Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 58)

58 Chapter 3. Boot Process, Init, and Shutdown

3.2.2. Init

The kernel ﬁnds /sbin/init and executes it. It is the init command which coordinates the rest of

the boot process.

When init starts, it becomes the parent or grandparent of all of the processes that start up automati-

cally on a Red Hat Linux system. First, it runs the /etc/rc.d/rc.sysinit script, which sets your

environment path, starts swap, checks the ﬁlesystems, and so on. Basically, rc.sysinit takes care

of everything that your system needs to have done at system initialization. Most systems use a clock,

so on them rc.sysinit uses the /etc/sysconfig/clock ﬁle to initialize the clock. If you have

special serial port processes that need to be initialized, rc.sysinit may also run rc.serial.

Then, init runs the /etc/inittab script, which describes how the system should be set up in each

runlevel and sets the default runlevel. (See Section 3.4 for more information on init runlevels.) This

ﬁle states, among other things, that /sbin/update should be run whenever a runlevel starts. The

update program is used to ﬂush dirty buffers back to disk.

Whenever the runlevel changes, init uses the scripts in the /etc/rc.d/init.d/ directory to start

and stop various services, such as a Web server, DNS server, and so on. First, init sets the source

function library for the system (commonly /etc/rc.d/init.d/functions), which spells out how

to start or kill a program and how to ﬁnd out the PID of a program. Then, init determines the current

and the previous runlevel.

Next, init starts all of the background processes necessary for the system to run by looking in the

appropriate rc directory for that runlevel (/etc/rc.d/rc

X .d/, where the X is a numerical

value between 0 and 6). The init command then runs each of the kill scripts (their ﬁle name starts

with a K) with a stop parameter. Then, init runs all of the start scripts (their ﬁle names start with an

S) in the appropriate runlevel directory with a start so that all services and applications are started

correctly. In fact, after the system is ﬁnished booting you can log in as root and execute these same

scripts manually with the command /etc/rc.d/init.d/httpd stop or /sbin/service httpd

stop. This will stop the httpd server.

None of the scripts that actually start and stop the services are located in the /etc/rc.d/rc X .d/

directory. Rather, all of the ﬁles in /etc/rc.d/rc

X .d/ are symbolic links that point to actual

scripts located in the /etc/rc.d/init.d/ directory. A symbolic link is nothing more than a ﬁle

that points to another ﬁle, and they are used in this case because they can be created and deleted

without affecting the actual script that kills or starts the service. The symbolic links to the various

scripts are numbered in a particular order so that they start in that order. You can change the order in

which the services start up or are killed by changing the name of the symbolic link that refers to the

script that actually starts or kills the service. You can give symbolic links the same number as other

symbolic links if you want that service start or stop right before or after another service.

For example, for runlevel 5, init looks into the /etc/rc.d/rc5.d/ directory and might ﬁnds the

following (your system and conﬁguration may vary):

K01pppoe -> ../init.d/pppoe

K05innd -> ../init.d/innd

K10ntpd -> ../init.d/ntpd

K15httpd -> ../init.d/httpd

K15mysqld -> ../init.d/mysqld

K15pvmd -> ../init.d/pvmd

K16rarpd -> ../init.d/rarpd

K20bootparamd -> ../init.d/bootparamd

K20nfs -> ../init.d/nfs

K20rstatd -> ../init.d/rstatd

K20rusersd -> ../init.d/rusersd

K20rwalld -> ../init.d/rwalld

K20rwhod -> ../init.d/rwhod

K25squid -> ../init.d/squid

K28amd -> ../init.d/amd

1 2 ... 53 54 55 56 57 58 59 60 61 62 63 ... 281 282

Commenti su questo manuale

Nessun commento

Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 58

Commenti su questo manuale

Prodotti e manuali riguardandi Software di system management Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE