In Linux systems, daemons (Daemons) refer to special processes that run in the background, have no terminal control, and are usually synchronized with system startup and shutdown. The daemon does not rely on the direct interaction of users and is responsible for executing system-level or backend services tasks, such as network services, log management, timing tasks, etc.
Several typical characteristics of daemon
Background operation: Daemons have nothing to do with the foreground interaction. They run silently in the background and are not directly controlled by the user.
No terminal association: The daemon is usually started at the system boot, independent of any user session, and is not bound to the control terminal.
Long-term operation: The daemon keeps running during the system life cycle and is generally only stopped under system shutdown or special circumstances.
Autonomous startup: Some daemons will be automatically restarted by the system after crashing to ensure high availability of services.
Use the `-daemon` or `nohup` command
In Linux or Unix-like systems, there are several ways to start a service and put it in the background, including using the `-daemon` or `nohup` command. The functions of the two are slightly different. Here is a brief description of their differences:
1. -daemon parameters:
- Some programs allow the daemon to be started by adding the `-daemon` or `--daemon` parameter to the startup command. The daemon runs in the background and is usually out of terminal control.
- When you start the program with the `-daemon` parameter, the program may perform background operations on its own, that is, turn itself into a daemon. In this case, the program handles its background execution itself without requiring additional tools.
- Some programs support the `-daemon` parameter, while others may not.
2. nohup command:
- `nohup` is a command used to continue running the command after the terminal is closed. The purpose of `nohup` is to ignore the SIGHUP (terminal suspend signal) commands, so that the command continues to run after the terminal is closed without being affected by terminal suspend.
- When using the `nohup` command, you usually need to add `&` to the end of the command to enable the command to be executed in the background.
- `nohup` is designed for long-running tasks, not just in the background.
In general, some programs may support the `-daemon` parameter, through which they can run in the background, while others may need to use `nohup` to ensure they are executed in the background and are not affected by terminal hangs. The specific behavior depends on the design and support of the program. If a program supports both `-daemon` and `nohup`, you can choose one of them according to your specific needs.
systemd starts and manages daemons
systemd, as a widely used system and service manager in Linux, can easily manage daemons. Use the systemctl command to start, stop, restart and view the status of the daemon. For example:
systemctl start # Start the daemonsystemctl stop # Stop the daemonsystemctl restart # Restart the daemonsystemctl status # Check daemon status
A simple systemd daemon configuration file might be as follows:
[Unit] Description=My Daemon Service [Service] ExecStart=/usr/local/bin/mydaemon Restart=on-failure [Install] WantedBy=
Daemon monitoring and logging
The logging of the daemon is usually handled by syslog or journald. The daemon can write log information into the system log through the syslog() function for subsequent analysis and troubleshooting.
Log files are usually located in the /var/log/ directory, such as /var/log/syslog or /var/log/messages.
Daemon management advantages of systemd
With the popularity of systemd, the management of daemon processes has also undergone great changes. systemd provides many advanced features such as:
Parallel Startup: Compared to the old init system, systemd can start multiple services in parallel, thereby speeding up system startup time.
Dependency Management: systemd can define dependencies between daemons through configuration files, ensuring that services are started and shut down in the correct order.
Status monitoring and automatic restart: If a daemon crashes, systemd can automatically restart it according to the configuration file to enhance the reliability of the system.
Summarize
As a key backend service in Linux systems, the daemon process undertakes many important tasks required for the normal operation of the system. Through detailed design and implementation of daemon processes, developers can create efficient and stable service processes, and using service managers of modern Linux systems such as systemd, it can greatly simplify the management, startup and monitoring of daemon processes.
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