How to List All Running Services Under Systemd in Linux

Linux systems provide various system services (such as process management, login , syslog, cron, etc.) and network services (such as remote login, email, printer, web hosting, data storage, file transfer, domain name resolution (using DNS), dynamic IP address allocation (using DHCP ), etc.).

Technically, a service is a process or group of process (often called a daemon ) that runs continuously in the background, waiting for incoming requests (especially from the client).

Linux supports different ways to manage services (start, stop, restart, enable automatic startup at system startup, etc.), usually through a process or service manager. Almost all modern Linux distributions now use the same process manager: systemd .

[You may also like: Why Linux needs to replace 'init' with 'systemd' ]

What is Systemd?

Systemd is the system and service manager of Linux; it is a direct replacement for the init process, compatible with SysV and LSB init scripts, and the systemctl command is the main tool for managing systemd .

Why list services running in Linux?

It is important to know which services are running on your Linux system because:

• Monitor resource utilization
• Troubleshoot performance issues
• Ensure critical services are active
• Optimize system performance and security

Systemd simplifies service management using powerful systemctl commands (also known as basic commands), making it easy to list, monitor, and manage active services.

In this guide, we will demonstrate the process of listing all running services under Systemd in Linux, providing a comprehensive walkthrough for users of all experience levels.

List the running services under SystemD in Linux

When you run the systemctl command without any parameters, it displays a list of all loaded systemd units (read the systemd documentation for more information about the systemd unit), including services, showing their status (active or not).

 <code># systemctl</code>

List all loaded services in Linux

To list all loaded services on the system (whether active, running, exited, or failed, use the list-units subcommand and --type switch with the value of service.

 <code># systemctl list-units --type=service OR # systemctl --type=service</code>

List only active services in Linux

To list all loaded but active services, including running and exited services, you can add the --state option with the value active as shown below.

 <code># systemctl list-units --type=service --state=active OR # systemctl --type=service --state=active</code>

Use systemctl to list running services in Linux

However, to quickly see all running services (i.e. all loaded and actively running services), run the following command.

 <code># systemctl list-units --type=service --state=running OR # systemctl --type=service --state=running</code>

Let's explore the key terms related to Systemd units and their state:

• Unit – A unit can be a service, socket, device, or various other entities.
• Load – It indicates whether the unit is loaded. The cell can be loaded, but not necessarily active.
• Active – It shows if the unit is running actively, or if it is having problems and is in a failed or inactive state.
• SUB – It provides more details about unit-specific status. For a service, it may indicate whether the service is running (running), stopped (exited), or encountered a problem (failed).
• Description – It helps users identify and understand the purpose of the unit without delving into detailed configuration files.

Create an alias for the systemctl command

If you use previous commands frequently, you can create an alias command in the ~/.bashrc file as shown to make it easy to call it.

 <code># vim ~/.bashrc</code>

Then, add the following line under the alias list as shown in the screenshot.

 <code>alias running_services='systemctl list-units  --type=service  --state=running'</code>

Save changes in the file and close it. From now on, use the " running_services " command to view all loaded and actively running services on the server.

 <code># running_services #use the Tab completion</code>

Find the port that the service is using

Furthermore, another important aspect of services is the ports they use. To determine which port the daemon is listening on, you can use the netstat or ss command as shown below.

Where the flag -l means printing all listening sockets, -t shows all TCP connections, -u shows all UDP connections, -n means printing the digital port number (not the application name), -p means displaying the application name.

 <code>netstat -ltup | grep zabbix_agentd OR ss -ltup | grep zabbix_agentd</code>

The fifth column shows the socket: Local address: Port. In this case, the process zabbix_agentd is listening on port 10050 .

List open firewall services and ports

Additionally, if your server is running a firewall service that controls how to block or allow data flow to a selected service or port, you can use the firewall-cmd or ufw command (depending on the Linux distribution you are using) to list the services or ports that have been opened in the firewall, as shown below.

 <code>firewall-cmd --list-services [FirewallD] firewall-cmd --list-ports sudo ufw status [UFW Firewall]</code>

Automating service monitoring in Linux

Manually checking for running services can be cumbersome, especially on production servers. Automating this process ensures that you are always aware of changes in service status without manual checking.

Check running services every 5 minutes using Cron jobs

A cron job is a scheduled task in Linux that runs at specific intervals. You can use it to log running services regularly and view them later in the event of a failure or unexpected shutdown.

 <code>crontab -e</code>

Add this line to record the running service every 5 minutes.

 <code>*/5 * * * * systemctl list-units --type=service --state=running > /tmp/running_services.log</code>

The output will be saved in the /tmp/running_services.log file, you can check the latest recorded services using the following command:

 <code>cat /tmp/running_services.log OR tail -f /tmp/running_services.log</code>

Restart the service when the service fails

By default, if the service crashes or stops unexpectedly, it will not automatically restart unless explicitly configured. To ensure that the service restarts on failure, you can modify its systemd service unit file.

For example, edit the service configuration using the following command (replace apache2 with the actual service name you want to restart automatically):

 <code>systemctl edit apache2</code>

After entering the editor, add the following lines.

 <code>[Service] Restart=always RestartSec=5s</code>

Now reload systemd to apply the changes.

 <code>systemctl daemon-reload</code>

Then restart the service to make sure it picks up the new settings

 <code>systemctl restart apache2</code>

Confirm that systemd is set to automatically restart the service.

 <code>systemctl show apache2 --property=Restart</code>

in conclusion

That's it now! In this guide, we demonstrate how to view the running services under systemd in Linux. We also covered how to check the ports the service is listening on and how to view the service or port that is open in the system firewall.

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