Daemons

A daemon is a type of program on UNIX-like operating systems that runs unobtrusively in the background rather than being directly invoked by a user. It operates passively, waiting for specific conditions or events to trigger its execution.

UNIX-like systems typically run numerous daemons to handle various tasks, such as responding to network service requests, monitoring hardware activity, or interacting with other software. Examples of conditions or events that might activate a daemon include a scheduled date or time, the passage of a defined time interval, the receipt of a web or email request, or the arrival of a file in a designated directory.

Daemons operate independently of the user who triggers the associated condition or event. While the user initiating the action may not be aware of the daemon’s presence, some programs are explicitly designed to perform actions that implicitly trigger daemons.

Daemons, also referred to as background processes, typically have names ending with the letter “d.” For instance, sshd handles SSH remote access connections, and httpd manages the Apache web server. Many daemons are initialized during system startup.

On Linux systems, shell scripts located in the /etc/init.d directory are commonly used

to start, stop, or manage daemons. These scripts enable administrators to control the behavior of background processes as needed.

Processes

Daemons in Linux are typically implemented as processes, which are instances of programs in execution. These processes are managed by the kernel—the core of the operating system—which assigns each one a unique process identification number (PID).

In Linux, there are three main types of processes:

1.  Batch Processes

These are submitted to a process queue and operate independently of the command line. Batch processes are well-suited for repetitive tasks, especially during periods of low system usage.

2. Interactive Processes

These are initiated directly by a user via the command line and are executed interactively.

3. Daemon Processes

These are background processes distinguished by the fact that their parent process has a PID of 1. This indicates that the init process is their parent. The init process is the first process launched when the system boots and remains active until the system shuts down.

The init process also adopts orphaned processes—that is, processes whose parent has terminated—ensuring they can continue running without disruption.

A common technique for creating a daemon involves forking the process (often once or twice) and allowing the parent process to terminate. This ensures the child process runs independently in the background, performing its intended function without direct association with the user or terminal session.