Init
From Wikipedia, the free encyclopedia
- The correct title of this article is init. The initial letter is shown capitalized due to technical restrictions.
init (short for "initialization") is the program on Unix and Unix-like systems which spawns all other processes. It runs as a daemon and typically has PID 1.
The functionality diverged, in Unixes such as System III and System V, from the functionality provided by the init in Research Unix and its BSD derivatives. The usage on most Linux distributions is compatible with System V, but some distributions, such as Slackware, use a BSD-style and others, such as Gentoo Linux, have their own customized version.
Opinions on the relative merits of different schemes can be strongly held, leading both to occasional "flame wars", and also to the development of several alternatives.
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[edit] BSD-style
BSD init runs the initialization shell script located in '/etc/rc', then launches getty on text-based terminals or a windowing system such as X on graphical terminals. There are no runlevels; the 'rc' file determines how init is to be run.
Advantages: Simple and easy to edit manually.
Problems: If a 3rd-party package needs to have an initialization script run during the boot procedure, it needs to edit one of the existing boot scripts, but a simple mistake in that process could lead to an unbootable system.
[edit] SysV-style
System V init examines the '/etc/inittab' file for an 'initdefault' entry, which tells init whether there is a default runlevel. If there is no default runlevel, then the user is dumped to a system console, where a runlevel must be entered manually.
Advantages: Flexibility and scalability.
Problems: Complexity.
[edit] Runlevels
The Runlevels in System V describe certain states of a machine, characterized by the processes run. There are generally 8 runlevels. These are the runlevels 0 to 6 and S or s, which are aliased to the same runlevel. Of these eight, 3 are so-called "reserved" runlevels:
- 0. Halt
- 1. Single user mode
- 6. Reboot
Aside from runlevels 0, 1, and 6, every Unix system treats runlevels a little differently. The common denominator is the /etc/inittab file, which defines what each runlevel does (if they do anything at all).
[edit] Default runlevels
| OS | Default runlevel |
|---|---|
| AIX | 2 |
| Debian | 2 |
| Gentoo Linux | 3 |
| Mandriva Linux | 5 |
| Solaris | 3 |
| SUSE Linux | 5 |
| Ubuntu | 2 |
| Redhat Linux | 3 or 5 |
On the two Linux distributions defaulting to runlevel 5 in the table above, runlevel 5 is a multiuser graphical environment running the X Window System, usually with a display manager. However, in the Solaris operating system, runlevel 5 is typically reserved to shutdown and automatically poweroff the machine.
On most systems users can check the current runlevel with either of the following commands:
$ runlevel $ who -r
The current runlevel is typically changed by root running the telinit or init commands. The default runlevel is set in the /etc/inittab file with the :initdefault: entry.
[edit] Skipping init
In Linux systems, with most modern bootloaders (such as LILO or GRUB), users can change which process the kernel spawns at the end of its initialization from the normal default of /sbin/init. This is generally done by typing init=/foo/bar at the bootloader's prompt. Appending init=/bin/bash, for example, will bring up a single root shell, without a password. If the system administrator feels that this is insecure, they may setup a BIOS password. Or in the case of GRUB, MD5 hashed boot passwords.
[edit] See also
- Unix System V
- Linux
- SystemStarter and launchd — Apple Mac OS X's process spawner
- Initng, a full replacement of init designed to start processes asynchronously
- Upstart, a full replacement of init designed to start processes asynchronously initiated by Ubuntu
- eINIT, a full replacement of init designed to start processes asynchronously, but with the potential of doing things without shell-scripts
[edit] External links
- man pages for init
- Alternative init schemes:
