init(1M) init(1M)
NAME
init, telinit - process control initialization
SYNOPSIS
/etc/init [0123456SsQq]
/sbin/telinit [0123456sSQqabc]
DESCRIPTION
Init
init is a general process spawner. Its primary role is to
create processes from a script stored in the file
/etc/inittab (see inittab(4)). This file usually has init
spawn getty's on each line that a user may log in on. It
also controls autonomous processes required by any particu-
lar system.
init considers the system to be in a run-level at any given
time. A run-level can be viewed as a software configuration
of the system where each configuration allows only a
selected group of processes to exist. The processes spawned
by init for each of these run-levels is defined in the init-
tab file. init can be in one of eight run-levels, 0-6 and S
or s. The run-level is changed by having a privileged user
run /etc/init (which is linked to /sbin/telinit). This
user-spawned init sends appropriate signals to the original
init spawned by the operating system when the system was
rebooted, telling it which run-level to change to.
init is invoked inside the UNIX system as the last step in
the boot procedure. The first thing init does is to look
for /etc/inittab and see if there is an entry of the type
initdefault (see inittab(4)). If there is, init uses the
run-level specified in that entry as the initial run-level
to enter. If this entry is not in inittab or inittab is not
found, init requests that the user enter a run-level from
the virtual system console, /dev/syscon. If an S (s) is
entered, init goes into the SINGLE USER level. This is the
only run-level that doesn't require the existence of a prop-
erly formatted inittab file. If /etc/inittab doesn't exist,
then by default the only legal run-level that init can enter
is the SINGLE USER level. In the SINGLE USER level the vir-
tual console terminal /dev/syscon is opened for reading and
writing and the command /bin/su is invoked immediately. To
exit from the SINGLE USER run-level one of two options can
be elected. First, if the shell is terminated (via an end-
of-file), init will reprompt for a new run-level. Second,
the init or telinit command can signal init and force it to
change the run-level of the system.
When attempting to boot the system, failure of init to
prompt for a new run-level may be due to the fact that the
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device /dev/syscon is linked to a device other than the phy-
sical system console (/dev/systty). If this occurs, init
can be forced to relink /dev/syscon by typing a delete on
the system console which is colocated with the processor.
When init prompts for the new run-level, the operator may
enter only one of the digits 0 through 6 or the letters S or
s. If S or s are entered init operates as previously
described in SINGLE USER mode with the additional result
that /dev/syscon is linked to the user's terminal line, thus
making it the virtual system console. A message is gen-
erated on the physical console, /dev/systty, saying where
the virtual terminal has been relocated. /dev/console is a
synonym for /dev/systty.
When init comes up initially and whenever it switches out of
SINGLE USER state to normal run states, it sets the ioctl(2)
states of the virtual console, /dev/syscon, to those modes
saved in the file /etc/ioctl.syscon. This file is written
by init whenever SINGLE USER mode is entered. If this file
does not exist when init wants to read it, a warning is
printed and default settings are assumed. If the terminal
modes set in /etc/ioctl.syscon unsatisfactory (e.g., the
delete character is not what you're used to), from single
user mode you can change your terminal modes with stty(1),
then save them permanently in /etc/ioctl.syscon by typing
"init s".
If a 0 through 6 is entered init enters the corresponding
run-level. Any other input will be rejected and the user
will be re-prompted. If this is the first time init has
entered a run-level other than SINGLE USER, init first scans
inittab for special entries of the type boot and bootwait.
These entries are performed, providing the run-level entered
matches that of the entry before any normal processing of
inittab takes place. 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. The
inittab file is scanned to find all entries that are to be
processed for that run-level.
Run-level 2 is usually defined by the user to contain all of
the terminal processes and daemons that are spawned in the
multi-user environment.
In a multi-user environment, the inittab file is usually set
up so that init will create a process for each terminal on
the system.
For terminal processes, ultimately the shell will terminate
because of an end-of-file either typed explicitly or gen-
erated as the result of hanging up. When init receives a
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child death signal, telling it that a process it spawned has
died, it records the fact and the reason it died in
/etc/utmp and /etc/wtmp if it exists (see who(1)). A his-
tory of the processes spawned is kept in /etc/wtmp if such a
file exists.
To spawn each process in the inittab file, init reads each
entry and for each entry which should be respawned, it forks
a child process. After it has spawned all of the processes
specified by the inittab file, init waits for one of its
descendant processes to die, a powerfail signal, a signal by
init or telinit to change the system's run-level, or until
the scan interval, as specified in the inittab file,
expires. When one of the above four conditions occurs, init
re-examines the inittab file. New entries can be added to
the inittab file at any time; however, init still waits for
one of the above four conditions to occur. To provide for
an instantaneous response the init Q or init q command can
wake init to re-examine the inittab file.
If init receives a powerfail signal (SIGPWR) and is not in
SINGLE USER mode, it scans inittab for special powerfail
entries. 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
whenever the operating system experiences a power failure.
When init is requested to change run-levels (via telinit),
init sends the warning signal (SIGTERM) to all processes
that are undefined in the target run-level. init waits 20
seconds before forcibly terminating these processes via the
kill signal (SIGKILL).
Telinit
Telinit, which is linked to /etc/init , is used to direct
the actions of init. It takes a one-character argument and
signals init via the kill system call to perform the
appropriate action. The following arguments serve as direc-
tives to init.
0-6 tells init to place the system in one of the
run-levels 0-6.
a,b,c tells init to process only those /etc/inittab
file entries having the a, b or c run-level
set.
Q,q tells init to re-examine the /etc/inittab
file.
s,S tells init to enter the single user environ-
ment. When this level change is effected, the
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virtual system console, /dev/syscon, is
changed to the terminal from which the command
was executed.
Telinit can only be run by someone who is super-user or a
member of group sys.
FILES
/etc/inittab
/etc/utmp
/etc/wtmp
/etc/ioctl.syscon
/dev/syscon
/dev/systty
SEE ALSO
getty(1M), login(1), sh(1), who(1), kill(2), inittab(4),
utmp(4).
DIAGNOSTICS
If init finds that it is continuously respawning an entry
from /etc/inittab more than 10 times in 2 minutes, it will
assume that there is an error in the command string, and
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 from a user init (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 the inittab.
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