Chapter 9. The Operating System

64

65535:

(uid_t)( 1) == (gid_t)( 1)

must not be used, because it is the error return sen 

tinel value.

9.3 System run levels and

init.d

scripts

9.3.1 Introduction

The

/etc/init.d

directory contains the scripts executed by

init

at boot time and when the

init state (or  runlevel ) is changed (see

init(8)

).

There are at least two different, yet functionally equivalent, ways of handling these scripts. For

the sake of simplicity, this document describes only the symbolic link method. However, it

must not be assumed by maintainer scripts that this method is being used, and any automated

manipulation of the various runlevel behaviours by maintainer scripts must be performed

using

update rc.d

as described below and not by manually installing or removing sym 

links. For information on the implementation details of the other method, implemented in the

file rc

package, please refer to the documentation of that package.

These scripts are referenced by symbolic links in the

/etc/rc

n

.d

directories. When changing

runlevels,

init

looks in the directory

/etc/rc

n

.d

for the scripts it should execute, where

n

is the runlevel that is being changed to, or

S

for the boot up scripts.

The names of the links all have the form

S

mmscript

or

K

mmscript

where mm is a two digit

number and script is the name of the script (this should be the same as the name of the actual

script in

/etc/init.d

).

When

init

changes runlevel first the targets of the links whose names start with a

K

are ex 

ecuted, each with the single argument

stop

, followed by the scripts prefixed with an

S

, each

with the single argument

start

. (The links are those in the

/etc/rc

n

.d

directory corre 

sponding to the new runlevel.) The

K

links are responsible for killing services and the

S

link

for starting services upon entering the runlevel.

For example, if we are changing from runlevel 2 to runlevel 3, init will first execute all of the

K

prefixed scripts it finds in

/etc/rc3.d

, and then all of the

S

prefixed scripts in that directory.

The links starting with

K

will cause the referred to file to be executed with an argument of

stop

, and the

S

links with an argument of

start

.

The two digit number mm is used to determine the order in which to run the scripts: low 

numbered links have their scripts run first. For example, the

K20

scripts will be executed

before the

K30

scripts. This is used when a certain service must be started before another. For

example, the name server

bind

might need to be started before the news server

inn

so that

inn

can set up its access lists. In this case, the script that starts

bind

would have a lower

number than the script that starts

inn

so that it runs first:

/etc/rc2.d/S17bind

/etc/rc2.d/S70inn