Table of Contents
Understanding System Services and Daemons
In Linux, many important tasks run quietly in the background without any visible window or user interaction. These background components are known as services and daemons. They are essential for networking, logging, scheduled jobs, and many other system functions.
What Is a Daemon?
A daemon is a background process that is not directly tied to a terminal or a logged-in user. It usually starts when the system boots and keeps running to provide some ongoing functionality, such as handling network connections or writing system logs.
Traditionally, daemon program names often end with the letter d. For example, sshd is the Secure Shell daemon, and httpd is a common name for a web server daemon.
Daemons usually follow a few characteristics. They run without a user interface, do not expect input from the keyboard, and continue running regardless of which users log in or out. They are typically managed by the system’s init system, which on most modern Linux distributions is systemd. Details of systemd and its tools belong to their own chapters, but it is important here that services and daemons are often started, stopped, and supervised by this central system manager.
What Is a System Service?
A system service is a more general term for a managed background task that provides a specific function to the system or to users. In practice, the words "service" and "daemon" are often used almost interchangeably, but there is a subtle distinction.
The term "daemon" refers to the process itself, the program running in the background. The term "service" usually refers to the combination of that process and the configuration that defines how it should be started, stopped, and restarted. This includes things like dependency information, what user it should run as, and where its log messages should go.
On systems that use systemd, each service is described by a unit file that has a .service extension. This unit file is what systemd uses to know how to start the daemon process. On older systems or different init systems, services might be represented as scripts in specific directories, but the overall idea is the same. The service definition is a formal description of how to manage a daemon.
Important: A daemon is the actual background process. A service is the managed entity that defines how such a process is started, stopped, and supervised by the init system.
How Services and Daemons Interact With the System
Services and daemons often start very early during the boot process, before any users have logged in. They are responsible for providing essential system capabilities.
For example, a network service will bring up network interfaces and allow the system to communicate over the network. A logging service will collect log messages from the kernel and from user programs and store them in log files or structured journals. A print service will manage communication with printers and handle print queues.
Most of these services run with special permissions and carefully controlled environments. This makes them different from regular user processes such as text editors or web browsers. Because they are so important, the system’s init system keeps track of them and can restart them automatically if they fail.
Long-running and On-demand Services
Not all services run all the time. Some daemons are long-running. They start at boot and stay active. Others are started only when needed. The system can be configured so that a service starts on demand, perhaps when a request arrives at a particular network port or when a specific hardware event occurs.
From the user’s perspective, both kinds still appear as services that provide capabilities. The difference is in when and how the processes are created. Long-running services use more memory continuously but are always ready to respond. On-demand services can reduce resource usage, but they take some time to start up when first used.
Recognizing Services and Daemons as Processes
When you view processes using tools like ps or top, daemons appear just like other processes, but a few clues can help identify them.
The parent process of a daemon is often the init system, for example systemd, which usually has process ID 1. This means the daemon has been started by the system, not by a user shell. The controlling terminal field for such a process is often ? instead of a specific terminal device, which indicates it does not belong to any user terminal.
Service processes may run under specific, often restricted, user accounts. For example, a web server daemon might run as the www-data user for security reasons, even though it is started by the system as root. This separation is common in well designed services.
When you list running processes, you might see a main daemon process plus one or more worker or helper processes that it has spawned. Some daemons are single process programs, while others use multiple processes or threads to handle higher workloads.
Service Management Concepts
Services and daemons are not just started once and forgotten. They are actively managed by the init system, which keeps track of their state. A service can be in various states such as active, inactive, failed, or reloading.
An active service is one whose associated daemon process is currently running. An inactive service is defined on the system but its process is not currently running. A failed service is one whose process has exited in an unexpected way. Some configurations instruct the init system to attempt to restart a failed service automatically.
In addition to the current state, services have enablement settings. An enabled service is configured to start automatically when the system boots or when certain conditions are met. A disabled service will not start automatically but can still be started manually.
When you interact with services from the command line, you typically use a tool that communicates with the init system. With systemd this tool is systemctl. With older systems, it may be another command or a script in /etc/init.d. The details of these tools and their commands are covered in their own chapter, but for services and daemons the key is understanding that the init system provides a standard interface for managing them.
Configuration of Services and Daemons
Each service or daemon usually has its own configuration files. These files are often stored in /etc, sometimes under a dedicated subdirectory. For example, a web server might read a main configuration file in /etc/httpd or /etc/apache2.
The service manager itself, such as systemd, has its own configuration that defines how it manages services in a general way. Individual service unit files tell the init system which executable to start, what arguments to pass, what environment variables to set, and what other services must be running before this one can begin.
The separation of responsibilities is important. The daemon’s own configuration controls what the program does, such as which ports it listens on or what directories it serves. The service’s unit file controls how that program is started and supervised by the system.
When an administrator wants to change how a service behaves, they must be aware of both layers. Changing the daemon’s own configuration requires editing its config files and then telling the service manager to reload or restart the service. Changing how the init system manages the service involves editing or creating its unit file or other init configuration.
Logging and Diagnostics for Services
Services and daemons typically write log messages that help administrators understand what is going on internally. These messages might describe successful actions, warnings, or errors.
Depending on the distribution and init system, service logs may be written to plain text files in directories under /var/log, or they may be written to a central logging system managed by the init system. For systemd, there is an integrated journal that collects logs from many sources.
When diagnosing problems with a service, it is important to check its logs as well as its process status. Errors in configuration files often show up as messages in the logs when the service attempts to start. If a daemon crashes, information about the reason may be recorded there.
The ability to tell whether a service is running, why it is failing, and what it has recently done depends greatly on these logging mechanisms. Learning where a specific service writes its logs is an important skill for troubleshooting.
Service Dependencies and Startup Order
Many services rely on other services. For instance, a service that accesses a database may require the database service to be running first. The init system needs to know about these dependencies to start services in the correct order.
Service definitions can contain dependency information that tells the init system which services must be started before or after others. This allows the system to boot in a coordinated way. It also affects what happens when one service is stopped or restarted.
If a service that others depend on fails, it can cause errors in the dependent services. A careful design and configuration of service dependencies helps prevent such chain reactions, or at least makes the behavior predictable.
Services can also be bound to specific system states, such as particular boot targets or runlevels, which represent groups of services that should be running at the same time. Working with these targets is part of more advanced system administration, but the central point here is that services rarely exist in isolation.
Security Considerations for Daemons
Because daemons often run with elevated privileges and accept input from networks or other untrusted sources, they are common targets for attacks. For this reason, service and daemon design pays careful attention to security.
Some services are split into several processes running with different privilege levels. The main daemon might start as root only long enough to open privileged resources, then drop to a less privileged user. Others use specific confinement mechanisms like access control profiles or namespaces to limit what they can do.
From a user or administrator perspective, keeping daemons up to date and properly configured is essential. Misconfigured or outdated services can expose vulnerabilities. Understanding that services and daemons are often the exposed surface of the system helps explain why administrators need to monitor them carefully.
Interaction Between Users and Services
Although services run in the background, users interact with them indirectly. When a user connects with an SSH client, they are really talking to the sshd daemon. When a user loads a webpage hosted on the system, they interact with a web server daemon. When a user prints a document, a print service manages the job.
User commands can instruct the init system to start or stop services, but regular users may have limited permissions to manage them. Often, only administrative users can change the state or configuration of important system services. However, regular users can still check the status of some services or interact with them through their network interfaces or sockets.
This separation lets the system remain stable while still offering services to many users at once. It is a core element of multiuser design.
Summary of Services and Daemons in Daily Use
In everyday Linux use, system services and daemons work quietly in the background to provide networking, storage sharing, logging, scheduling, and many other features. They differ from ordinary programs because they are designed to run continuously, to start automatically, and to be controlled centrally by the init system.
Understanding that a daemon is a background process, and that a service is the managed entity that governs such a process, gives you a mental model of how background functionality is organized on a Linux system. Later chapters will explore the tools used to control these services in detail. For now, it is enough to know that these background components are central to how Linux operates, even when you do not see them directly.