecStart=/usr/bin/echo Hello from systemd timer

--- myjob.service ---

[Unit]
Description=Timer for myjob

[Timer]
OnBootSec=10s
Unit=myjob.service

[Install]
WantedBy=timers.target

--- myjob.timer ---

This example defines a service that echoes a message and a timer that triggers it 10 seconds after boot.

Execution Table

Step	Action	Evaluation	Result
1	Create myjob.service file	File created with echo command	Service ready to run
2	Create myjob.timer file	Timer set to trigger 10 seconds after boot	Timer ready to trigger service
3	Enable timer with systemctl	systemctl enable myjob.timer	Timer enabled and started
4	System boots	Timer counts 10 seconds	Timer triggers service
5	systemd runs myjob.service	ExecStart runs echo command	Output: Hello from systemd timer
6	Timer waits for next trigger	No repeat set, timer stops	Timer inactive until next boot or manual start

💡 Timer stops because no repeat interval is set; service ran once after 10 seconds from boot

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Timer State	inactive	active (enabled)	counting down	triggered	inactive (no repeat)
Service State	not running	not running	not running	running	completed

Key Moments - 3 Insights

Why doesn't the service run immediately after enabling the timer?

Because the timer is set with OnBootSec=10s, it waits 10 seconds after boot before triggering the service, as shown in execution_table step 4.

What happens if the timer has no repeat interval?

The timer triggers the service once and then becomes inactive, as seen in execution_table step 6.

How does systemd know which service to run when the timer triggers?

The timer file specifies the Unit=myjob.service, linking the timer to the service, as shown in the execution_sample code and step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Timer State after step 4?

A inactive

B triggered

C counting down

D active but not counting

💡 Hint

Check variable_tracker Timer State column after Step 4

At which step does the service actually run the echo command?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at execution_table Action and Result columns for when ExecStart runs

If you want the timer to run the service every minute, what would change in the timer file?

A Change OnBootSec=10s to OnUnitActiveSec=1min

B Change Unit=myjob.service to Unit=myjob.timer

C Remove the [Timer] section

D Add Type=oneshot to the timer file

💡 Hint

Refer to execution_sample and think about timer scheduling options

Concept Snapshot

systemd timers run tasks on schedule.
Create a .service file for the task.
Create a .timer file to schedule it.
Enable timer with systemctl.
Timer triggers service at set times.
Useful for automation without cron.

Full Transcript

Systemd timers automate running tasks by linking a timer file to a service file. First, you create a service file that defines what to run, like a script or command. Then, you create a timer file that tells systemd when to run that service, for example, 10 seconds after boot. You enable the timer with systemctl, which starts counting. When the timer reaches the set time, systemd runs the service. If no repeat is set, the timer stops after running once. This method is a modern alternative to cron for scheduling tasks.