Exercise: Control a service

Control a service

• Read systemctl status and the is-active and is-enabled answers.
• Separate start/stop from enable/disable.
• Mask and unmask a service.

Start the exercise

On workstation, start the exercise. It makes sure crond runs and is enabled, and that bluetooth is not masked.

[student@workstation ~]$ lab start sa-services

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Read the status

On servera, open a root shell with sudo -i. Show the status of crond. What are its main PID, its state, and its enabled state? Then ask the two short questions.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sudo -i
[root@servera ~]# systemctl status crond | head -5
● crond.service - Command Scheduler
     Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; preset: enabled)
     Active: active (running) since Sat 2026-10-03 16:36:40 UTC; 4s ago
   Main PID: 642 (crond)
      Tasks: 1 (limit: 5820)
[root@servera ~]# systemctl is-active crond
active
[root@servera ~]# systemctl is-enabled crond
enabled

Stop it, and check what survives a reboot

Stop crond and show its status. Is it still enabled? What will happen at the next boot? Then start it again.

[root@servera ~]# systemctl stop crond
[root@servera ~]# systemctl status crond | head -3
○ crond.service - Command Scheduler
     Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; preset: enabled)
     Active: inactive (dead) since Sat 2026-10-03 16:36:45 UTC; 10ms ago
[root@servera ~]# systemctl start crond

It is stopped but still enabled, so it would start again at boot. Stopping changes now, not boot.

Disable it, and check that it keeps running

Disable crond without stopping it. Compare is-active and is-enabled. Then re-enable it and start it with one command.

[root@servera ~]# systemctl disable crond
Removed "/etc/systemd/system/multi-user.target.wants/crond.service".
[root@servera ~]# systemctl is-active crond; systemctl is-enabled crond
active
disabled
[root@servera ~]# systemctl enable --now crond
Created symlink /etc/systemd/system/multi-user.target.wants/crond.service → /usr/lib/systemd/system/crond.service.

Running but disabled is the "temporary" state: it works until the next reboot, then disappears.

Inspect how the service is defined

Show the unit file that defines crond with systemctl cat, and find its start command and its restart policy.

[root@servera ~]# systemctl cat crond | grep -E 'ExecStart|Restart'
ExecStart=/usr/sbin/crond -n $CRONDARGS
Restart=on-failure

systemd runs /usr/sbin/crond itself and restarts it if it crashes. You will write a unit like this in the next lesson.

Mask a service you do not want

A server has no use for Bluetooth. Mask bluetooth, try to start it, check is-enabled, and finally unmask it.

[root@servera ~]# systemctl mask bluetooth
Created symlink /etc/systemd/system/bluetooth.service → /dev/null.
[root@servera ~]# systemctl start bluetooth
Failed to start bluetooth.service: Unit bluetooth.service is masked.
[root@servera ~]# systemctl is-enabled bluetooth
masked
[root@servera ~]# systemctl unmask bluetooth
Removed "/etc/systemd/system/bluetooth.service".

Look for trouble

List the failed units, then the services that are running.

[root@servera ~]# systemctl --failed
  UNIT LOAD ACTIVE SUB DESCRIPTION
0 loaded units listed.
[root@servera ~]# systemctl list-units --type=service --state=running | head -5
  UNIT                              LOAD   ACTIVE SUB     DESCRIPTION
  chronyd.service                   loaded active running NTP client/server
  crond.service                     loaded active running Command Scheduler
  dbus-broker.service               loaded active running D-Bus System Message Bus
[root@servera ~]# exit
[student@servera ~]$ exit

Grade and finish

Check your work with lab grade sa-services, then put the exercise away with lab finish sa-services.