The ps command can be used to determine if your system is using init or systemd. If PID 1 is systemd, then you will use systemctl to start, stop, enable, disable, and check the status of a systemd service.

ps -ef | head
UID         PID   PPID  C STIME TTY          TIME CMD
root          1      0  0 Mar01 ?        00:06:47 /usr/lib/systemd/systemd --switched-root --system --deserialize 22

If PID 1 is init, then you will use the chkconfig command.

UID         PID   PPID  C STIME TTY          TIME CMD
root          1      0  0 Mar08 ?        00:00:01 /sbin/init

IMPORTANT

The service command is used by systems using init. systemctl is used by systems using systemd.

When issuing a systemctl command, if the following is displayed, this means that the service file was updated, such as /etc/systemd/system/example.service.

Warning: example.service changed on disk. Run 'systemctl daemon-reload' to reload units.

As the warning suggested, the systemctl daemon-reload command can be used to resolve this issue.

systemctl daemon-reload

systemctl status

The systemctl status command will show if a service is running. In this example, systemctl is used to check the status of the nginx service.

systemctl status nginx

Following is an example of what will be displayed. In this example, nginx is active.

 nginx.service - The nginx HTTP and reverse proxy server
   Loaded: loaded (/usr/lib/systemd/system/nginx.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2020-03-04 00:00:17 CST; 3 weeks 4 days ago
  Process: 1097 ExecStart=/usr/sbin/nginx (code=exited, status=0/SUCCESS)
  Process: 1034 ExecStartPre=/usr/sbin/nginx -t (code=exited, status=0/SUCCESS)
  Process: 1026 ExecStartPre=/usr/bin/rm -f /run/nginx.pid (code=exited, status=0/SUCCESS)
 Main PID: 1123 (nginx)
   CGroup: /system.slice/nginx.service
           ├─1123 nginx: master process /usr/sbin/nginx
           ├─1137 nginx: worker process
           └─1138 nginx: worker process

Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.

In this example, nginx is inactive.

systemctl status example.service
. . .
 example.service - Example Service
  Loaded: loaded (/etc/systemd/system/example.service; disabled; vendor preset: disabled)
  Active: inactive (dead)

systemctl start

The systemctl start command is used to start a service.

systemctl start example.service

If the service failed the start, something like this will be display. As this message suggested, the systemctl status command can be used to start understanding why the service failed to start.

Job for example.service failed because the control process exited with error code. See "systemctl status example.service" and "journalctl -xe" for details.

systemctl stop

The systemctl stop command is used to stop a service. The stop command will not return any output. Typically, after issuing the stop command, the status command is used to ensure the status of the service is inactive.

systemctl stop example.service

systemctl get-default

The systemctl get-default command is used to determine the target for the entire system. In this example, the entire system is in graphical.target.

systemctl get-default
. . .
multi-user.target

systemctl set-default

The systemctl set-default command is used to set the target. In this example, the target is set to graphical.target.

systemctl set-default graphical.target