F.2.4. The /sbin/init Program

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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 processes the jobs in the /etc/event.d directory, which describe 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 F.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 F.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, Upstart forks an /sbin/mingetty process for each virtual console (login prompt) allocated to the runlevel by the job definition in the /etc/event.d directory. 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[18], sets their modes, prints the login prompt, accepts the user's username and password, and initiates the login process.
In runlevel 5, Upstart runs a script called /etc/X11/prefdm. The prefdm script executes the preferred X display manager[19]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.

[18] Refer to the Red Hat Enterprise Linux Deployment Guide for more information about tty devices.
[19] Refer to the Red Hat Enterprise Linux Deployment Guide for more information about display managers.
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