Linux startup process
Encyclopedia
The Linux startup process is the process of Linux
-operating system
initialization. It is in many ways similar to the BSD and other Unix
style boot processes, from which it derives.
, to boot loader, to kernel. The kernel then starts the scheduler (to allow multi-tasking) and runs the first userland (i.e. outside kernel space) program Init
(which sets up the user environment and allows user interaction and login), at which point the kernel goes idle unless called externally.
In detail:
On shutdown, Init is called to close down all user space functionality in a controlled manner, again via scripted directions, following which Init terminates and the Kernel executes its own shutdown.
s are needed to load the main user space file system and load them from a temporary filesystem.
From that point, the boot process continues as follows:
The first stage boot loader (in the MBR
or the volume boot record
) loads the remainder of the boot loader, which typically gives a prompt asking which operating system
(or type of session) the user wishes to initialize. LILO and GRUB differ in some ways:
GRUB supports both direct and chain-loading boot methods, LBA
, ext2, ext3, ext4
and "a true command-based, pre-OS environment on x86 machines". It contains three interfaces: a selection menu, a configurot loader (second stage). If the second stage is on a large drive, sometimes an intermediate 1.5 stage is loaded, which contains extra code to allow cylinders above 1024, or LBA
type drives, to be read. The 1.5 boot loader is stored (if needed) in the MBR or the boot partition. GRUB does this by storing disk location information (cylinder/head/sector addresses, sector numbers, etc.) inside itself, starting within the boot sector itself. This is a lot like the LILO map installer (see elsewhereAT, ext2, etc.), so GRUB can load any arbitrary file at boot time, including its configuration file, a kernel, an initrd, or in this article). The key difference is the code which is loaded is able to interpret the structure of several filesysation editor, and a command line console.
etc.) containing a program capable of fixing this. Additionally, it does not understand filesystems. Instead, locations of image files are stored within the boot sector and map file directly and the BIOS is used to access them directly.
or Windows 9x
, where the Linux kernel completely replaces the running copy of this operating system. This can be useful in the case of hardware which needs to be switched on via software and for which such configuration programs are only available for DOS, whereas not for Linux, those being proprietary to the manufacturer and kept an industry secret. This tedious booting method is less necessary nowadays, as Linux has drivers for a multitude of hardware devices, but it has seen some use in mobile devices.
Another case is when the Linux is located on a storage device which is not available to the BIOS for booting: DOS or Windows can load the appropriate drivers to make up for the BIOS limitation, and boot Linux from there.
in Linux handles all operating system processes, such as memory management
, task scheduling
, I/O
, interprocess communication, and overall system control. This is loaded in two stages - in the first stage the kernel (as a compressed image file) is loaded into memory and decompressed, and a few fundamental functions such as basic memory management are set up. Control is then switched one final time to the main kernel start process. Once the kernel is fully operational – and as part of its startup, upon being loaded and executing – the kernel looks for an init process to run, which (separately) sets up a user space and the processes needed for a user environment and ultimate login. The kernel itself is then allowed to go idle, subject to calls from other processes.
formats with zlib
. A routine at the head of it does a minimal amount of hardware setup, decompresses the image fully into high memory
, and takes note of any RAM disk
if configured. It then executes kernel startup via
The startup function for the kernel (also called the swapper or process 0) establishes memory management
(paging tables and memory paging), detects the type of CPU
and any additional functionality such as floating point
capabilities, and then switches to non-architecture specific Linux kernel functionality via a call to
start_kernel executes a wide range of initialization functions. It sets up interrupt handling (IRQs), further configures memory, starts the Init process (the first user-space process), and then starts the idle task via
("initrd") that was loaded previously as the temporary root filing system during the boot phase. This allows driver modules to be loaded without reliance upon other physical devices and drivers, and keeps the kernel smaller. The root file system is later switched via a call to
Thus, the kernel initializes devices, mounts the root filesystem specified by the boot loader as read only, and runs Init (
= 1). A message is printed by the kernel upon mounting the file system, and by Init upon starting the Init process. It may also optionally run Initrd
to allow setup and device related matters (RAM disk
or similar) to be handled before the root file system is mounted.
According to Red Hat
, the detailed kernel process at this stage is therefore summarized as follows:
At this point, with interrupts enabled, the scheduler can take control of the overall management of the system, to provide pre-emptive multi-tasking, and the init process is left to continue booting the user environment in user space.
. This includes checking and mounting file system
s, starting up necessary user services, and ultimately switching to a user-environment when system startup is completed. It is similar to the Unix
and BSD init processes, from which it derived, but in some cases has diverged or became customized. In a standard Linux system, Init is executed with a parameter, known as a runlevel
, that takes a value from 1 to 6, and that determines which subsystems are to be made operational. Each runlevel has its own scripts which codify the various processes involved in setting up or leaving the given runlevel, and it is these scripts which are referenced as necessary in the boot process. Init scripts are typically held in directories with names such as
During system boot, it checks whether a default runlevel is specified in /etc/inittab, and requests the runlevel to enter via the system console
if not. It then proceeds to run all the relevant boot scripts for the given runlevel, including loading modules
, checking the integrity of the root file system (which was mounted read-only) and then remounting it for full read-write access, and sets up the network
.
After it has spawned all of the processes specified, init goes dormant, and waits for one of three events to happen:- processes it started to end or die, a power failure signal, or a request via
This applies to SysV
-style init. Other init binaries, such as systemd
or Upstart
, may behave differently.
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
-operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
initialization. It is in many ways similar to the BSD and other Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
style boot processes, from which it derives.
Overview of typical process
In Linux, the flow of control during a boot is from BIOSBIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
, to boot loader, to kernel. The kernel then starts the scheduler (to allow multi-tasking) and runs the first userland (i.e. outside kernel space) program Init
Init
init is a program for Unix-based computer operating systems that spawns all other processes. It runs as a daemon and typically has PID 1. The boot loader starts the kernel and the kernel starts init...
(which sets up the user environment and allows user interaction and login), at which point the kernel goes idle unless called externally.
In detail:
- The BIOSBIOSIn IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
performs hardwareHardwareHardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....
-platform specific startup tasks - Once the hardware is recognized and started correctly, the BIOS loads and executes the partition boot code from the designated boot device, which contains phase 1 of a Linux boot loader. Phase 1 loads phase 2 (the bulk of the boot loader code). Some loaders may use an intermediate phase (known as phase 1.5) to achieve this since modern large disks may not be fully readable without further code.
- The boot loader often presents the user with a menu of possible boot options. It then loads the operating system, which decompresses into memory, and sets up system functions such as essential hardware and memory paging, before calling
start_kernel. -
start_kernelthen performs the majority of system setup (interrupts, the rest of memory management, device initialization, drivers, etc.) before spawning separately, the idle process and scheduler, and the Init process (which is executed in user spaceUser spaceA conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
). - The scheduler effectively takes control of the system management, as the kernel goes dormant (idle).
- The Init process executes scripts as needed that set up all non-operating system services and structures in order to allow a user environment to be created, and then presents the user with a login screen.
On shutdown, Init is called to close down all user space functionality in a controlled manner, again via scripted directions, following which Init terminates and the Kernel executes its own shutdown.
Early user space
Early user space has been available since version 2.5.46 of the Linux kernel, with the intent to replace as many functions as possible that previously the kernel would have performed during the start-up process. Typical uses of early user space are to detect what device driverDevice driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
s are needed to load the main user space file system and load them from a temporary filesystem.
Boot loader phase
The boot loader phase varies by platform. Since the earlier phases are not specific to the OS, the boot process is considered to start:- For x86X86 architectureThe term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...
or x86-64X86-64x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
: when the partition boot sector code is executed in real modeReal modeReal mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
and loads the first stage boot loader (typically a part of LILOLILO (boot loader)LILO is a generic boot loader for Linux.-Overview:LILO does not depend on a specific file system, and can boot an operating system from floppy disks and hard disks. One of up to sixteen different images can be selected at boot time. Various parameters, such as the root device, can be set...
or GRUB).
From that point, the boot process continues as follows:
The first stage boot loader (in the MBR
Master boot record
A master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...
or the volume boot record
Volume Boot Record
A volume boot record is a type of boot sector introduced by the IBM Personal Computer...
) loads the remainder of the boot loader, which typically gives a prompt asking which operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
(or type of session) the user wishes to initialize. LILO and GRUB differ in some ways:
- Under LILO, the map installer,
lilo, reads the configuration fileConfiguration fileIn computing, configuration files, or config files configure the initial settings for some computer programs. They are used for user applications, server processes and operating system settings. The files are often written in ASCII and line-oriented, with lines terminated by a newline or carriage...
/etc/lilo.confto identify the available bootable systems (it is executed from a running Linux system). The configuration file can include data such as boot partitionDisk partitioningDisk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...
and kernel pathname for each, as well as customized options if needed. To be precise,/etc/lilo.confis prior-parsed and used to create fixed-offset information saved in the boot sector and the map file, which will be used at the next boot. This information is discovered by asking the Linux kernel, at map installer time, where (e.g. on which disk sectors) the object of interest (such as an initrdInitrdIn computing, initrd is a scheme for loading a temporary file system into memory in the boot process of the Linux kernel. initrd and initramfs refer to slightly different methods of achieving this...
, a kernel image file, or the like) is stored. At boot time, the default or selected operating system is loaded into RAM, a minimal initial file system is possibly set up in RAM from an image file ("initrdInitrdIn computing, initrd is a scheme for loading a temporary file system into memory in the boot process of the Linux kernel. initrd and initramfs refer to slightly different methods of achieving this...
"), and along with the appropriate parameters, control is passed to the newly loaded kernel. LILO does not "understand" file systems, so it uses raw disk offsets and the BIOS to load any needed code or data based on data in the boot sector and map file. It cannot "find"/etc/lilo.confat boot time because it does not understand file systems; instead it searches fixed locations on the disk memorized the last time the LILO map installer (lilo) was run to generate new offsets in the boot sector and the map file image. Boot time LILO logic loads the menu code, and then, depending on thelilo.confdirectives used to make the map file, along with any user interaction, loads either the boot sector for another system such as Microsoft WindowsMicrosoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, or the kernel image for Linux. - GRUB by contrast does have understanding of the common ext2Ext2The ext2 or second extended filesystem is a file system for the Linux kernel. It was initially designed by Rémy Card as a replacement for the extended file system ....
, ext3Ext3The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions, including Debian...
and ext4Ext4The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as the successor to ext3.It was born as a series of backward compatible extensions to ext3, many of them originally developed by Cluster File Systems for the Lustre file system between 2003 and 2006, meant to...
file systems. Because GRUB stores its data in a configuration file rather than the MBR and contains a command-line interfaceCommand-line interfaceA command-line interface is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks...
, it is often easier to rectify or modify GRUB if misconfigured or corrupt.
GRUB
- Source: Red Hat GRUB description.
- The first stage loader is read by the BIOSBIOSIn IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
from the MBRMaster boot recordA master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...
(master boot record) or the partition boot sector. - The first stage loads the rest of the botems (F a chainloaded OS boot sector(s). More recent versions can also interpret Logical Volume Manager volumes.
- The second stage boot loader executes, and displays the GRUB startup menu which allows the user to choose an operating system and examine and edit startup parameters.
- After an operating system is chosen, it is loaded and executed.
- The first stage loader is read by the BIOS
GRUB supports both direct and chain-loading boot methods, LBA
Logical block addressing
Logical block addressing is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks....
, ext2, ext3, ext4
Ext4
The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as the successor to ext3.It was born as a series of backward compatible extensions to ext3, many of them originally developed by Cluster File Systems for the Lustre file system between 2003 and 2006, meant to...
and "a true command-based, pre-OS environment on x86 machines". It contains three interfaces: a selection menu, a configurot loader (second stage). If the second stage is on a large drive, sometimes an intermediate 1.5 stage is loaded, which contains extra code to allow cylinders above 1024, or LBA
Logical block addressing
Logical block addressing is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks....
type drives, to be read. The 1.5 boot loader is stored (if needed) in the MBR or the boot partition. GRUB does this by storing disk location information (cylinder/head/sector addresses, sector numbers, etc.) inside itself, starting within the boot sector itself. This is a lot like the LILO map installer (see elsewhereAT, ext2, etc.), so GRUB can load any arbitrary file at boot time, including its configuration file, a kernel, an initrd, or in this article). The key difference is the code which is loaded is able to interpret the structure of several filesysation editor, and a command line console.
LILO
LILO, the older of the two boot loaders, is almost identical to GRUB in process, except that its command line interface allows only selection of options previously recorded in the boot sector and map file. Thus all changes must be made to its configuration and written to the boot sector and map file, and then the system restarted. An error in configuration can therefore leave a disk unable to be booted without use of a separate boot device (floppy diskFloppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
etc.) containing a program capable of fixing this. Additionally, it does not understand filesystems. Instead, locations of image files are stored within the boot sector and map file directly and the BIOS is used to access them directly.
Loadlin
Yet another way to boot Linux is from DOSDOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
or Windows 9x
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...
, where the Linux kernel completely replaces the running copy of this operating system. This can be useful in the case of hardware which needs to be switched on via software and for which such configuration programs are only available for DOS, whereas not for Linux, those being proprietary to the manufacturer and kept an industry secret. This tedious booting method is less necessary nowadays, as Linux has drivers for a multitude of hardware devices, but it has seen some use in mobile devices.
Another case is when the Linux is located on a storage device which is not available to the BIOS for booting: DOS or Windows can load the appropriate drivers to make up for the BIOS limitation, and boot Linux from there.
Kernel phase
The kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
in Linux handles all operating system processes, such as memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...
, task scheduling
Scheduling (computing)
In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
, I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
, interprocess communication, and overall system control. This is loaded in two stages - in the first stage the kernel (as a compressed image file) is loaded into memory and decompressed, and a few fundamental functions such as basic memory management are set up. Control is then switched one final time to the main kernel start process. Once the kernel is fully operational – and as part of its startup, upon being loaded and executing – the kernel looks for an init process to run, which (separately) sets up a user space and the processes needed for a user environment and ultimate login. The kernel itself is then allowed to go idle, subject to calls from other processes.
Kernel loading stage
The kernel as loaded is typically an image file, compressed into either zImage or bzImageVmlinux
On Linux systems, vmlinux is a statically linked executable file that contains the Linux kernel in one of the object file formats supported by Linux, including ELF, COFF and a.out...
formats with zlib
Zlib
zlib is a software library used for data compression. zlib was written by Jean-Loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. Zlib is also a crucial component of many software platforms including Linux, Mac OS X,...
. A routine at the head of it does a minimal amount of hardware setup, decompresses the image fully into high memory
High memory
High Memory is the part of physical memory in a computer which is not directly mapped by the page tables of its operating system kernel. The phrase is also sometimes used as shorthand for the High Memory Area, which is a different concept entirely....
, and takes note of any RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
if configured. It then executes kernel startup via
./arch/i386/boot/head and the startup_32 (for x86 based processors) process.Kernel startup stage
The startup function for the kernel (also called the swapper or process 0) establishes memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...
(paging tables and memory paging), detects the type of CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
and any additional functionality such as floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
capabilities, and then switches to non-architecture specific Linux kernel functionality via a call to
start_kernel .start_kernel executes a wide range of initialization functions. It sets up interrupt handling (IRQs), further configures memory, starts the Init process (the first user-space process), and then starts the idle task via
cpu_idle . Notably, the kernel startup process also mounts the initial RAM diskInitrd
In computing, initrd is a scheme for loading a temporary file system into memory in the boot process of the Linux kernel. initrd and initramfs refer to slightly different methods of achieving this...
("initrd") that was loaded previously as the temporary root filing system during the boot phase. This allows driver modules to be loaded without reliance upon other physical devices and drivers, and keeps the kernel smaller. The root file system is later switched via a call to
pivot_root which unmounts the temporary root file system and replaces it with the use of the real one, once the latter is accessible. The memory used by the temporary root file system is then reclaimed.Thus, the kernel initializes devices, mounts the root filesystem specified by the boot loader as read only, and runs Init (
/sbin/init) which is designated as the first process run by the system (PIDProcess identifier
In computing, the process identifier is a number used by most operating system kernels to uniquely identify a process...
= 1). A message is printed by the kernel upon mounting the file system, and by Init upon starting the Init process. It may also optionally run Initrd
Initrd
In computing, initrd is a scheme for loading a temporary file system into memory in the boot process of the Linux kernel. initrd and initramfs refer to slightly different methods of achieving this...
to allow setup and device related matters (RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
or similar) to be handled before the root file system is mounted.
According to Red Hat
Red Hat
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
, the detailed kernel process at this stage is therefore summarized as follows:
- "When the kernel is loaded, it immediately initializes and configures the computer's memory and configures the various hardware attached to the system, including all processors, I/O subsystems, and storage devices. It then looks for the compressed initrd image in a predetermined location in memory, decompresses it, mounts it, and loads all necessary drivers. Next, it initializes virtual devices related to the file system, such as LVMLogical Volume Manager (Linux)LVM is a logical volume manager for the Linux kernel; it manages disk drives and similar mass-storage devices, in particular large ones. The term "volume" refers to a disk drive or partition thereof...
or software RAIDRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
before unmounting the initrd disk imageDisk imageA disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drive, floppy disk, CD/DVD/BD, or USB flash drive, although an image of an optical disc may be referred to as an optical disc image...
and freeing up all the memory the disk image once occupied. The kernel then creates a root device, mounts the root partition read-only, and frees any unused memory. At this point, the kernel is loaded into memory and operational. However, since there are no user applications that allow meaningful input to the system, not much can be done with it."
At this point, with interrupts enabled, the scheduler can take control of the overall management of the system, to provide pre-emptive multi-tasking, and the init process is left to continue booting the user environment in user space.
Init process (SysV init style only)
Init's job is "to get everything running the way it should be" once the kernel is fully running. Essentially it establishes and operates the entire user spaceUser space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
. This includes checking and mounting file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
s, starting up necessary user services, and ultimately switching to a user-environment when system startup is completed. It is similar to the Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
and BSD init processes, from which it derived, but in some cases has diverged or became customized. In a standard Linux system, Init is executed with a parameter, known as a runlevel
Runlevel
The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine, may be used. S is sometimes used as a synonym for one...
, that takes a value from 1 to 6, and that determines which subsystems are to be made operational. Each runlevel has its own scripts which codify the various processes involved in setting up or leaving the given runlevel, and it is these scripts which are referenced as necessary in the boot process. Init scripts are typically held in directories with names such as
"/etc/rc...". The top level configuration file for init is at /etc/inittab.During system boot, it checks whether a default runlevel is specified in /etc/inittab, and requests the runlevel to enter via the system console
System console
The system console, root console or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger...
if not. It then proceeds to run all the relevant boot scripts for the given runlevel, including loading modules
Modular programming
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
, checking the integrity of the root file system (which was mounted read-only) and then remounting it for full read-write access, and sets up the network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
.
After it has spawned all of the processes specified, init goes dormant, and waits for one of three events to happen:- processes it started to end or die, a power failure signal, or a request via
/sbin/telinit to further change the runlevel.This applies to SysV
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
-style init. Other init binaries, such as systemd
Systemd
systemd is a replacement for the System V init daemon for Linux. It is intended to provide a better framework for expressing services' dependencies, allow more work to be done in parallel at system startup, and to reduce shell overhead....
or Upstart
Upstart
Upstart is an 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....
, may behave differently.