Getting Started with systemctl

systemd is the default init system and service manager for all EL systems since CentOS 7, including AlmaLinux and Rocky Linux. It replaces the traditional SysVinit and is responsible for bootstrapping the system, managing services, mounting filesystems, and other core tasks. systemctl is the primary command-line tool for interacting with systemd.

What is systemd

systemd manages system resources around the core concept of units. Common unit types include:

Unit Type	Suffix	Description
Service	.service	System services (e.g., Nginx, sshd)
Socket	.socket	Inter-process communication sockets
Target	.target	A collection of units, similar to runlevels
Timer	.timer	Scheduled task triggers
Mount	.mount	Filesystem mount points

systemd unit configuration files are typically stored in the following locations:

• /usr/lib/systemd/system/ — Unit files installed by packages
• /etc/systemd/system/ — Administrator-customized or overridden unit files (higher priority)

Starting and Stopping Services

Start a Service

Start the Nginx service

sudo systemctl start nginx

Stop a Service

Stop the Nginx service

sudo systemctl stop nginx

Restart a Service

Restarting stops and then starts the service, suitable for scenarios where a full reload is needed after configuration changes:

Restart the Nginx service

sudo systemctl restart nginx

Reload Configuration

Some services support reloading their configuration without interrupting operation:

Reload Nginx configuration (without interrupting the service)

sudo systemctl reload nginx

If you are unsure whether a service supports reload, you can use reload-or-restart, which attempts reload first and falls back to restart if it fails:

Try reload, fall back to restart if unsupported

sudo systemctl reload-or-restart nginx

Checking Service Status

The status subcommand displays the service’s running state, PID, memory usage, and recent log output:

Check Nginx service status

sudo systemctl status nginx

Example output:

● nginx.service - The nginx HTTP and reverse proxy server
     Loaded: loaded (/usr/lib/systemd/system/nginx.service; enabled; preset: disabled)
     Active: active (running) since Tue 2026-03-24 10:15:30 CST; 2h ago
   Main PID: 1234 (nginx)
      Tasks: 3 (limit: 23456)
     Memory: 5.2M
        CPU: 35ms
     CGroup: /system.slice/nginx.service
             ├─1234 "nginx: master process /usr/sbin/nginx"
             └─1235 "nginx: worker process"

You can also use the following commands to quickly check service status:

Check if a service is running

systemctl is-active nginx

Check if a service is enabled for boot

systemctl is-enabled nginx

Check if a service has failed

systemctl is-failed nginx

Configuring Services to Start at Boot

Enable a Service at Boot

Set Nginx to start automatically at boot

sudo systemctl enable nginx

This creates a symbolic link under /etc/systemd/system/ so the service starts automatically when the system boots.

If you want to enable and start the service immediately, add the --now flag:

Enable at boot and start immediately

sudo systemctl enable --now nginx

Disable a Service at Boot

Disable Nginx from starting at boot

sudo systemctl disable nginx

This also supports --now to stop the service at the same time:

Disable at boot and stop immediately

sudo systemctl disable --now nginx

Listing System Units

List All Running Services

List all active service units

systemctl list-units --type=service

List All Installed Services (Including Inactive)

List all installed service unit files

systemctl list-unit-files --type=service

List Failed Units

View all failed units

systemctl --failed

View Unit Dependencies

View the dependency tree of Nginx

systemctl list-dependencies nginx

Viewing Logs with journalctl

systemd collects all service logs through journald. You can use journalctl to quickly view a service’s log output:

View all logs for the Nginx service

sudo journalctl -u nginx

Follow Nginx log output in real time

sudo journalctl -u nginx -f

View Nginx logs since the current boot

sudo journalctl -u nginx -b

For more advanced journalctl usage, see the Journal Logs chapter.

Managing System Targets

systemd uses targets to replace traditional runlevels. Common targets and their correspondences:

Target	Traditional Runlevel	Description
poweroff.target	0	Power off
rescue.target	1	Single-user/rescue mode
multi-user.target	3	Multi-user command-line mode
graphical.target	5	Graphical interface mode
reboot.target	6	Reboot

View the current default boot target

systemctl get-default

Set the default boot target to multi-user command-line mode

sudo systemctl set-default multi-user.target

Temporarily switch to rescue mode (without changing the default target)

sudo systemctl isolate rescue.target

Quick Reference Table

Operation	Command
Start a service	sudo systemctl start <service>
Stop a service	sudo systemctl stop <service>
Restart a service	sudo systemctl restart <service>
View status	sudo systemctl status <service>
Enable at boot	sudo systemctl enable <service>
Disable at boot	sudo systemctl disable <service>
Enable and start	sudo systemctl enable --now <service>
List all services	systemctl list-units --type=service
View failed units	systemctl --failed
View service logs	sudo journalctl -u <service>