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.

Practice

1. What is the main purpose of a systemd timer in Linux?

easy

A. To manage user permissions

B. To schedule and run tasks automatically at specified times

C. To monitor system hardware status

D. To update software packages automatically

Solution

1. Step 1: Understand systemd timer role

   Systemd timers are designed to schedule tasks to run automatically without manual intervention.

2. Step 2: Compare with other options

   Monitoring hardware, managing permissions, and updating packages are not the primary roles of systemd timers.

3. Final Answer:

   To schedule and run tasks automatically at specified times -> Option B

4. Quick Check:

   Systemd timers = schedule tasks [OK]

Hint: Timers schedule tasks like alarms for programs [OK]

Common Mistakes:

• Confusing timers with monitoring tools
• Thinking timers manage user permissions
• Assuming timers update software automatically

2. Which of the following is the correct syntax to enable a systemd timer named backup.timer?

easy

A. systemctl enable backup.timer

B. systemctl start backup.timer

C. systemctl run backup.timer

D. systemctl activate backup.timer

Solution

1. Step 1: Identify enabling command for systemd timers

   The command systemctl enable sets a timer to start automatically on boot.

2. Step 2: Differentiate from other commands

   start runs the timer immediately but does not enable it; run and activate are invalid commands.

3. Final Answer:

   systemctl enable backup.timer -> Option A

4. Quick Check:

   Enable timer = systemctl enable [OK]

Hint: Use 'enable' to start timer on boot, 'start' to run now [OK]

Common Mistakes:

• Using 'start' instead of 'enable' to auto-run timers
• Using invalid commands like 'run' or 'activate'
• Confusing service commands with timer commands

3. Given this timer unit file snippet:

[Timer]
OnCalendar=*-*-* 12:00:00
Persistent=true

What does the Persistent=true setting do?

medium

A. Prevents the timer from running at boot

B. Runs the timer only once

C. Disables the timer after one run

D. Runs missed jobs immediately after boot

Solution

1. Step 1: Understand Persistent=true meaning

   This setting makes systemd run any missed timer jobs immediately after the system boots.

2. Step 2: Eliminate other options

   It does not limit runs to once, disable timers, or prevent running at boot.

3. Final Answer:

   Runs missed jobs immediately after boot -> Option D

4. Quick Check:

   Persistent=true = catch missed runs after reboot [OK]

Hint: Persistent=true runs missed tasks after reboot [OK]

Common Mistakes:

• Thinking it runs only once
• Assuming it disables timer after run
• Believing it prevents timer at boot

4. You created a timer file cleanup.timer but it never triggers the service. Which is the most likely mistake?

medium

A. The timer file is not named exactly like the service file

B. The timer file is missing the [Timer] section

C. The service file cleanup.service is not enabled

D. The timer file uses OnBootSec= instead of OnCalendar=

Solution

1. Step 1: Check timer and service file naming

   Systemd timers must have matching names with their service files except for the extension.

2. Step 2: Analyze other options

   Missing [Timer] section causes syntax errors; service enabling is not required for timers to trigger; both OnBootSec and OnCalendar are valid.

3. Final Answer:

   The timer file is not named exactly like the service file -> Option A

4. Quick Check:

   Timer and service names must match [OK]

Hint: Timer and service filenames must match except extension [OK]

Common Mistakes:

• Ignoring filename matching between timer and service
• Assuming service must be enabled for timer to run
• Confusing valid timer options as errors

5. You want a systemd timer to run a backup service every day at 3:30 AM but only if the system was off at that time. Which timer configuration achieves this?

hard

A. [Timer]
OnActiveSec=03:30:00
Persistent=true

B. [Timer]
OnCalendar=*-*-* 03:30:00
AccuracySec=1min

C. [Timer]
OnCalendar=*-*-* 03:30:00
Persistent=true

D. [Timer]
OnBootSec=03:30:00
Persistent=false

Solution

1. Step 1: Identify daily schedule at 3:30 AM

   The OnCalendar=*-*-* 03:30:00 sets the timer to run daily at 3:30 AM.

2. Step 2: Ensure missed runs execute after downtime

   Persistent=true makes systemd run missed jobs after system was off at scheduled time.

3. Step 3: Evaluate other options

   AccuracySec adjusts timing precision but doesn't affect missed runs; OnBootSec and OnActiveSec run relative to boot or activation, not fixed time.

4. Final Answer:

   [Timer] OnCalendar=*-*-* 03:30:00 Persistent=true -> Option C

5. Quick Check:

   Daily 3:30 AM with catch-up = OnCalendar + Persistent [OK]

Hint: Use OnCalendar with Persistent=true for daily missed runs [OK]

Common Mistakes:

• Using OnBootSec instead of OnCalendar for fixed time
• Forgetting Persistent=true to run missed jobs
• Confusing AccuracySec with scheduling behavior