How to use Systemctl to manage Systemd Services

Systemd is an init system and system manager that has widely become the new standard for Linux distributions.

Prerequisites

For this tutorial you need: 

• An active Fuga Cloud account
• Any Linux distro.

Service Management

The fundamental purpose of an init system is to initialise the components that must be started after the Linux kernel is booted. The init system is also used to manage services and daemons for the server at any point, while the system is running. With that in mind, lets start with some basic service management operations.

Start and Stop Service

If you want to start a service, you use the start command. You need to use sudo because it affects the state of your system:

$ sudo systemctl start apache2

As you can see, you have started the Apache service. If you wish to stop the apache services, you can replace the start command into stop, as seen in the following example:

$ sudo systemctl stop apache2

Restart and Reload Service

To restart the service, you use the following command:

$ sudo systemctl restart apache2

If you wish to reload the service, you use the reload command: 

$ sudo systemctl reload apache2

If you are not sure the service has to be reloaded or restarted, you can use both statements:

$ sudo systemctl reload-or-restart apache2

Enable and Disable Service

You have just learned how to start and stop any service. To start a service at boot, you need to enable it. 

$ sudo systemctl enable apache2

To disable the service from starting at boot, you use the following command:

$ sudo systemctl disable apache2

Check the status of your service

To check the status of the service you are working with, use the status command:

$ sudo systemctl status apache2

You will receive the following output:

apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; disabled; vendor preset: enabled)
     Active: active (running) since Thu 2022-03-24 15:19:10 UTC; 25min ago
       Docs: https://httpd.apache.org/docs/2.4/
   Main PID: 23884 (apache2)
      Tasks: 55 (limit: 8908)
     Memory: 5.6M
     CGroup: /system.slice/apache2.service
             ├─23884 /usr/sbin/apache2 -k start
             ├─23974 /usr/sbin/apache2 -k start
             └─23975 /usr/sbin/apache2 -k start

Mar 24 15:19:10 mysql systemd[1]: Starting The Apache HTTP Server...

This will give you an overview of the current status of the service you are running. As well as problems you are facing with which might require to take action.

System Status overview

You have learned how to manage a single service. If you want to get an overview of all the active services in your system, you can use the list-units command. 

$ sudo systemctl list-units

You receive the following output: 

UNIT                                    LOAD   ACTIVE SUB       DESCRIPTION
accounts-daemon.service                 loaded active running   Accounts Service
apache2.service                         loaded active running   The Apache HTTP Server
apparmor.service                        loaded active exited    Load AppArmor profiles
apport.service                          loaded active exited    LSB: automatic crash report generation
atd.service                             loaded active running   Deferred execution scheduler
blk-availability.service                loaded active exited    Availability of block devices
...  
...
...

If you wish the view all services, you can add the --all flag:

$ sudo systemctl list-units --all

You can use other flags to filter the output. Use the --state= flag to identify LOAD, ACTIVE, or SUB states as seen in the column above. An example would be:

$ systemctl list-units --state=active

$ systemctl list-units --state=loaded

List all unit files

To see every available unit file within the systemd paths, including those that systemd has not attempted to load, you can use the list-unit-files command:

$ sudo systemctl list-unit-files

Output example:

UNIT FILE                                  STATE           VENDOR PRESET
proc-sys-fs-binfmt_misc.automount          static          enabled
-.mount                                    generated       enabled
boot-efi.mount                             generated       enabled
dev-hugepages.mount                        static          enabled
dev-mqueue.mount                           static          enabled
proc-sys-fs-binfmt_misc.mount              disabled        enabled
snap-core20-1169.mount                     enabled         enabled
snap-core20-1376.mount                     enabled         enabled
...
...
...

Conclusion

In this tutorial you have learned how to start and stop any service in your system, by using Systemclt. You also learned how to enable and disable these services, and view the status of each service in your system.