4. Operate Running Systems

📘Red Hat Certified System Administrator (RHCSA – EX200)

This section is very important for the RHCSA (EX200) exam. In the exam, you may need to manage network services on a running system. You must know how to start, stop, restart, enable, disable, and check the status of services.

In Red Hat Enterprise Linux, services are managed by:

systemd (the init system)
The main command used: systemctl

You must be comfortable using these commands.

1. What is a Network Service?

A network service is a program that:

Runs in the background (daemon)
Waits for network requests
Provides services to other systems or users

Examples of network services in an IT environment:

A web server serving websites
An SSH server allowing remote login
A firewall service controlling traffic
A time synchronization service
A DNS server resolving domain names

These services run continuously in the background.

2. Understanding systemd and systemctl

In Red Hat Enterprise Linux 8/9, services are controlled by systemd.

The command used to manage services is:

systemctl

Services are controlled using unit files, usually located in:

/usr/lib/systemd/system/

Service files end with:

.service

For example:

sshd.service

When using systemctl, you usually do not need to type .service.

3. Start a Network Service

To start a service immediately:

sudo systemctl start service_name

Example:

sudo systemctl start sshd

This:

Starts the SSH server immediately
Does NOT enable it at boot

In the exam:

If a task says “Start the service now”, this is the correct command.

4. Stop a Network Service

To stop a running service:

sudo systemctl stop service_name

Example:

sudo systemctl stop sshd

This:

Stops the service immediately
It will no longer accept connections

5. Restart a Network Service

To restart a service:

sudo systemctl restart service_name

Example:

sudo systemctl restart sshd

This:

Stops the service
Starts it again

You usually restart a service after:

Changing configuration files
Applying new settings

6. Reload a Service (Very Important)

Some services support reload without fully restarting:

sudo systemctl reload service_name

Example:

sudo systemctl reload sshd

Reload:

Applies configuration changes
Does NOT completely stop the service

If reload is not supported, you must use restart.

To check if reload is supported:

systemctl status service_name

Look for “can reload”.

7. Check Service Status

To check whether a service is running:

systemctl status service_name

Example:

systemctl status sshd

Output shows:

Loaded (enabled/disabled)
Active (running/stopped/failed)
Process ID (PID)
Recent log messages

Important status values:

Status	Meaning
active (running)	Service is working
inactive	Service is stopped
failed	Service crashed or error

In the exam:
You must understand whether a service is running or not.

8. Enable a Service at Boot

To make a service start automatically at boot:

sudo systemctl enable service_name

Example:

sudo systemctl enable sshd

This:

Creates symbolic links
Ensures service starts when the system boots

To verify:

systemctl is-enabled sshd

Output:

enabled
disabled

9. Disable a Service at Boot

To prevent a service from starting at boot:

sudo systemctl disable service_name

Example:

sudo systemctl disable sshd

This does NOT stop it immediately.
It only prevents auto-start at next boot.

10. Start and Enable at the Same Time

Very useful for the exam:

sudo systemctl enable --now service_name

Example:

sudo systemctl enable --now sshd

This:

Starts the service immediately
Enables it at boot

11. Check All Services

To list all active services:

systemctl list-units --type=service

To list all services (including inactive):

systemctl list-unit-files --type=service

12. Mask and Unmask a Service (Advanced but Exam-Relevant)

To completely block a service from starting:

sudo systemctl mask service_name

This:

Prevents starting manually
Prevents starting at boot

To allow it again:

sudo systemctl unmask service_name

13. Common Network Services in RHCSA

You should recognize these:

Service	Purpose
sshd	Remote login
firewalld	Firewall management
chronyd	Time synchronization
httpd	Web server
vsftpd	FTP server
NetworkManager	Network configuration

You may need to start or enable them during the exam.

14. Check if a Service is Listening on a Port

In real IT environments, after starting a service, you must verify that it is listening.

Use:

ss -tuln

Or:

ss -tulnp

This shows:

Listening ports
Associated services

Example:

SSH listens on port 22
HTTP listens on port 80

This is very important in troubleshooting.

15. Troubleshooting a Failed Service

If a service fails:

Step 1: Check status

systemctl status service_name

Step 2: Check logs

journalctl -u service_name

This shows detailed error messages.

16. Difference Between Active and Enabled

This is a common exam confusion.

Term	Meaning
Active	Running now
Enabled	Will start at boot

A service can be:

Active but not enabled
Enabled but not currently running
Disabled and inactive

You must understand this clearly.

17. Targets (Replacement for Runlevels)

Older Linux used runlevels.
Now systemd uses targets.

Example:

systemctl get-default

Common target:

multi-user.target

Network services usually run in multi-user target.

18. Exam Tips for RHCSA

For this section, you must be able to:

✔ Start a service
✔ Stop a service
✔ Restart a service
✔ Reload a service
✔ Enable a service at boot
✔ Disable a service
✔ Check status
✔ Verify service is listening
✔ Troubleshoot using journalctl

Practice these commands many times.

19. Step-by-Step Example Scenario (Exam Style)

If the exam says:

Ensure the SSH service is running and enabled at boot.

You should do:

sudo systemctl enable --now sshd

Then verify:

systemctl status sshd
systemctl is-enabled sshd

20. Important Summary Commands

systemctl start service
systemctl stop service
systemctl restart service
systemctl reload service
systemctl status service
systemctl enable service
systemctl disable service
systemctl enable --now service
systemctl is-enabled service
systemctl mask service
systemctl unmask service
journalctl -u service
ss -tulnp

You must remember all of these for the exam.

Final Advice for RHCSA Students

This topic is practical. In the exam:

You will not get multiple choice questions.
You must actually perform these tasks.
Typing mistakes will cost marks.
Always verify after making changes.

Practice on a RHEL 8 or 9 system repeatedly until you are fully comfortable.