How to Enable/Disable Services on Ubuntu Systemd. (Update !!!)

What is Systemd and Why should you Care?

Systemd is a init system (first process started during booting) and a system manager providing a standard provides a standard process for controlling what programs run when a Linux system boots up. While systemd is compatible with SysV and Linux Standard Base (LSB) init scripts, systemd is meant to be a drop-in replacement for these older ways of getting a Linux system running. Systemd does more than starting the core programs running. It also starts a journal of system activity, manages services, automounts filesystems, logs events, set up a hostname, and other system tasks. That may sound good to you, but some developers hate it.

Manage Services

The main role of an init system is to set up all the components that need to be started once the kernel has been booted and manage services and daemons. It is itself a daemon process that continues running until the system is shut down.

For service management tasks, Systemd uses the concept of units, which can be services, sockets, mount points, devices, etc. Units are defined using text files in ini format. These files include information about the unit, its settings, and commands to execute. The unit file type is determined by the filename extensions. For ie. (.service), (.mount), (.device) and (.socket).

However, for most service management tasks, you may omit the .service suffix since Systemd is clever enough to recognize that you're presumably trying to operate on a service.
Mount points will automatically be translated into the appropriate .mount unit. For example, specifying /home is equivalent to home.mount. Similar to mount points, devices are automatically translated into the appropriate .device unit, therefore specifying /dev/sda2 is equivalent to dev-sda2.device.

Systemctl, on the other hand, is a command-line program for managing services and controlling Systemd. It is a component of the Systemd ecosystem, and it is installed by default on all systems.

List Current Units

We can use the list-units --type service --all command to obtain a list of all active units that systemd is aware of.

$sudo systemctl list-units --type service --all

OUTPUT

 UNIT                               LOAD      ACTIVE   SUB     DESCRIPTION                                         
  apparmor.service                  loaded    active   exited  Load AppArmor profiles
  apport-autoreport.service         loaded    inactive dead    Process error reports when automatic reporting is en
  apport.service                    loaded    active   exited  LSB: automatic crash report generation
  apt-daily-upgrade.service         loaded    inactive dead    Daily apt upgrade and clean activities
  apt-daily.service                 loaded    inactive dead    Daily apt download activities
● auditd.service                    not-found inactive dead    auditd.service

Here the output has the above columns:

• UNIT: The systemd unit name
• LOAD: Whether the unit’s configuration has been parsed by systemd. The configuration of loaded units is kept in memory.
• ACTIVE: A summary state about whether the unit is active or not. This is usually a fairly basic way to tell if the unit has started successfully or not.
• SUB: This is a lower-level state that indicates more detailed information about the unit. This often varies by unit type, state, and the actual method in which the unit runs.
• DESCRIPTION: A short textual description of what the unit is/does.

How to Enable and Disable Services in Systemd init

To start a service in systemd run the command as follow:

$ sudo systemctl start service-name

for instance; to start apache web service, run

$ sudo systemctl start apache2

to verify that the service is running:

$ sudo systemctl status apache2

OUTPUT

apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2021-06-09 09:24:14 UTC; 1 day 2h ago
       Docs: https://httpd.apache.org/docs/2.4/
   Main PID: 3015 (apache2)
      Tasks: 55 (limit: 2101)
     CGroup: /system.slice/apache2.service
             ├─ 3015 /usr/sbin/apache2 -k start
             ├─98358 /usr/sbin/apache2 -k start
             └─98359 /usr/sbin/apache2 -k start

Jun 09 09:24:03 WebServer systemd[1]: Starting The Apache HTTP Server...
Jun 09 09:24:14 WebServer systemd[1]: Started The Apache HTTP Server.
Jun 10 00:00:03 WebServer systemd[1]: Reloading The Apache HTTP Server.
Jun 10 00:00:04 WebServer systemd[1]: Reloaded The Apache HTTP Server.

Notice that the dot ("●") uses color on supported terminals to summarize the unit state at a glance.
White color indicates an "inactive" or "deactivating" state.
Red color indicates a "failed" or "error" state.
Green indicates an "active", "reloading" or "activating" state.

To Stop the Service

$ sudo systemctl stop apache2

Confirm that the service is not running

$ sudo systemctl stop apache2

OUTPUT

 ● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: inactive (dead) since Thu 2021-06-10 12:12:58 UTC; 2s ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 315777 ExecStop=/usr/sbin/apachectl stop (code=exited, status=0/SUCCESS)
   Main PID: 3015 (code=exited, status=0/SUCCESS)

Jun 09 09:24:03 WebServer systemd[1]: Starting The Apache HTTP Server...
Jun 09 09:24:14 WebServer systemd[1]: Started The Apache HTTP Server.
Jun 10 00:00:03 WebServer systemd[1]: Reloading The Apache HTTP Server.
Jun 10 00:00:04 WebServer systemd[1]: Reloaded The Apache HTTP Server.
Jun 10 12:12:57 WebServer systemd[1]: Stopping The Apache HTTP Server...
Jun 10 12:12:58 WebServer systemd[1]: apache2.service: Succeeded.
Jun 10 12:12:58 WebServer systemd[1]: Stopped The Apache HTTP Server.

Enable apache2 Service on bootup

$ sudo systemctl enable apache2

Disable apache2 Service on bootup

$ sudo systemctl disable apache2

Restart the Service

$ sudo systemctl restart apache2

Alternatively, if you also wish to enable and start the service at the same time you may execute

$ sudo systemctl enable -- now apache2

To Check whether the Service is currently configured to Start on the next bootup

$ sudo systemctl is-enabled apache2

OUPUT

enabled

To Check whether the Service is active

$ sudo systemctl is-active apache2

OUTPUT

active

How to remove Systemd Services completely (Bonus)

What if you installed a package, and later decide that you no longer need it.
How do you go about removing it completely?

First, stop the service

$ sudo systemctl stop service-name

Second, disable the service

$ sudo systemctl disable service-name

Removing the service in systemd

$ sudo rm /etc/systemd/system/service-name

$ sudo rm /etc/systemd/system/service-name/[related symlinks]

Reload systemd

$ sudo systemctl daemon-reload

Then, run reset-failed to reset any units from failed state

$ sudo systemctl reset-failed

You should be familiar with the systemctl command's fundamental features, which enable you to interface with and control your systemd instance.
While systemctl primarily manages the core systemd process, it also manages various components of the systemd ecosystem.
Also, other features, such as log management and user sessions, are handled by separate daemons and administration programs (journald/journalctl and logind/loginctl).

Taking the effort to become acquainted with these additional tools and daemons will make management easier and effective.