init is the first process to run after the system boots, and in many ways it is the most important daemon. It always has a PID of 1 and is an ancestor of all user processes and all but a few system processes.

At startup, init either places the system in single-user mode or spawns a shell to read the system’s startup files. When you boot the system into single-user mode, init reads the startup files after you terminate the single-user shell, usually by typing exit or <Control-D>.

After processing the startup files, init consults a configuration file (/etc/inittab on most systems, /etc/ttys on FreeBSD) to determine on which physical ports it should expect users to log in. It opens these ports and spawns a getty ...

Get UNIX System Administration Handbook, Third Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.