30.2.4. The /sbin/init Program

30.2.4. The /sbin/init Program

The /sbin/init program (also called init) coordinates the rest of the boot process and configures the environment for the user.

When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on the system. First, it runs the /etc/rc.d/rc.sysinit script, which sets the environment path, starts swap, checks the file systems, and executes all other steps required for system initialization. For example, most systems use a clock, so rc.sysinit reads the /etc/sysconfig/clock configuration file to initialize the hardware clock. Another example is if there are special serial port processes which must be initialized, rc.sysinit executes the /etc/rc.serial file.

The init command then runs the /etc/inittab script, which describes how the system should be set up in each SysV init runlevel. Runlevels are a state, or mode, defined by the services listed in the SysV /etc/rc.d/rc<x>.d/ directory, where <x> is the number of the runlevel. For more information on SysV init runlevels, refer to Section 30.4, “SysV Init Runlevels”.

Next, the init command sets the source function library, /etc/rc.d/init.d/functions, for the system, which configures how to start, kill, and determine the PID of a program.

The init program starts all of the background processes by looking in the appropriate rc directory for the runlevel specified as the default in /etc/inittab. The rc directories are numbered to correspond to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel 5.

When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to determine which processes to start and stop.

Below is an example listing of the /etc/rc.d/rc5.d/ directory:

K05innd -> ../init.d/innd K05saslauthd -> ../init.d/saslauthd K10dc_server -> ../init.d/dc_server K10psacct -> ../init.d/psacct K10radiusd -> ../init.d/radiusd K12dc_client -> ../init.d/dc_client K12FreeWnn -> ../init.d/FreeWnn K12mailman -> ../init.d/mailman K12mysqld -> ../init.d/mysqld K15httpd -> ../init.d/httpd K20netdump-server -> ../init.d/netdump-server K20rstatd -> ../init.d/rstatd K20rusersd -> ../init.d/rusersd K20rwhod -> ../init.d/rwhod K24irda -> ../init.d/irda K25squid -> ../init.d/squid K28amd -> ../init.d/amd K30spamassassin -> ../init.d/spamassassin K34dhcrelay -> ../init.d/dhcrelay K34yppasswdd -> ../init.d/yppasswdd K35dhcpd -> ../init.d/dhcpd K35smb -> ../init.d/smb K35vncserver -> ../init.d/vncserver K36lisa -> ../init.d/lisa K45arpwatch -> ../init.d/arpwatch K45named -> ../init.d/named K46radvd -> ../init.d/radvd K50netdump -> ../init.d/netdump K50snmpd -> ../init.d/snmpd K50snmptrapd -> ../init.d/snmptrapd K50tux -> ../init.d/tux K50vsftpd -> ../init.d/vsftpd K54dovecot -> ../init.d/dovecot K61ldap -> ../init.d/ldap K65kadmin -> ../init.d/kadmin K65kprop -> ../init.d/kprop K65krb524 -> ../init.d/krb524 K65krb5kdc -> ../init.d/krb5kdc K70aep1000 -> ../init.d/aep1000 K70bcm5820 -> ../init.d/bcm5820 K74ypserv -> ../init.d/ypserv K74ypxfrd -> ../init.d/ypxfrd K85mdmpd -> ../init.d/mdmpd K89netplugd -> ../init.d/netplugd K99microcode_ctl -> ../init.d/microcode_ctl S04readahead_early -> ../init.d/readahead_early S05kudzu -> ../init.d/kudzu S06cpuspeed -> ../init.d/cpuspeed S08ip6tables -> ../init.d/ip6tables S08iptables -> ../init.d/iptables S09isdn -> ../init.d/isdn S10network -> ../init.d/network S12syslog -> ../init.d/syslog S13irqbalance -> ../init.d/irqbalance S13portmap -> ../init.d/portmap S15mdmonitor -> ../init.d/mdmonitor S15zebra -> ../init.d/zebra S16bgpd -> ../init.d/bgpd S16ospf6d -> ../init.d/ospf6d S16ospfd -> ../init.d/ospfd S16ripd -> ../init.d/ripd S16ripngd -> ../init.d/ripngd S20random -> ../init.d/random S24pcmcia -> ../init.d/pcmcia S25netfs -> ../init.d/netfs S26apmd -> ../init.d/apmd S27ypbind -> ../init.d/ypbind S28autofs -> ../init.d/autofs S40smartd -> ../init.d/smartd S44acpid -> ../init.d/acpid S54hpoj -> ../init.d/hpoj S55cups -> ../init.d/cups S55sshd -> ../init.d/sshd S56rawdevices -> ../init.d/rawdevices S56xinetd -> ../init.d/xinetd S58ntpd -> ../init.d/ntpd S75postgresql -> ../init.d/postgresql S80sendmail -> ../init.d/sendmail S85gpm -> ../init.d/gpm S87iiim -> ../init.d/iiim S90canna -> ../init.d/canna S90crond -> ../init.d/crond S90xfs -> ../init.d/xfs S95atd -> ../init.d/atd S96readahead -> ../init.d/readahead S97messagebus -> ../init.d/messagebus S97rhnsd -> ../init.d/rhnsd S99local -> ../rc.local

As illustrated in this listing, none of the scripts that actually start and stop the services are located in the /etc/rc.d/rc5.d/ directory. Rather, all of the files in /etc/rc.d/rc5.d/ are symbolic links pointing to scripts located in the /etc/rc.d/init.d/ directory. Symbolic links are used in each of the rc directories so that the runlevels can be reconfigured by creating, modifying, and deleting the symbolic links without affecting the actual scripts they reference.

The name of each symbolic link begins with either a K or an S. The K links are processes that are killed on that runlevel, while those beginning with an S are started.

The init command first stops all of the K symbolic links in the directory by issuing the /etc/rc.d/init.d/<command> stop command, where <command> is the process to be killed. It then starts all of the S symbolic links by issuing /etc/rc.d/init.d/<command> start.


After the system is finished booting, it is possible to log in as root and execute these same scripts to start and stop services. For instance, the command /etc/rc.d/init.d/httpd stop stops the Apache HTTP Server.

Each of the symbolic links are numbered to dictate start order. The order in which the services are started or stopped can be altered by changing this number. The lower the number, the earlier it is started. Symbolic links with the same number are started alphabetically.


One of the last things the init program executes is the /etc/rc.d/rc.local file. This file is useful for system customization. Refer to Section 30.3, “Running Additional Programs at Boot Time” for more information about using the rc.local file.

After the init command has progressed through the appropriate rc directory for the runlevel, the /etc/inittab script forks an /sbin/mingetty process for each virtual console (login prompt) allocated to the runlevel. Runlevels 2 through 5 have all six virtual consoles, while runlevel 1 (single user mode) has one, and runlevels 0 and 6 have none. The /sbin/mingetty process opens communication pathways to tty devices[14], sets their modes, prints the login prompt, accepts the user's username and password, and initiates the login process.

In runlevel 5, the /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script executes the preferred X display manager[15]gdm, kdm, or xdm, depending on the contents of the /etc/sysconfig/desktop file.

Once finished, the system operates on runlevel 5 and displays a login screen.

[14] Refer to the Red Hat Enterprise Linux Deployment Guide for more information about tty devices.

[15] Refer to the Red Hat Enterprise Linux Deployment Guide for more information about display managers.

Note: This documentation is provided {and copyrighted} by Red Hat®, Inc. and is released via the Open Publication License. The copyright holder has added the further requirement that Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder. The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-5 because CentOS-5 is built from publicly available, open source SRPMS. The documentation is unmodified to be compliant with upstream distribution policy. Neither CentOS-5 nor the CentOS Project are in any way affiliated with or sponsored by Red Hat®, Inc.