Inittab

In Unix-based computer operating systems, init (short for initialization) is the first process started during booting of the computer system. Init is a daemon process that continues running until the system is shut down. It is the direct or indirect ancestor of all other processes and automatically adopts all orphaned processes. Init is started by the kernel using a hard-coded filename; a kernel panic will occur if the kernel is unable to start it. Init is typically assigned process identifier 1.

In Unix systems such as System III and System V, the design of init has diverged from the functionality provided by the init in Research Unix and its BSD derivatives. The usage in most Linux distributions employing a traditional init rather than a recent variant such as systemd is somewhat compatible with System V, while some distributions such as Slackware use BSD-style startup scripts, and others such as Gentoo have their own customized versions.

Several replacement init implementations have been created, attempting to address design limitations in the standard versions. These include launchd, the Service Management Facility, systemd and Upstart; as of March 2015^[update], systemd has been adopted by several major Linux distributions although it remains controversial.

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