daemon

A daemon is simply a background process that runs without direct user interaction. It usually starts at boot and keeps running in the background.

systemd

systemd is the modern init system used by most Linux distributions.

An init system is the first process started by the kernel (PID 1), and it:

• Starts all other services (daemons) and manages them

So:

• daemon = the service itself
• systemd = the manager of services

systemd manages things called units. Common unit types:

• service daemons
• socket network sockets
• target groups of services (like runlevels)

systemctl

systemctl is the command-line tool used to interact with systemd

Start a service:

sudo systemctl start sshd

Stop a service:

sudo systemctl stop sshd

Restart a service, kills the current process and starts a brand-new one:

sudo systemctl restart sshd

Reload a service, sends a signal to the service to re-read its configuration files without dropping connections or stopping the service:

Not all services support reload.

sudo systemctl reload sshd

Check a service status:

systemctl status sshd

Enable service at boot:

systemctl enable sshd

Disable service at boot:

systemctl disable sshd

Where services are defined

Service files live in:

/etc/systemd/system/
/usr/lib/systemd/system/

There are two locations because systemd separates vendor-provided units from administrator-customized units. This is mainly about package management safety and override behavior.

Example service file:

sshd.service

/etc/rc

This comes from older init systems (before systemd), mainly:

• SysVinit
• BSD-style init

rc stands for run commands. It refers to scripts that run during boot.