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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 during the booting process; a kernel panic will occur if the kernel is unable to start it, or it should die for any reason.
Once the kernel has started, it starts the init process, [20] a daemon which then bootstraps the user space, for example by checking and mounting file systems, and starting up other processes. The init system is the first daemon to start (during booting) and the last daemon to terminate (during shutdown). Systemd load is a runlevel target to ...
After the Linux kernel has booted, the /sbin/init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter , the system will attempt to enter (start) the default runlevel.
The name systemd adheres to the Unix convention of naming daemons by appending the letter d. [9] It also plays on the term "System D", which refers to a person's ability to adapt quickly and improvise to solve problems. [10] Since 2015, the majority of Linux distributions have adopted systemd, having replaced other init systems such as SysV ...
Instead, the system simply redefines the "parent PID" field in the child process's data to be the process that is the "ancestor" of every other process in the system, whose PID generally has the value of 1 (one), and whose name is traditionally "init" (except in the Linux kernel 3.4 and above [more info below]).
Linux distributions and other operating systems based on the Linux kernel which use Upstart as the default init system: Upstart is used in Google's ChromeOS and ChromiumOS. [8] Linux distributions that support or have supported Upstart to some extent, but moved away since or no longer use it as their default init system:
Most Unix systems have historically used init as the system process to which orphans are reparented, but in modern DragonFly BSD, FreeBSD, and Linux systems, an orphan process may be reparented to a "subreaper" process instead of init. [1] [2] A process can be orphaned unintentionally, such as when the parent process terminates or crashes.
launchd has two main tasks. The first is to boot the system, and the second is to load and maintain services.. Here is a simplified view of the Mac OS X Tiger system startup on a PowerPC Mac (on an Intel Mac, EFI replaces Open Firmware and boot.efi replaces BootX):