Services and daemons in Linux

In the realm of Linux operating systems, daemons are a fundamental component that runs quietly in the background, silently executing essential tasks without the direct involvement of an interactive user. These processes are traditionally identified by their names ending with the letter d, such as sshd for the Secure Shell (SSH) daemon and httpd for the web server daemon. They play a vital role in handling system-level tasks crucial for both the operating system and the applications running on it.

Daemons serve an array of purposes, ranging from file serving, web serving, and network communications to logging and monitoring services. They are designed to be autonomous and resilient, starting at system boot and running continuously until the system is shut down. Unlike regular processes initiated and controlled by users, daemons possess distinct characteristics:

• Background operation:
  • Daemons operate in the background
  • They lack a controlling terminal for direct user interaction
  • They do not require a user interface or manual intervention to perform their tasks
• User independence:
  • Daemons operate independently of user sessions
  • They function autonomously without direct user involvement
  • They wait for system events or specific requests to trigger their actions
• Task-oriented focus:
  • Each daemon is tailored to execute a specific task or a set of tasks
  • They are designed to handle specific functions or listen for particular events or requests
  • This ensures efficient task execution

Creating a daemon process involves more than merely running a process in the background. To ensure effective operation as a daemon, developers must consider several key steps:

1. Detaching from the terminal: The fork() system call is employed to detach the daemon from the terminal. The parent process exits after the fork, leaving the child process running in the background.
2. Session creation: The setsid() system call creates a new session and designates the calling process as the leader of both the session and the process group. This step is crucial for complete detachment from the terminal.
3. Working directory change: To prevent blocking the unmounting of the filesystem, daemons typically change their working directory to the root directory.
4. File descriptor handling: Inherited file descriptors are closed by daemons, and stdin, stdout, and stderr are often redirected to /dev/null.
5. Signal handling: Proper handling of signals, such as SIGHUP for configuration reloading or SIGTERM for graceful shutdown, is essential for effective daemon management.

Daemons communicate with other processes or daemons through various IPC mechanisms.

Daemons are integral to the architecture of many asynchronous systems, providing essential services without direct user interaction. Some prominent use cases of daemons include the following:

• Web servers: Daemons such as httpd and nginx serve web pages in response to client requests, handling multiple requests concurrently and ensuring seamless web browsing.
• Database servers: Daemons such as mysqld and postgresql manage database services, allowing for asynchronous access and manipulation of databases by various applications.
• File servers: Daemons such as smbd and nfsd provide networked file services, enabling asynchronous file sharing and access across different systems.
• Logging and monitoring: Daemons such as syslogd and snmpd collect and log system events, providing asynchronous monitoring of system health and performance.

In summary, daemons are essential components of Linux systems, silently performing critical tasks in the background to ensure smooth system operation and efficient application execution. Their autonomous nature and resilience make them indispensable for maintaining system stability and providing essential services to users and applications.

We have seen processes and demons, a special type of process. A process can have one or more threads of execution. In the next section, we will be introducing threads.