A daemon is a long-running background process on Unix-like operating systems that starts at boot or on demand and provides a service, such as a web server or SSH server, without a controlling terminal.
Why it matters
Most server software runs as a daemon, and names often end with d, such as sshd, crond and httpd. Init systems like systemd start, stop and restart them and can bring them back up after a crash.
When a daemon stops, the service it provides disappears even though the machine is running, which is a common cause of outages.
Example
sudo systemctl status nginx
sudo systemctl restart nginx
sudo journalctl -u nginx --since "10 min ago"These commands check the nginx daemon, restart it and show its recent logs.
Common mistakes
- Starting a service manually in a terminal so it dies when the session ends.
- Not enabling the service at boot, so it is missing after a reboot.
- Restarting repeatedly without reading the logs to find the cause.
Related
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