In Unix-based computer operating systems, init (short for initialization) is the first process started during booting of the operating 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 during the booting process; a kernel panic will occur if the kernel is unable to start it, or it should die for any reason. 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. Up until the early 2010s, [1] [ failed verification ] most Linux distributions employed a traditional init that was somewhat compatible with System V, while some distributions such as Slackware use BSD-style startup scripts, and other distributions such as Gentoo have their own customized versions.
Since then, several additional init implementations have been created, attempting to address design limitations in the traditional versions. These include launchd, the Service Management Facility, systemd, Runit and OpenRC.
Research Unix init runs the initialization shell script located at /etc/rc
, [2] then launches getty on terminals under the control of /etc/ttys
. [3] There are no runlevels; the /etc/rc
file determines what programs are run by init. The advantage of this system is that it is simple and easy to edit manually. However, new software added to the system may require changes to existing files that risk producing an unbootable system.
BSD init was, prior to 4.3BSD, the same as Research UNIX's init; [4] [5] in 4.3BSD, it added support for running a windowing system such as X on graphical terminals under the control of /etc/ttys
. [6] [7] To remove the requirement to edit /etc/rc
, BSD variants have long supported a site-specific /etc/rc.local
file that is run in a sub-shell near the end of the boot sequence.
A fully modular system was introduced with NetBSD 1.5 and ported to FreeBSD 5.0 and successors. This system executes scripts in the /etc/rc.d
directory. Unlike System V's script ordering, which is derived from the filename of each script, this system uses explicit dependency tags placed within each script. [8] The order in which scripts are executed is determined by the rcorder utility based on the requirements stated in these tags.
When compared to its predecessors, AT&T's UNIX System III introduced a new style of system startup configuration, [9] which survived (with modifications) into UNIX System V and is therefore called the "SysV-style init".
At any moment, a running System V is in one of the predetermined number of states, called runlevels . At least one runlevel is the normal operating state of the system; typically, other runlevels represent single-user mode (used for repairing a faulty system), system shutdown, and various other states. Switching from one runlevel to another causes a per-runlevel set of scripts to be run, which typically mount filesystems, start or stop daemons, start or stop the X Window System, shutdown the machine, etc.
The runlevels in System V describe certain states of a machine, characterized by the processes and daemons running in each of them. In general, there are seven runlevels, out of which three runlevels are considered "standard", as they are essential to the operation of a system:
Aside from these standard ones, Unix and Unix-like systems treat runlevels somewhat differently. The common denominator, the /etc/inittab
file, defines what each configured runlevel does in a given system.
Operating system | Default runlevel |
---|---|
AIX | 2 |
antiX | 5 |
Gentoo Linux | 3 [10] |
HP-UX | 3 (console/server/multiuser) or 4 (graphical) |
Linux From Scratch | 3 |
Slackware Linux | 3 |
Solaris / illumos | 3 [11] |
UNIX System V Releases 3.x, 4.x | 2 |
UnixWare 7.x | 3 |
On Linux distributions defaulting to runlevel 5 in the table on the right, runlevel 5 invokes a multiuser graphical environment running the X Window System, usually with a display manager like GDM or KDM. However, the Solaris and illumos operating systems typically reserve runlevel 5 to shut down and automatically power off the machine.
On most systems, all users can check the current runlevel with either the runlevel
or who -r
command. [12] The root user typically changes the current runlevel by running the telinit
or init
commands. The /etc/inittab
file sets the default runlevel with the :initdefault:
entry.
On Unix systems, changing the runlevel is achieved by starting only the missing services (as each level defines only those that are started / stopped).[ citation needed ] For example, changing a system from runlevel 3 to 4 might only start the local X server. Going back to runlevel 3, it would be stopped again.
Traditionally, one of the major drawbacks of init is that it starts tasks serially, waiting for each to finish loading before moving on to the next. When startup processes end up Input/output (I/O) blocked, this can result in long delays during boot. Speeding up I/O, e.g. by using SSDs, may shorten the delays but it does not address the root cause.
Various efforts have been made to replace the traditional init daemons to address this and other design problems, including:
As of February 2019 [update] , systemd has been adopted by most major Linux distributions. [24]
The Filesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout of Unix-like systems. It has been made popular by its use in Linux distributions, but it is used by other Unix-like systems as well. It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.
Portage is a package management system originally created for and used by Gentoo Linux and also by ChromeOS, Calculate, and Funtoo Linux among others. Portage is based on the concept of ports collections. Gentoo is sometimes referred to as a meta-distribution due to the extreme flexibility of Portage, which makes it operating-system-independent. The Gentoo/Alt project was concerned with using Portage to manage other operating systems, such as BSDs, macOS and Solaris. The most notable of these implementations is the Gentoo/FreeBSD project.
In multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.
chroot
is an operation on Unix and Unix-like operating systems that changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree. The term "chroot" may refer to the chroot(2) system call or the chroot(8) wrapper program. The modified environment is called a chroot jail.
ifconfig is a system administration utility in Unix-like operating systems for network interface configuration.
A runlevel is a mode of operation in the computer operating systems that implements Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six. S is sometimes used as a synonym for one of the levels. Only one runlevel is executed on startup; run levels are not executed one after another.
udev is a device manager for the Linux kernel. As the successor of devfsd and hotplug, udev primarily manages device nodes in the /dev directory. At the same time, udev also handles all user space events raised when hardware devices are added into the system or removed from it, including firmware loading as required by certain devices.
Unix-like operating systems identify a user by a value called a user identifier, often abbreviated to user ID or UID. The UID, along with the group identifier (GID) and other access control criteria, is used to determine which system resources a user can access. The password file maps textual user names to UIDs. UIDs are stored in the inodes of the Unix file system, running processes, tar archives, and the now-obsolete Network Information Service. In POSIX-compliant environments, the shell command id
gives the current user's UID, as well as more information such as the user name, primary user group and group identifier (GID).
launchd is an init and operating system service management daemon created by Apple Inc. as part of macOS to replace its BSD-style init and SystemStarter. There have been efforts to port launchd to FreeBSD and derived systems.
Service Management Facility (SMF) is a feature of the Solaris operating system as of version 10 and OpenSolaris-descendant illumos with its illumos distributions, that creates a supported, unified model for services and service management on each Solaris or illumos system and replaces init.d scripts. SMF introduces:
Upstart is a discontinued event-based replacement for the traditional init daemon—the method by which several Unix-like computer operating systems perform tasks when the computer is started. It was written by Scott James Remnant, a former employee of Canonical Ltd. In 2014, Upstart was placed in maintenance mode, and other init daemons, such as systemd, were recommended in place of Upstart. Ubuntu moved away from Upstart with the release of version 15.04 in favor of migrating to systemd. As of June 2024, there have been no updates released for Upstart since September 2014.
Single-user mode is a mode in which a multiuser computer operating system boots into a single superuser. It is mainly used for maintenance of multi-user environments such as network servers. Some tasks may require exclusive access to shared resources, for example running fsck
on a network share. This mode can also be used for security purposes – network services are not run, eliminating the possibility of outside interference. On some systems a lost superuser password can be changed by switching to single-user mode, but not asking for the password in such circumstances is viewed as a security vulnerability.
The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, bootloader execution, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process.
In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.
ptrace is a system call found in Unix and several Unix-like operating systems. By using ptrace one process can control another, enabling the controller to inspect and manipulate the internal state of its target. ptrace is used by debuggers and other code-analysis tools, mostly as aids to software development.
Calculate Linux is a Linux distribution optimized for fast deployment in an organization environment. It is based on the Gentoo Linux project and includes many preconfigured functions.
runit is an init and service management scheme for Unix-like operating systems that initializes, supervises, and ends processes throughout the operating system. Runit is a reimplementation of the daemontools process supervision toolkit that runs on many Linux-based operating systems, as well as BSD, and Solaris operating systems. Runit features parallelization of the start up of system services, which can speed up the boot time of the operating system.
systemd is a software suite that provides an array of system components for Linux operating systems. The main aim is to unify service configuration and behavior across Linux distributions. Its primary component is a "system and service manager" — an init system used to bootstrap user space and manage user processes. It also provides replacements for various daemons and utilities, including device management, login management, network connection management, and event logging. The name systemd adheres to the Unix convention of naming daemons by appending the letter d. It also plays on the term "System D", which refers to a person's ability to adapt quickly and improvise to solve problems.
OpenRC is a dependency-based init system for Unix-like computer operating systems. It was created by Roy Marples, a NetBSD developer who was also active in the Gentoo project. It became more broadly adopted as an init system outside of Gentoo following the decision by some Linux distributions not to adopt systemd.
Container Linux is a discontinued open-source lightweight operating system based on the Linux kernel and designed for providing infrastructure for clustered deployments. One of its focuses was scalability. As an operating system, Container Linux provided only the minimal functionality required for deploying applications inside software containers, together with built-in mechanisms for service discovery and configuration sharing.
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