Lab 23: Start Stop Automate Services

Objective Focus

Deploy, configure, and maintain systems: Start and stop services and configure services to start automatically at boot

Systemd aka System Daemon

To understand services, we need to understand systemd.

Systemd is the software that initializes and manages the other software that provide services. It is the first process on boot, starting as PID 1.

We can use the command top to see this in action.

$ top

Units

Systemd has many components under it and those components are managed using Units.

Units are Systemd objects that are used to configure and maintain the many software components in a RHEL system. That also includes services.

I am going to list the types of Units below a table but our focus are the Service Unit type.

Unit Type	Extension	Description
Service	`.service`	Defines how to manage a system service or application. This is the most common unit type.
Socket	`.socket`	Describes a network or IPC socket that systemd can listen on. Services can be activated on demand when traffic arrives on these sockets.
Device	`.device`	Represents a kernel device exposed in the `sysfs` filesystem. These units can be used to manage device-specific actions or dependencies.
Mount	`.mount`	Defines a filesystem mount point, similar to an entry in `/etc/fstab`. Systemd can manage mounting and unmounting.
Automount	`.automount`	Defines a mount point that will be automatically mounted when accessed, similar to `autofs`.
Swap	`.swap`	Specifies a swap device or swap file that systemd should activate.
Target	`.target`	Used to group other units together and act as synchronization points during the boot process or for specific system states (e.g., `multi-user.target`, `graphical.target`).
Path	`.path`	Monitors a filesystem path for changes. When a change occurs, it can trigger the activation of another unit (e.g., a service).
Timer	`.timer`	Defines a timer that can activate another unit (typically a service) at a specific time or on a recurring basis, similar to `cron` jobs.
Slice	`.slice`	Used for resource management. Slices group processes together in a hierarchical manner within the Linux control groups (cgroups) for resource allocation.
Scope	`.scope`	Similar to slices, but created dynamically for external processes or sets of processes that are not managed by systemd itself (e.g., user sessions).

Systemctl Command

$ man systemctl

Control the systemd system and service manager

Systemctl provides a plethora of subcommands and switches for managing the system.

$ systemctl

The default output shows the services in memory. That is with no options.

Finding what we need!

$ systemctl list-unit-files -t service

Example: List unit files that are classified as a service type.

Check the Status of a Service

$ systemctl status *.service

$ systemctl status sshd.service

$ systemctl status NetworkManager.service

Start a Service

$ systemctl start *.service 

$ systemctl start sshd.service

Stop a Service

$ systemctl stop *.service 

$ systemctl stop sshd.service

Automate a Service to start at boot

$ systemctl enable *.service 

$ systemctl enable sshd.service

Edit a service

$ systemctl show sshd.service

This will show all the available properties of the sshd service. These properties can be edited or added the unit pertaining to the sshd.service.

$ systemctl edit sshd.service

Here we can add, remove, and edit and properties of a specified service.

Table of Systemctl subcommands

Category	Subcommand	Description	Example Usage
Service Control	`start <unit>`	Initiates a specified service or unit.	`sudo systemctl start apache2.service`
	`stop <unit>`	Halts a running service or unit.	`sudo systemctl stop apache2.service`
	`restart <unit>`	Stops a service (if running) and then starts it again.	`sudo systemctl restart apache2.service`
	`reload <unit>`	Tells a service to reload its config without restarting (if supported).	`sudo systemctl reload apache2.service`
	`reload-or-restart <unit>`	Reloads config if possible, otherwise restarts.	`sudo systemctl reload-or-restart nginx.service`
	`status <unit>`	Displays the current status of a service (active, loaded, logs).	`systemctl status sshd.service`
Boot-Time Config	`enable <unit>`	Configures a service to start automatically at boot.	`sudo systemctl enable firewalld.service`
	`disable <unit>`	Prevents a service from starting automatically at boot.	`sudo systemctl disable firewalld.service`
	`is-enabled <unit>`	Checks if a service is configured to start at boot.	`systemctl is-enabled ufw.service`
	`mask <unit>`	Completely disables a service, preventing any start.	`sudo systemctl mask apache2.service`
	`unmask <unit>`	Reverses the effect of `mask`.	`sudo systemctl unmask apache2.service`
System/Unit File Mgmt.	`daemon-reload`	Reloads the systemd manager configuration (after unit file changes).	`sudo systemctl daemon-reload`
	`list-units`	Displays all currently active units.	`systemctl list-units`
	`list-unit-files`	Lists all installed unit files and their enabled/disabled state.	`systemctl list-unit-files`
	`list-dependencies <unit>`	Shows the dependency tree for a specific unit.	`systemctl list-dependencies graphical.target`
	`--failed` (option)	Used with `list-units` to show only failed units.	`systemctl --failed`
	`get-default`	Shows the default systemd target.	`systemctl get-default`
	`set-default <target>`	Sets the default systemd target for booting.	`sudo systemctl set-default multi-user.target`
System Power Control	`poweroff`	Shuts down and powers off the system.	`sudo systemctl poweroff`
	`reboot`	Reboots the system.	`sudo systemctl reboot`
	`suspend`	Puts the system into a suspend (sleep) state.	`sudo systemctl suspend`
	`hibernate`	Puts the system into a hibernate state.	`sudo systemctl hibernate`

Systemctl

That is all for this lab. Understanding services makes your Linux system more useful and flexible.