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Chapter 2.7 - Services and Daemons

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand Linux services and daemons
  • Manage services with systemd
  • Create custom services
  • Understand systemd unit files

What is a Service/Daemon?​

A service (or daemon) is a program that runs in the background, usually without a user interface, to provide system or network functionalities.

Types of Services​

System Services​

Essential services for system operation:

  • sshd: SSH Server
  • systemd-logind: Session management
  • NetworkManager: Network management

Network Services​

Services that listen on the network:

  • apache2/nginx: Web servers
  • mysql/postgresql: Databases
  • bind9: DNS server

User Services​

Services launched by the user:

  • pulseaudio: Audio
  • dbus: Inter-process communication

Management with systemd​

systemctl Commands​

# View the status of a service
systemctl status apache2

# Start a service
systemctl start apache2

# Stop a service
systemctl stop apache2

# Restart a service
systemctl restart apache2

# Reload the configuration
systemctl reload apache2

# Enable at startup
systemctl enable apache2

# Disable at startup
systemctl disable apache2

# View all services
systemctl list-units --type=service

# View active services
systemctl list-units --type=service --state=running

Unit Files​

Services are defined by unit files in /etc/systemd/system/ or /lib/systemd/system/:

Example of a simple service:

[Unit]
Description=My Custom Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/my-service
Restart=always
User=myuser

[Install]
WantedBy=multi-user.target

Sections:

  • [Unit]: Metadata and dependencies
  • [Service]: Service configuration
  • [Install]: Installation and activation

Types of Services​

Simple Type​

Service that starts and remains active:

Type=simple
ExecStart=/usr/bin/my-service

Forking Type​

Service that forks and the parent terminates:

Type=forking
ExecStart=/usr/bin/my-service
PIDFile=/var/run/my-service.pid

Oneshot Type​

Service that runs once and terminates:

Type=oneshot
ExecStart=/usr/bin/script-init
RemainAfterExit=yes

Dependencies​

Services can have dependencies:

[Unit]
Description=Web Service
After=network.target mysql.service
Requires=mysql.service
Wants=redis.service

Directives:

  • After: Starts after these services
  • Before: Starts before these services
  • Requires: Mandatory dependency
  • Wants: Optional dependency

Summary​

In this chapter, you learned:

Services/Daemons: Background programs
systemd: Modern service manager
systemctl: Management commands
Unit files: Service configuration
Dependencies: After, Before, Requires, Wants
Types: simple, forking, oneshot

Next Steps​

Chapter 2.8: Runlevels
Chapter 2.9: Boot Process

Chapter created on: December 2024