Table of Contents

Controlling Services and the Boot Process

What is systemd ?

systemd uses units to manage different types of objects. Some common unit types are listed below:

Type	Description
Service	Represents system services. Used to start frequently accessed daemons, such as a web server.
Socket	Represents IPC sockets monitored by systemd. Starts the daemon when a client connects. Useful for on-demand or delayed service start at boot.
Path	Used to delay the activation of a service until a specific path condition is met.

A specific file system change occurs. This is commonly used for services which use spool directories such as a printing system. The systemctl command is used to manage units. For example, display available unit types with the systemctl -t help command

systemctl -t help

To list current running active services run

systemctl list-units --type=service

UNIT                               LOAD   ACTIVE SUB     DESCRIPTION                                                                  
accounts-daemon.service            loaded active running Accounts Service
alsa-state.service                 loaded active running Manage Sound Card State (restore and store)
atd.service                        loaded active running Deferred execution scheduler
auditd.service                     loaded active running Security Auditing Service
avahi-daemon.service               loaded active running Avahi mDNS/DNS-SD Stack
chronyd.service                    loaded active running NTP client/server
...Output Omitted...

To specify a specific type for LOAD, ACTIVE, SUB use flag --statue= and specify the type needed., You can also add --all to show all services despite their state as the command by default shows ONLY active

Illustration for the output above

Column	Description
UNIT	The service unit name.
LOAD	Whether systemd properly parsed the unit’s configuration and loaded the unit into memory.
ACTIVE	The high-level activation state of the unit. This indicates whether the unit has started successfully or not.
SUB	The low-level activation state of the unit. Provides more detailed information about the unit, varying based on unit type, state, and how the unit is executed.
DESCRIPTION	The short description of the unit.

Note that the running command systemctl without arguments show all active running units

systemctl list-unit-files --type=service

The command above list files shows enabled & disabled units, valid entries for the STATE field are enabled, disabled, static, and masked.

To show service status run systemctl status UNIT.TYPE

systemctl status UNIT.service

Check is service is enables, output values areenabled, disabled

systemctl is-enabled sshd.service

Check is service is active, output values areactive, inactive

systemctl is-active sshd.service

Check is service is failed, output values areactive, failed, inactive

systemctl is-failed sshd.service

Manipulating units state

Task	UNIT
View detailed information about a unit state.	systemctl status UNIT
Stop a service on a running system.	systemctl stop UNIT
Start a service on a running system.	systemctl start UNIT
Restart a service on a running system.	systemctl restart UNIT
Reload the configuration file of a running service.	systemctl reload UNIT
Completely disable a service from being started, both manually and at boot.	systemctl mask UNIT
Make a masked service available.	systemctl unmask UNIT
Configure a service to start at boot time.	systemctl enable UNIT
Disable a service from starting at boot time.	systemctl disable UNIT
List units required and wanted by the specified unit.	systemctl list-dependencies UNIT

Rebooting and Shutting Down

To power off or reboot a running system from the command line, you can use the systemctl command.

systemctl poweroff stops all running services, unmounts all file systems (or remounts them read-only when they cannot be unmounted), and then powers down the system.

systemctl reboot stops all running services, unmounts all file systems, and then reboots the system.

You can also use the shorter version of these commands, poweroff and reboot, which are symbolic links to their systemctl equivalents.

System boot targets

Target	Purpose
graphical.target	System supports multiple users, graphical- and text-based logins.
multi-user.target	System supports multiple users, text-based logins only.
rescue.target	sulogin prompt, basic system initialization completed.
emergency.target	sulogin prompt, initramfs pivot complete, and system root mounted on / read only

To get the default running target use

systemctl get-default

To change the default boot target use

systemctl set-default TARGET_NAME

Note that TARGET_NAME can only be 1 from the 4 types specified above.