NAME
init, telinit - process control initialization
控制初始化的进程。
SYNOPSIS
/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
/sbin/telinit [ -t sec ] [ 0123456sSQqabcUu ]
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.
init是所有进程的父进程。她主要的角色就是从/etc/inittab这个脚本中创建许多的进程。inittab配置文档中通常具有一些条目，这些条目能初始化每行从spawn gettys登录的用户。她也能够控制特定系统要求的自治进程。
RUNLEVELS
A runlevel is a software configuration of the system which
allows only a selected group of processes to exist. The pro-
cesses spawned by init for each of these runlevels are defined
in the /etc/inittab file. Init can be in one of eight run-
levels: 0-6 and S or s. The runlevel is changed by having a
privileged user run telinit, which sends appropriate signals to
init, telling it which runlevel to change to.
运行级别是系统定义的软件，只允许一个被选定的进程组存在。通过初始化每一个在inittab配置文件定义了的运行级别，运行级别产生大量进程。init被八个运行级别定义，这八个运行级别是0-6 和S ，s。一个私有用户通过telinit改变运行级别。telinit 发送适当信号给init，告诉init哪一个运行级别被改变。
Runlevels 0, 1, and 6 are reserved. Runlevel 0 is used to halt
the system, runlevel 6 is used to reboot the system, and run-
level 1 is used to get the system down into single user mode.
Runlevel S is not really meant to be used directly, but more for
the scripts that are executed when entering runlevel 1. For more
information on this, see the manpages for shutdown(8) and init-
tab(5).
运行级别0，1和6是被保留的。运行级别0被用来终止系统。运行级别6被用来重新启动系统。运行级别1用来使用单人模式登录系统。运行级别S表示不是直接被使用。使用运行级别1的时候，脚本里的都不被执行。想知道更多的信息请参考shutdown（8）和inittab（5）
Runlevels 7-9 are also valid, though not really documented. This
is because "traditional" Unix variants don’t use them. In case
you‘re curious, runlevels S and s are in fact the same. Internally they are aliases for the same runlevel.
运行级别7-9也是有效的，尽管没有真实的文档。这是因为传统的unix系统变种不会使用7-9，假使你好奇，运行级别S和s实际上是一样的。S和s是同一个运行级别的别名。
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 initde-
fault 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.
作为系统内核启动顺序的最后一步， 在init被请求之后，init寻找/etc/inittab，看inittab里是否有默认运行级别类型的条目，默认运行级别的条目决定了系统初始化运行级别。如果没有这样的条目或者根本没有inittab，则在系统控制台内必须输入运行级别。
Runlevel S or s bring the system to single user mode and do not
require an /etc/inittab file. In single user mode, a root shell
is opened on /dev/console.
运行级别S或者s将系统带到单人用户模式，而不要求inittab配置文件，在单人模式下，一个root用户shell被打开。
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.
当进入单人用户模式init初始化控制台设定一个健全的登录环境，设置本地模式，不改变硬件速度和握手。
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.
当第一次进入多用户模式，init执行boot 和bootwait条目，允许在用户登录前挂载文件系统。那么所有的条目合并成运行级别被处理。
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.
当开始一个新进程，init先检查/etc/initscript文件是否存在。如果存在，init使用这个脚本文件启动这个进程。
Each time a child terminates, init records the fact and the rea-
son it died in /var/run/utmp and /var/log/wtmp, provided that
these files exist.
在每一子终端，假如utmp和wtmp存在，init在 /var/run/utmp 和 /var/log/wtmp记录终端的死亡和死亡的原因，
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鈥檚 run-
level. 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 instanta-
neous 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 sig-
nal (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. Exe-
cute the powerfailnow entries.
If /etc/powerstatus doesn鈥檛 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 /dev/initctl con-
trol 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 ter-
minating these processes 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 ter-
minated separately.
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 happens. Run level
should be one of Ss12345, otherwise request would be
silently ignored.
telinit can also tell init how long it should wait between send-
ing processes the SIGTERM and SIGKILL signals. The default is 5
seconds, but this can be changed with the -t sec option.
telinit can be invoked only by users with appropriate privi-
leges.
The init binary checks if it is init or telinit by looking at
its process id; the real init鈥檚 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 chil-
dren:
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 exam-
ined 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" environment 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 manu-
ally.
-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 /dev, /dev/initctl, for messages.
Telinit uses this to communicate with init. The interface is not
very well documented or finished. Those interested should study
the initreq.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, /dev/initctl. Useful for bootscripts when
/dev is remounted.
SIGINT
Normally the kernel sends this signal to init when CTRL-
ALT-DEL is pressed. It activates the ctrlaltdel 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 directories /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
/dev/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鈥檛 kill them and
you may end up with two processes reading from one terminal
line.
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 prevents it
from eating up system resources when someone makes a typographi-
cal error in the /etc/inittab file or the program for the entry
is removed.
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