Title: Linux Services and Daemons: A Complete Guide

When working with Linux, you’ll often hear terms like services and daemons. These processes are the backbone of a Linux system, quietly running in the background to ensure everything from networking to web hosting functions properly. Whether you’re a beginner or an experienced sysadmin, understanding how services and daemons work - and how to control them - is essential for effective Linux management.

What are Services and Daemons in Linux?

In Linux, many essential tasks happen behind the scenes - long before you even log in or open a terminal. These background workers are known as services and daemons, and they play a critical role in keeping your system reliable, secure, and functional. Let’s look at what they are, how they differ, and why they matter.

What is a Daemon?

A daemon (pronounced as DEE-muhn) is a background process that runs quietly without requiring direct user interaction. Its main purpose is to either continuously listen for system events or perform routine tasks automatically.

Think of a daemon as an invisible helper that never needs a break. It just waits in the background until it’s needed.

Common examples of daemons include:

• sshd – Handles secure remote logins over SSH.
• httpd (Apache) – Runs web servers to deliver websites and APIs.
• crond – Executes scheduled tasks, like automatic backups or cleanup jobs.
• mysqld – Manages MySQL databases and handles database queries.
• cupsd – Manages print jobs in a Linux printing environment.
  
  

Tip: Most daemons in Linux end with the letter “d”, which is short for daemon.

What is a Service?

A service in Linux refers to any background process that provides a function to the system or to users. While many services are powered by daemons, a service can also be something that runs only when needed instead of continuously.

Services are the backbone of many day-to-day Linux operations, such as:

• NetworkManager – Configures and manages your system’s network connections.
• firewalld – Provides firewall management and security filtering.
• rsyslog – Handles system logging for troubleshooting and auditing.
• docker – Runs containerized applications and manages container services.
• postfix – Manages email delivery and mail server functions.
  
  

Unlike daemons, which typically run all the time, services can be:

• Set to start automatically at boot.
• Manually started, stopped, or restarted by the user.
• Configured to run persistently or on-demand.

How Linux Manages Services: systemd and More

On modern Linux systems, the systemd init system has become the standard for managing services and daemons.

Here’s why it’s important:

• systemd is PID 1: It’s the very first process that starts when your system boots, and it remains running for the lifetime of the system.
• It’s responsible for initialising hardware, mounting filesystems, and launching essential daemons.
• It also provides powerful tools (systemctl) to start, stop, restart, enable, or disable services.

It also ensures that processes launch in the correct order, dependencies are handled, and system resources are efficiently managed. Most major Linux distributions - including Ubuntu, Fedora, Debian, CentOS, and Arch Linux - have adopted systemd as their default init system because of its speed, efficiency, and extensive feature set.

Other Init Systems You Might Encounter

While systemd dominates today’s Linux ecosystem, it’s not the only init system out there. Depending on the distribution, you may still encounter alternatives:

• SysVinit – The traditional init system found in older Linux versions. It relied on simple shell scripts stored in /etc/init.d/ to start and stop services.
  
  
• Upstart – Developed by Ubuntu to improve on SysVinit with event-driven capabilities. It was eventually phased out in favor of systemd.
  
  
• OpenRC – A lightweight, flexible init system still used by distributions like Alpine Linux and Gentoo, popular in minimal and container - focused environments.

Controlling Services with systemd

Today, most Linux distributions (Ubuntu, CentOS, Fedora, Debian, etc.) use systemd as their init system. With systemd, managing services is straightforward.

Here are some essential commands:

Start a service

sudo systemctl start apache2

Start a service

sudo systemctl stop apache2

Restart a service

sudo systemctl restart apache2

Check the status of a service

sudo systemctl status apache2

Enable a service to start at boot

sudo systemctl enable apache2

Disable a service at boot

sudo systemctl disable apache2

This makes it simple to control when and how a daemon should run.

Checking Running Services

To view all active services on your Linux system, use:

systemctl list-units --type=service --state=running

This will show you which daemons are currently active and performing tasks.

How to View Daemons with ps, top, and htop

While systemctl is excellent for managing services, sometimes you’ll want to look directly at what’s happening at the process level.  That’s where commands like ps, top, and htop come in. 

This is useful when you need to:

• Confirm whether a daemon is running.
• Check its resource usage.
• Troubleshoot performance issues.
  
  

Using ps

The ps aux command gives you a snapshot of all running processes:

ps aux

If you look for a specific daemon, you need to use a combination of ps with grep to filter the results.

Sample output might look like this. Here, you can clearly see sshd running as a background daemon.

Using top

For a real-time view of what’s happening on your system, you can use the top command, which comes pre-installed on almost every Linux distribution.

Once you run it, you’ll see an updated list of processes sorted by CPU usage.

Using htop: 

If you prefer something more user-friendly and colourful, try htop -  a modern, interactive alternative to top. Unlike top, htop lets you:

• Scroll through the full list of processes
• Filter and search by name
• Sort by CPU, memory, or other criteria with a single key press

If you try running htop and get an error like: -bash: htop: command not found,

Then you may need to install it first.

Once installed, just type htop and you’ll have an interactive, colourful view of running processes.

Why controlling Daemons Matters

Efficient service management isn’t just about starting or stopping daemons - it’s about system performance and security. For instance:

• Disabling unused services reduces the attack surface.
• Restarting services helps apply configuration changes.
• Monitoring ensures critical services (like SSH or databases) stay up and running.
  
  

Conclusion

Linux services and daemons may work silently in the background, but they are the backbone of every Linux system. They take care of essential background tasks - like handling SSH logins, scheduling automated jobs, serving websites, syncing time, and managing firewall rules - without requiring constant user input.

By learning how they function and how to control them with tools like systemd, you gain better control over your server’s stability, performance, and security. Additionally, by checking how these components operate and mastering tools like systemctl, ps, top, and htop, you gain the ability to troubleshoot problems, fine-tune performance, and take full control of your Linux environment.

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