Server operations · Lesson 22 of 36 · 8 minutes

Unit dependency reading

Read service definitions and list units together.

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Your objectives

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1
List units
Run systemctl list-units.
Hint & worked example
Inspect or change a simulated systemd service. Status, active state, and unit configuration reveal different evidence.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
systemctl list-units
```
Remember: A service can be active while an application endpoint is unhealthy; verify both layers.
2
Read cron unit
Run cat /etc/systemd/system/cron.service.
Hint & worked example
Read one or more files to standard output without editing their contents.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
cat /etc/systemd/system/cron.service
```
Remember: Reading a source file should precede changing it; avoid printing real secrets.
3
Inspect cron status
Run systemctl status cron.
Hint & worked example
Inspect or change a simulated systemd service. Status, active state, and unit configuration reveal different evidence.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
systemctl status cron
```
Remember: A service can be active while an application endpoint is unhealthy; verify both layers.

Understand the pattern

Junior operators build confidence by gathering evidence before changing systems.

systemctl: Inspect or change a simulated systemd service. Status, active state, and unit configuration reveal different evidence.
A service can be active while an application endpoint is unhealthy; verify both layers.
cat: Read one or more files to standard output without editing their contents.
Reading a source file should precede changing it; avoid printing real secrets.

Worked example and expected result

$ systemctl list-units

UNIT                  LOAD   ACTIVE
nginx.service loaded active simulated serving the simulated static learn site
ssh.service loaded active simulated accepting key-only training logins
cron.service loaded active simulated running scheduled simulated maintenance
backup.service loaded inactive simulated waiting for a configured backup target
database.service loaded inactive simulated not installed in this lab

Exit status: 0. The command completed successfully in the training fixture.

$ cat /etc/systemd/system/cron.service

[Unit]
Description=fake cron
[Service]
ExecStart=/usr/sbin/cron -f

Exit status: 0. The command completed successfully in the training fixture.

$ systemctl status cron

cron.service - simulated cron
   Loaded: loaded
   Active: active
   Notes: running scheduled simulated maintenance

Exit status: 0. The command completed successfully in the training fixture.

Try a variation

Repeat this workflow in the sandbox with a different scratch filename. Keep the system fixtures unchanged and compare the observed output with your expectation before moving on.

Explain it

In “Unit dependency reading”, which interpretation is safe?A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.Reading a source file should precede changing it; avoid printing real secrets.

Apply it elsewhere

Before adapting the cat workflow from “Unit dependency reading” to another server, what should you do?Skip output inspection whenever the command is familiar.Confirm the current directory, target identity, permissions, and the installed command behavior first.Paste the training paths unchanged into production.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

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