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Init is the father of all processes in a UNIX system (usually it hase a pid of 1). It is responsible for starting all other programs and daemons. On most systems, init provides a set of "runlevels". Each runlevel contains a set of processes, and the root may switch between them at any time.

Name used to describe an extension under Mac OS 6.x.x. Inits are small chunks of code that load right after the operating system loads, modifying the behavior of the Finder or adding functionality to the OS.

init is also a method in the Java java.applet.Applet class. init is used by the web browser to inform the Applet it has been loaded. A good place to initialize variables, as one can probably guess.

UNIX systems (including HP-UX, IRIX, Linux and Solaris flavors) all use the init command to set the initialization process.

  • init 0
    Shut down the entire system to a halted state. Basically powers off the system or PROM monitor mode.

  • init 1
    Places the machine into admin mode. All file systems are accessible, but only a superuser console can access the system.

  • init 2
    Normal multiuser mode.

  • init 3
    Normal multiuser mode with remote file sharing.

  • init 4
    Custom user-defined multiuser environment. In HP-UX, the VUE is loaded.

  • init 5
    Shut down the system like init 0 but do not power off.

  • init 6
    Shut down the machine and restart as init 2 or 3.

  • init a,b,c
    Not a true init state, but allows programs to be set to run.

  • init S,s
    Normal single-user mode. This mode is the default should the inittab be missing or corrupted.

  • init Q,q
    Do not change init levels, but re-examine the inittab file. This way you can use changes without rebooting.
  • The idea of runlevels in init is found in System V, one of the two chief genera of Unix. The other genus, BSD, uses a much simpler init process. The BSD init program sets the kernel's securelevel, runs the boot script /etc/rc, and maintains getty processes for the terminals specified in /etc/ttys. And ... well ... that's about it.

    Linux, incidentally, uses a SysV-style init, but with a different set of runlevels than RancidPickle gives. Notably, runlevel 5 is used for X11 mode, in which console logins are managed by xdm (or a variant such as gdm or kdm). Debian uses runlevel 3 for normal multi-user text console mode; some other distributions use runlevel 2 for this.

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