「init」

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内容:Linux下init、telinit介绍。

init用于进程控制初始化,而telinit是指向init的软链接。
init是所有进程的父母(翻译成父母进程挺奇怪的),它的主要作用是从文件/etc/inittab中存储的脚本创建进程。inittab文件中的描述条目使init在用户可登录的每一行上产生gettys,它还可以控制任何特定系统所需的自治进程。

相关引用内容:

/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
/sbin/telinit [ -t SECONDS ] [ 0123456sSQqabcUu ]
/sbin/telinit [ -e VAR[=VAL] ]

关于RUNLEVEL相关内容,查看:《runlevel》
关于inittab的相关内容,查看:《inittab》

NAME

init, telinit – process control initialization

SYNOPSIS

/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]

/sbin/telinit [ -t SECONDS ] [ 0123456sSQqabcUu ]

/sbin/telinit [ -e VAR[=VAL] ]

DESCRIPTION
Init

Init is the parent of all processes. Its primary role is to create processes from a script stored in

the file /etc/inittab (see inittab(5)). This file usually has entries which cause init to spawn gettys

on each line that users can log in. It also controls autonomous processes required by any particular

system.

RUNLEVELS

A runlevel is a software configuration of the system which allows only a selected group of processes to

exist. The processes spawned by init for each of these runlevels are defined in the /etc/inittab file.

Init can be in one of eight runlevels: 0–6 and S (a.k.a. s). The runlevel is changed by having a priv‐

ileged user run telinit, which sends appropriate signals to init, telling it which runlevel to change

to.

Runlevels S, 0, 1, and 6 are reserved. Runlevel S is used to initialize the system on boot. When

starting runlevel S (on boot) or runlevel 1 (switching from a multi-user runlevel) the system is enter‐

ing “single-user mode”, after which the current runlevel is S. Runlevel 0 is used to halt the sys‐

tem; runlevel 6 is used to reboot the system.

After booting through S the system automatically enters one of the multi-user runlevels 2 through 5,

unless there was some problem that needs to be fixed by the administrator in single-user mode. Nor‐

mally after entering single-user mode the administrator performs maintenance and then reboots the sys‐

tem.

For more information, see the manpages for shutdown(8) and inittab(5).

Runlevels 7-9 are also valid, though not really documented. This is because “traditional” Unix variants

don’t use them.

Runlevels S and s are the same. Internally they are aliases for the same runlevel.

BOOTING

After init is invoked as the last step of the kernel boot sequence, it looks for the file /etc/inittab

to see if there is an entry of the type initdefault (see inittab(5)). The initdefault entry determines

the initial runlevel of the system. If there is no such entry (or no /etc/inittab at all), a runlevel

must be entered at the system console.

Runlevel S or s initialize the system and do not require an /etc/inittab file.

In single user mode, /sbin/sulogin is invoked on /dev/console.

When entering single user mode, init initializes the consoles stty settings to sane values. Clocal mode

is set. Hardware speed and handshaking are not changed.

When entering a multi-user mode for the first time, init performs the boot and bootwait entries to

allow file systems to be mounted before users can log in. Then all entries matching the runlevel are

processed.

When starting a new process, init first checks whether the file /etc/initscript exists. If it does, it

uses this script to start the process.

Each time a child terminates, init records the fact and the reason it died in /var/run/utmp and

/var/log/wtmp, provided that these files exist.

CHANGING RUNLEVELS

After it has spawned all of the processes specified, init waits for one of its descendant processes to

die, a powerfail signal, or until it is signaled by telinit to change the system’s runlevel. When one

of the above three conditions occurs, it re-examines the /etc/inittab file. New entries can be added

to this file at any time. However, init still waits for one of the above three conditions to occur.

To provide for an instantaneous response, the telinit Q or q command can wake up init to re-examine the

/etc/inittab file.

If init is not in single user mode and receives a powerfail signal (SIGPWR), it reads the file

/etc/powerstatus. It then starts a command based on the contents of this file:

F(AIL) Power is failing, UPS is providing the power. Execute the powerwait and powerfail entries.

O(K) The power has been restored, execute the powerokwait entries.

L(OW) The power is failing and the UPS has a low battery. Execute the powerfailnow entries.

If /etc/powerstatus doesn’t exist or contains anything else then the letters F, O or L, init will

behave as if it has read the letter F.

Usage of SIGPWR and /etc/powerstatus is discouraged. Someone wanting to interact with init should use

the /run/initctl control channel – see the source code of the sysvinit package for more documentation

about this.

When init is requested to change the runlevel, it sends the warning signal SIGTERM to all processes

that are undefined in the new runlevel. It then waits 5 seconds before forcibly terminating these pro‐

cesses via the SIGKILL signal. Note that init assumes that all these processes (and their descendants)

remain in the same process group which init originally created for them. If any process changes its

process group affiliation it will not receive these signals. Such processes need to be terminated sep‐

arately.

TELINIT

/sbin/telinit is linked to /sbin/init. It takes a one-character argument and signals init to perform

the appropriate action. The following arguments serve as directives to telinit:

0,1,2,3,4,5 or 6
tell init to switch to the specified run level.

a,b,c tell init to process only those /etc/inittab file entries having runlevel a,b or c.

Q or q tell init to re-examine the /etc/inittab file.

S or s tell init to switch to single user mode.

U or u tell init to re-execute itself (preserving the state). No re-examining of /etc/inittab file hap‐
pens. Run level should be one of Ss0123456 otherwise request would be silently ignored.

telinit can tell init how long it should wait between sending processes the SIGTERM and SIGKILL sig‐

nals. The default is 5 seconds, but this can be changed with the -t option.

telinit -e tells init to change the environment for processes it spawns. The argument of -e is either

of the form VAR=VAL which sets variable VAR to value VAL, or of the form VAR (without an equality sign)

which unsets variable VAR.

telinit can be invoked only by users with appropriate privileges.

The init binary checks if it is init or telinit by looking at its process id; the real init’s process

id is always 1. From this it follows that instead of calling telinit one can also just use init

instead as a shortcut.

ENVIRONMENT

Init sets the following environment variables for all its children:

PATH /bin:/usr/bin:/sbin:/usr/sbin

INIT_VERSION
As the name says. Useful to determine if a script runs directly from init.

RUNLEVEL
The current system runlevel.

PREVLEVEL
The previous runlevel (useful after a runlevel switch).

CONSOLE
The system console. This is really inherited from the kernel; however if it is not set init will

set it to /dev/console by default.

BOOTFLAGS

It is possible to pass a number of flags to init from the boot monitor (eg. LILO). Init accepts the

following flags:

-s, S, single
Single user mode boot. In this mode /etc/inittab is examined and the bootup rc scripts are usually

run before the single user mode shell is started.

1-5 Runlevel to boot into.

-b, emergency
Boot directly into a single user shell without running any other startup scripts.

-a, auto
The LILO boot loader adds the word “auto” to the command line if it booted the kernel with the

default command line (without user intervention). If this is found init sets the “AUTOBOOT” envi‐

ronment variable to “yes”. Note that you cannot use this for any security measures – of course the

user could specify “auto” or -a on the command line manually.

-z xxx
The argument to -z is ignored. You can use this to expand the command line a bit, so that it takes

some more space on the stack. Init can then manipulate the command line so that ps(1) shows the

current runlevel.

INTERFACE

Init listens on a fifo in /run, /run/initctl, for messages. Telinit uses this to communicate with

init. The interface is not very well documented or finished. Those interested should study the ini‐

treq.h file in the src/ subdirectory of the init source code tar archive.

SIGNALS

Init reacts to several signals:

SIGHUP
Has the same effect as telinit q.

SIGUSR1
On receipt of this signals, init closes and re-opens its control fifo, /run/initctl.

SIGINT
Normally the kernel sends this signal to init when CTRL-ALT-DEL is pressed. It activates the ctr‐

laltdel action.

SIGWINCH
The kernel sends this signal when the KeyboardSignal key is hit. It activates the kbrequest

action.

CONFORMING TO

Init is compatible with the System V init. It works closely together with the scripts in the directo‐

ries /etc/init.d and /etc/rc{runlevel}.d. If your system uses this convention, there should be a

README file in the directory /etc/init.d explaining how these scripts work.

FILES

/etc/inittab

/etc/initscript

/dev/console

/var/run/utmp

/var/log/wtmp

/run/initctl

WARNINGS

Init assumes that processes and descendants of processes remain in the same process group which was

originally created for them. If the processes change their group, init can’t kill them and you may end

up with two processes reading from one terminal line.

On a Debian system, entering runlevel 1 causes all processes to be killed except for kernel threads and

the script that does the killing and other processes in its session. As a consequence of this, it

isn’t safe to return from runlevel 1 to a multi-user runlevel: daemons that were started in runlevel S

and are needed for normal operation are no longer running. The system should be rebooted.

DIAGNOSTICS

If init finds that it is continuously respawning an entry more than 10 times in 2 minutes, it will

assume that there is an error in the command string, generate an error message on the system console,

and refuse to respawn this entry until either 5 minutes has elapsed or it receives a signal. This pre‐

vents it from eating up system resources when someone makes a typographical error in the /etc/inittab

file or the program for the entry is removed.

SEE ALSO

getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), inittab(5), initscript(5), utmp(5)

参考文献

  • man 8 init

更新日志

05/06/2017 首次创建