Red Hat [Electronic resources] : The Complete Reference Enterprise Linux Fedora Edition؛ The Complete Reference

SysV Init: init.d Scripts

You can manage the startup and shutdown of server daemons with special startup scripts located in the

/etc/rc.d/init.d directory. These scripts often have the same name as the server's program. For example, for the

/usr/sbin/httpd Web server program, the corresponding script is called

/etc/rc.d/init.d/httpd . This script starts and stops the Web server. This method of using

init.d startup scripts to start servers is called SysV Init, after the method used in Unix System V.

The startup scripts in the

/etc/rc.d/init.d directory can be executed automatically whenever you boot your system. Be careful when accessing these scripts, however. These start essential programs, such as your network interface and your printer daemon. These init scripts are accessed from links in subdirectories set up for each possible runlevel. The

/etc/rc.d directory holds a set of subdirectories whose names have the format

rc

N

.d , where

N is a number referring to a runlevel. The

rc script detects the runlevel in which the system was started, and then executes only the startup scripts specified in the subdirectory for that runlevel. When you start your system, the

rc script executes the startup scripts specified in the

rc3.d directory, if you are performing a command line login, and the

rc5.d directory, if you are using a graphical login. The

rc3.d and

rc5.d directories hold symbolic links to certain startup scripts in the

/etc/rc.d/init.d directory. So, the

httpd script in the

/etc/rc.d/init.d directory is actually called through a symbolic link in the

rc3.d or the

rc5.d directory. The symbolic link for the

/etc/rc.d/httpd script in the

rc3.d directory is

S85httpd . The S prefixing the link stands for "startup"; thus, the link calls the corresponding

init.d script with the

start option. The number indicates the order in which startup scripts are run; lower numbers run first.

S85httpd invokes

/etc/rc.d/init.d/httpd with the option

start . If you change the name of the link to start with a K, the script is invoked with the

stop option, stopping it. Such links are used in the runlevels 0 and 6 directories,

rc6.d and

rc0.d . Runlevel 0 halts the system and runlevel 6 reboots it. You can use the

runlevel command to find out what runlevel you are currently operating at (see Chapter 27 for more details on runlevels). A listing of runlevels is shown in Table 20-2.