How UNIX/LINUX boot/initialization works

Folks,

Although, it is a 'man init' from the Solaris prompt but it great read to understand the process of initialization.

System Administration Commands init(1M)

NAME
init, telinit - process control initialization

SYNOPSIS
/sbin/init [0123456abcQqSs]

/etc/telinit [0123456abcQqSs]

DESCRIPTION
init is a general process spawner. Its primary role is to
create processes from information stored in the file
/etc/inittab.

Run Level Defined
At any given time, the system is in one of eight possible
run levels. A run level is a software configuration under
which only a selected group of processes exists. Processes
spawned by init for each of these run levels are defined in
/etc/inittab. init can be in one of eight run levels, 0-6
and S or s (S and s are identical). The run level changes
when a privileged user runs /sbin/init. This sends appropri-
ate signals to the original init spawned by the operating
system at boot time, saying which run level to invoke.

init and System Booting
When the system is booted, init is invoked and the following
occurs. First, it reads /etc/default/init to set environment
variables. This is typically where TZ (time zone) and
locale-related environments such as LANG or LC_CTYPE get
set. (See the FILES section at the end of this page.) init
then looks in /etc/inittab for the initdefault entry (see
inittab(4)). If the initdefault entry:

exists
init usually uses the run level specified in that
entry as the initial run level to enter.

does not exist
/etc/inittab, init asks the user to enter a run level
from the system console.

S or s
init goes to the single-user state. In this
state, the system console device (/dev/console)
is opened for reading and writing and the com-
mand /sbin/su, (see su(1M)), is invoked. Use
either init or telinit to change the run level
of the system. Note that if the shell is ter-
minated (using an end-of-file), init only re-
initializes to the single-user state if
/etc/inittab does not exist.

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System Administration Commands init(1M)

0-6 init enters the corresponding run level. Run
levels 0, 5, and 6 are reserved states for shut-
ting the system down. Run levels 2, 3, and 4 are
available as multi-user operating states.

If this is the first time since power up that init has
entered a run level other than single-user state, init first
scans /etc/inittab for boot and bootwait entries (see init-
tab(4)). These entries are performed before any other pro-
cessing of /etc/inittab takes place, providing that the run
level entered matches that of the entry. In this way any
special initialization of the operating system, such as
mounting file systems, can take place before users are
allowed onto the system. init then scans /etc/inittab and
executes all other entries that are to be processed for that
run level.

To spawn each process in /etc/inittab, init reads each entry
and for each entry that should be respawned, it forks a
child process. After it has spawned all of the processes
specified by /etc/inittab, init waits for one of its descen-
dant processes to die, a powerfail signal, or a signal from
another init or telinit process to change the system's run
level. When one of these conditions occurs, init re-examines
/etc/inittab.

inittab Additions
New entries can be added to /etc/inittab at any time; how-
ever, init still waits for one of the above three conditions
to occur before re-examining /etc/inittab. To get around
this, init Q or init q command wakes init to re-examine
/etc/inittab immediately.

When init comes up at boot time and whenever the system
changes from the single-user state to another run state,
init sets the ioctl(2) states of the console to those modes
saved in the file /etc/ioctl.syscon. init writes this file
whenever the single-user state is entered.

Run Level Changes
When a run level change request is made, init sends the
warning signal (SIGTERM) to all processes that are undefined
in the target run level. init waits five seconds before for-
cibly terminating these processes by sending a kill signal
(SIGKILL).

When init receives a signal telling it that a process it
spawned has died, it records the fact and the reason it died
in /var/adm/utmpx and /var/adm/wtmpx if it exists (see
who(1)). A history of the processes spawned is kept in
/var/adm/wtmpx.

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System Administration Commands init(1M)

If init receives a powerfail signal (SIGPWR) it scans
/etc/inittab for special entries of the type powerfail and
powerwait. These entries are invoked (if the run levels per-
mit) before any further processing takes place. In this way
init can perform various cleanup and recording functions
during the powerdown of the operating system.

Environment Variables in /etc/defaults/init
You can set default values for environment variables, for
such items as timezone and character formatting, in
/etc/default/init. See the FILES section, below, for a list
of these variables.

telinit
telinit, which is linked to /sbin/init, is used to direct
the actions of init. It takes a one-character argument and
signals init to take the appropriate action.

SECURITY
init uses pam(3PAM) for session management. The PAM confi-
guration policy, listed through /etc/pam.conf, specifies the
session management module to be used for init. Here is a
partial pam.conf file with entries for init using the UNIX
session management module.

init session required pam_unix_session.so.1

If there are no entries for the init service, then the
entries for the "other" service will be used.

OPTIONS
0 Go into firmware.

1 Put the system in system administrator mode. All local
file systems are mounted. Only a small set of essen-
tial kernel processes are left running. This mode is
for administrative tasks such as installing optional
utility packages. All files are accessible and no
users are logged in on the system.

2 Put the system in multi-user mode. All multi-user
environment terminal processes and daemons are
spawned. This state is commonly referred to as the
multi-user state.

3 Extend multi-user mode by making local resources
available over the network.

4 Is available to be defined as an alternative multi-
user environment configuration. It is not necessary
for system operation and is usually not used.

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System Administration Commands init(1M)

5 Shut the machine down so that it is safe to remove the
power. Have the machine remove power, if possible.

6 Stop the operating system and reboot to the state
defined by the initdefault entry in /etc/inittab.

a, b, c
process only those /etc/inittab entries having the a,
b, or c run level set. These are pseudo-states, which
may be defined to run certain commands, but which do
not cause the current run level to change.

Q, q Re-examine /etc/inittab.

S, s Enter single-user mode. This is the only run level
that doesn't require the existence of a properly for-
matted /etc/inittab file. If this file does not exist,
then by default, the only legal run level that init
can enter is the single-user mode. When in single-user
mode, the filesystems required for basic system opera-
tion will be mounted. When the system comes down to
single-user mode, these file systems will remain
mounted (even if provided by a remote file server),
and any other local filesystems will also be left
mounted. During the transition down to single-user
mode, all processes started by init or init.d scripts
that should only be running in multi-user mode are
killed. In addition, any process that has a utmpx
entry will be killed. This last condition insures that
all port monitors started by the SAC are killed and
all services started by these port monitors, including
ttymon login services, are killed.

FILES
/dev/console
System console device.

/etc/default/init
Contains environment variables and their default
values. For example, for the timezone variable, TZ,
you might specify TZ=US/Pacific. The variables are:

TZ Either specifies the timezone information (see
ctime(3C)) or the name of a timezone information
file /usr/share/lib/zoneinfo.

Refer to the TIMEZONE(4) man page before chang-
ing this setting.

CMASK The mask (see umask(1)) that init uses and that
every process inherits from the init process. If
not set, init uses the mask it inherits from the

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System Administration Commands init(1M)
kernel. Note that init always attempts to apply
a umask of 022 before creating a file, regard-
less of the setting of CMASK.

LC_CTYPE
Character characterization information.

LC_MESSAGES
Message translation.

LC_MONETARY
Monetary formatting information.

LC_NUMERIC
Numeric formatting information.

LC_TIME
Time formatting information.

LC_ALL
If set, all other LC_* environmental variables
take-on this value.

LANG If LC_ALL is not set, and any particular LC_* is
also not set, the value of LANG is used for that
particular environmental variable.

/etc/initpipe
A named pipe used for internal communication.

/etc/inittab
Controls process dispatching by init.

/etc/ioctl.syscon
ioctl states of the console, as saved by init when
single-user state is entered.

/var/adm/utmpx
User access and administration information.

/var/adm/wtmpx
History of user access and administration information.

ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:

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System Administration Commands init(1M)

____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Availability | SUNWcsu |
|_____________________________|_____________________________|

SEE ALSO
login(1), sh(1), stty(1), who(1), shutdown(1M), su(1M),
ttymon(1M), ioctl(2), kill(2), ctime(3C), pam(3PAM), init-
tab(4), pam.conf(4), TIMEZONE(4), utmpx(4), attributes(5),
pam_authtok_check(5), pam_authtok_get(5),
pam_authtok_store(5), pam_dhkeys(5), pam_passwd_auth(5),
pam_unix(5), pam_unix_account(5), pam_unix_auth(5),
pam_unix_session(5), termio(7I)

DIAGNOSTICS
If init finds that it is respawning an entry from
/etc/inittab more than ten times in two minutes, assumes
that there is an error in the command string in the entry,
and generates an error message on the system console. It
will then refuse to respawn this entry until either five
minutes has elapsed or it receives a signal from a user-
spawned init or telinit. This prevents init from eating up
system resources when someone makes a typographical error in
the inittab file, or a program is removed that is referenced
in /etc/inittab.


NOTES
init and telinit can be run only by a privileged user.

The S or s state must not be used indiscriminately in
/etc/inittab. When modifying this file, it is best to avoid
adding this state to any line other than initdefault.

If a default state is not specified in the initdefault entry
in /etc/inittab, state 6 is entered. Consequently, the sys-
tem will loop by going to firmware and rebooting continu-
ously.

If the utmpx file cannot be created when booting the system,
the system will boot to state "s" regardless of the state
specified in the initdefault entry in /etc/inittab. This can
occur if the /var file system is not accessible.

When a system transitions down to the S or s state, the
/etc/nologin file (see nologin(4)) is created. Upon subse-
quent transition to run level 2, this file is removed by a
script in the /etc/rc2.d directory.

init uses /etc/initpipe, a named pipe, for internal communi-
cation.

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System Administration Commands init(1M)

The pam_unix(5) module might not be supported in a future
release. Similar functionality is provided by
pam_authtok_check(5), pam_authtok_get(5),
pam_authtok_store(5), pam_dhkeys(5), pam_passwd_auth(5),
pam_unix_account(5), pam_unix_auth(5), and
pam_unix_session(5).