The Master Boot Record (MBR) stored on the first hard disk drive identifies the operating system to be loaded.

If the LILO program is used, the Linux kernel (operating system) is typically started (LILO allows the user to identify and start other operating systems, and can even default to those systems instead of Linux).

If a PC-style disk operating system (DOS) is loaded, as it is at PCFA.ORG, a program such as LOADLIN is run to start execution of the Linux kernel.

A parameter is passed to the Linux kernel identifying the "root file system".

The file system is a specific partition on a specific hard disk drive.

IDE drives are named beginning with /dev/hd and SCSI drives are named beginning with /dev/sd. This is followed by the sequential letter of the alphabet representing the drive (a for the first drive, b for the second drive, etc.). The last character of the partition name is the number representing the partion.

An IDE partition will have a name like /dev/hda1. A SCSI partition will have a name like /dev/sda1.

PCFA.ORG uses a DOS partition on /dev/sda1, a Linux partition on /dev/sda2 and a Linux swap partition on /dev/sda3. Other PCFA.ORG drives have similar partitions (/dev/sdb1, /dev/sdb2, and /dev/sdb3).

The Linux kernel runs the init process.

The ps listing, below, shows the init process with pid of 1. That never changes because the init process is the "father of all other processes".

The init program uses the /etc/inittab file in the "root file system".

/etc/inittab is utilized each time a login process needs to respawn, when a power failure notice is sent by the UPS, or when a shutdown is requested.

The /etc/inittab file sets the default runlevel and executes scripts in the /etc/rc.d/rc#.d folder, where # represents the runlevel:

PCFA.ORG uses 3 as the default.

The listings below are actually symbolic links to the scripts in the /etc/rc.d/init.d directory.

The letter (K or S) and number (00 to 99) before the name is used as part of the ASCII sort of the file to determine the order in which the scripts execute.

For security reasons, PCFA.ORG has elected to move several of the scripts in the /etc/rc.d/init.d directory in order that those services not be started. The moved scripts are in /etc/rc.d/init.d-UNUSED.

The /etc/rc.d/init.d scripts utilize the system configuration in the /etc/sysconfig directory.

Here are the runlevels:

0 - halt system
K00linuxconf, K08autofs, K15httpd, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K28amd, K30sendmail, K45named, K50inet, K50snmpd, K55routed, K60atd, K60crond, K60lpd, K80random, K85netfs, K87ypbind, K89portmap, K90killall, K90network, K92apmd, K96pcmcia, K99syslog, S00halt

1 - single user mode
K00linuxconf, K08autofs, K15httpd, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K28amd, K30sendmail, K45named, K50inet, K50snmpd, K55routed, K60atd, K60crond, K60lpd, K85netfs, K87ypbind, K89portmap, K90network, K92apmd, K96pcmcia, K99syslog, S00single, S20random

2 - multiuser mode without NFS
K08autofs, K15httpd, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K28amd, K45named, K50inet, K50snmpd, K55routed, K60atd, K85netfs, K87ypbind, K89portmap, S05apmd, S10network, S20random, S30syslog, S40crond, S45pcmcia, S50inet, S60lpd, S80sendmail, S99linuxconf, S99local

3 - full multiuser mode
K08autofs, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K50snmpd, K55routed, K87ypbind, S05apmd, S100postgres, S10network, S11portmap, S15netfs, S20random, S30syslog, S40atd, S40crond, S45pcmcia, S50inet, S55named, S60lpd, S72amd, S80sendmail, S85httpd, S99linuxconf, S99local

4 - not used
K08autofs, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K50snmpd, K55routed, K87ypbind, S05apmd, S10network, S11portmap, S15netfs, S20random, S30syslog, S40atd, S40crond, S45pcmcia, S50inet, S55named, S60lpd, S60nfs S72amd, S80sendmail, S85gpm, S85httpd, S99linuxconf

5 - X11 (X-Windows)
K08autofs, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K50snmpd, K55routed, K87ypbind, S05apmd, S10network, S11portmap, S15netfs, S20random, S30syslog, S40atd, S40crond, S45pcmcia, S50inet, S55named, S60lpd, S72amd, S80sendmail, S85httpd, S99linuxconf, S99local

6 - reboot system
K00linuxconf, K08autofs, K15httpd, K20rstatd, K20rusersd, K20rwalld, K20rwhod, K28amd, K30sendmail, K45named, K50inet, K50snmpd, K55routed, K60atd, K60crond, K60lpd, K80random, K85netfs, K87ypbind, K89portmap, K90killall, K90network, K92apmd, K96pcmcia, K99syslog, S00reboot

S - used to run scripts when in runlevel 1

s - alias to S

Part of /etc/inittab file requires the /etc/rc.d/rc.sysinit script to be executed.

The /etc/rc.d/rc.sysinit script begins many other processes. Among them are:

/sbin/initlog,

/sbin/loglevel,

the default keymap,

swap partition,

set hostname,

file system check (fsck) (if needed),

mount the /proc filesystem,

plug and play,

remount the / filesystem,

quotas,

load modules (basic, sound, and raid),

reset tmp log files,

set the system clock,

turn on the swap partition

run the /etc/rc.d/rc.serial script

copy the dmesg (kernel ring) to a log file.

The rc.serial script (run by /etc/rc.d/rc.sysinit, above,

initializes the serial ports. On PCFA.ORG they are ttS16 through ttS31. Modems are attached to ttS16 through ttS20, even though telephone lines are no longer connected to the modems.

Finally, the /etc/rc.d/rc.local script is executed.

At PCFA.ORG, this recreates the /etc/issue and /etc/issue.net files, which are sent as the first lines of a login ("Welcome to PCFA . . ."), initializes the unipowerd daemon for communication with the APC uninterruptable power supply, and records the time to a log (a log of reboots).

  PID TTY      STAT   TIME COMMAND
    1 ?        S      0:12 init
    2 ?        SW     1:33 [kflushd]
    3 ?        SW     0:52 [kupdate]
    4 ?        SW     0:00 [kpiod]
    5 ?        SW     4:10 [kswapd]
  279 ?        SW     0:00 [portmap]
  302 ?        S      4:46 syslogd -m 0
  313 ?        S      0:00 klogd
  327 ?        S      0:00 /usr/sbin/atd
  341 ?        S      0:03 crond
  359 ?        S      0:14 inetd
  487 ?        S      0:00 /usr/local/sbin/unipowerd /dev/ttyS0
  490 tty1     SW     0:00 [mingetty]
  491 tty2     SW     0:00 [mingetty]
  492 tty3     SW     0:00 [mingetty]
  493 ?        SW     0:00 [uugetty]
11018 ?        SW     0:00 [uugetty]
32308 ?        SW     0:00 [uugetty]
  548 ?        SW     0:00 [uugetty]
 4835 ?        SW     0:00 [uugetty]
 1801 ?        S      0:01 sendmail: accepting connections on port 25
11592 ?        S      0:01 /usr/sbin/named
13160 ?        S      0:00 /usr/local/pgsql/bin/postmaster -D/usr/local/pgsql/database
13171 ?        S      0:00 /usr/local/apache/bin/httpd -f /usr/local/apache/conf

       D   uninterruptible sleep (usually IO)
       R   runnable (on run queue)
       S   sleeping
       T   traced or stopped
       Z   a defunct ("zombie") process
 
       For  BSD  formats  and when the "stat" keyword is used, additional letters may be
       displayed:
       W   has no resident pages