Frequently Asked Question

What is a daemon and how is it different from a normal background process?

A daemon is a long-running background process designed to run unattended, typically starting at boot and surviving the lifetime of any user session. The classic test is that it has no controlling terminal, ps shows ? in the TTY column, and its parent is PID 1 because it was launched by the init system rather than by a login shell. Names traditionally end in d: sshd, crond, systemd, httpd, dockerd.

Historically a program "became a daemon" by going through a ritual (fork, leave the process group, fork again, close all file descriptors, chdir to /, set the umask). On a systemd system you almost never write that code yourself: you write a .service unit declaring Type=simple (or notify), and systemd handles the backgrounding, logging to the journal, restart on failure, and resource limits. A backgrounded shell job, by contrast, still has a parent shell, a controlling terminal, and dies when the user logs out, those are the things daemonisation strips away.