Answer Posted / shree
The booting process undergoes the following phases afterwards :
1) init phase
2) inittab file
3) rc scripts & Run Level
4.) Next Steps
INIT phase
Init phase is started by the execution of /sbin/init program
and starts other processes after reading the /etc/inittab
file as per the directives in the /etc/inittab file .
Two most important functions of init are
a) It runs the processes to bring the system to the default
run level state ( Run level 3 in Solaris , defined by
initdefault parameter in /etc/inittab )
b) It controls the transition between different run levels
by executing appropriate rc scripts to start and the stop
the processes for that run level.
/etc/inittab file
This file states the default run level and some actions to
be performed while the system reaches up to that level. The
fields and their explanation are as follows :
S3:3:wait:/sbin/rc3 > /dev/console 2>&1 < /dev/console
S3 denotes a identification if the line
3 is run level
wait is action to be performed
/sbin/rc3 is the command to be run.
So the fields in the inittab are
Identification : run level : action : process
The complete line thus means run the command /sbin/rc3 at
run level 3 and wait until the rc3 process is complete.
The action field can have any of the following keywords :
Initdefault : default run level of the system
Respawn : start and restart the process if it stops.
Powerfail : stop on powerfail
Sysinit : start and wait till console in accessible .
Wait : wait till the process ends before going on to the
next line.
RC scripts & Run Levels
Rc scripts performs the following functions :
a) They check and mount the file systems
b) Start and stop the various processes like network , nfs etc.
c) Perform some of the house keeping jobs.
System goes in to one of the following run level after
booting depending on default run level and the commands
issued for changing the run level to some other one.
0 Boot prom level ok> or > prompt in Sun.
1 Administrative run level . Single user mode
2 Multiuser mode with no resource sharing .
3 Multiuser level with nfs resource sharing
4 Not used
5 Shutdown & power off (Sun 4m and 4u architecture )
6 Reboot to default run level
S s Single user mode user logins are disabled.
Broadly speaking the running system can be in any of the
folloing state
Single user – Minimum processes running , user logins
disabled and root password is required to gain access to the
shell .
Multiuser - All system processes are running and user logins
are permitted
Run level of a desired state is achieved by a number of
scripts executed by the rc program the rc scripts are
located in /etc/rc0.d , /etc/rc1.d , /etc/rc2.d , /etc/rc3.d
& /etc/rcS.d directories . All the files of a particular run
level are executed in the alphanumeric order .Those files
beginning with letter S starts the processes and those
beginning with K stops the processes.
These files are hard linked to the files in /etc/init.d in
order to provide a central location for all these files and
eliminating the need to change the run level in case these
scripts needs to be run separately . The files in
/etc/init.d directory are without any S , K and numeric
prefix instead a stop / start argument has to be supplied
whenever these scripts are to be executed .
By default system has a number of rc scripts needed for run
level transition but sometimes it becomes necessary to start
some custom scripts at the booting time and turn them off at
the shutdown . Custom scripts can be put in any of the
required rc directory but following major considerations has
to be kept in mind :
* The sequence number of the file should not conflict
with other files.
* The sevices needed should be available by previously
executed scripts.
* File should be hard linked to the /etc/init.d directory .
* The system looks for only those files beginning with
letter K & S , any thing else is ignored , therefore, to
make a file inactive simply changing uppercase K or S to
lower case will cause system to ignore it .
| Is This Answer Correct ? | 7 Yes | 3 No |
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