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YoLinux.com: Linux Init Process / PC Boot Procedure

This tutorial covers the PC boot process and the Linux Operating System initiation of background applications (daemons/services). The sequence, configuration and administration of the Linux boot process is covered.

Also see the YoLinux Tutorial on Recovery and Boot disks.

PC Boot and Linux Init Process:

  1. BIOS: The Basic Input/Output System is the lowest level interface between the computer and peripherals.
    The BIOS performs integrity checks on memory and seeks instructions on the Master Boor Record (MBR) on the floppy drive or hard drive.
  2. The MBR points to the boot loader (GRUB or LILO: Linux boot loader).
  3. Boot loader (GRUB or LILO) will then ask for the OS label which will identify which kernel to run and where it is located (hard drive and partition specified). The installation process requires to creation/identification of partitions and where to install the OS. GRUB/LILO are also configured during this process. The boot loader then loads the Linux operating system.
    • See the YoLinux tutorial on creating a boot disk for more information on GRUB and LILO and also to learn how to put the MBR and boot loader on a floppy for system recovery.
  4. The first thing the kernel does is to execute init program. Init is the root/parent of all processes executing on Linux.
  5. The first processes that init starts is a script /etc/rc.d/rc.sysinit
  6. Based on the appropriate run-level, scripts are executed to start various processes to run the system and make it functional.

The Linux Init Processes:

SysV init or systemd:

The Linux and Unix SysV heritage began with the "sysvinit" process which used the /etc/inittab configuration file to drive the execution of init scripts and configuration of terminal and GUI interfaces. SysV init launches scripts and "daemon" processes one at a time, in order for a given run level found in /etc/rc.#/ (Where # is a run level number from 0 to 6).

Systemd is used in later versions of Linux (Fedora 15+, RHEL 7, OpenSUSE 12.1+, ...) which is compatible with Unix SysV and Linux Standard Base (LSB) init scripts. Systemd supports parallelization (faster boot), socket management, process tracking, mounting of file systems and dependency-based service control. Daemon processes are categorized into "cgroups" which have categorized resource limits.

Ubuntu 12.04+, 14.04 uses a hybrid system called upstart which includes SysV runlevel specific start scripts in /etc/rc.#/ but also systemd scripts for those services which have been ported in /lib/systemd/system/. See "man 7 upstart-events" for details.

Launching processes:

The init process is the last step in the boot procedure and identified by process id "1". Init is responsible for starting system processes as defined in:

  • SysV systems like RHEL 1-6, Fedora 1-14: /etc/inittab
  • Systemd Fedora 15+: /lib/systemd/system/default.target (This is a symbolic link)
    And the files in /etc/systemd/system/ and /lib/systemd/system/
Init typically will start multiple instances of "getty" which waits for console logins which spawn one's user shell process. Upon shutdown, init controls the sequence and processes for shutdown. The init process is never shut down. It is a user process and not a kernel system process although it does run as root.

System Processes:

Process ID Description
0 The Scheduler
1 The init process
2 kflushd
3 kupdate
4 kpiod
5 kswapd
6 mdrecoveryd
Systemd init:

Systemd is becoming the default init system for Linux and has brought the following changes:
  • Boot scripts in /etc/systemd/system/ and /lib/systemd/system/
  • Service control command:
    • Fedora 15+: systemctl
    • Ubuntu 12.04+, 14.04: initctl
      • initctl list: List all system services and their current state.
      • initctl stop service-name: Stop a specified service. See "list" above to list names.
  • Requires kernel support: (Kernel compile config options)
    • CONFIG_AUTOFS4_FS=[y|m]
  • The kernel command line in GRUB will include "init=/bin/systemd"
  • Gnome desktop D-Bus dependency on systemd
SysV init:

Init config file (Red Hat EL6): /etc/inittab


Init config file (Red Hat 7.3-9.0, Fedora 1-14, RHEL5): /etc/inittab


# System initialization.

l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6

# Things to run in every runlevel.  This line is only in Red Hat 7.X Used to flush disk buffers.

ca::ctrlaltdel:/sbin/shutdown -t3 -r now

# When our UPS tells us power has failed, schedule a shutdown for 2 minutes from now.
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"

# If power was restored before the shutdown kicked in, cancel it.
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Canceled"

# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6

# Run xdm in runlevel 5
x:5:respawn:/etc/X11/prefdm -nodaemon

Note that this config file directs the init process to run the shell script /etc/rc.d/rc.sysinit. This script should be used as is and NOT changed. Extend rc.local and NOT this script. This will (not in exact order):

  • Run /sbin/initlog (depricated after RHEL5)
  • Run devfs to generate/manage system devices
  • Run network scripts: /etc/sysconfig/network
  • Start graphical boot (If so configured): rhgb (RHEL5)
    rhgb is started by the initscripts which drives the boot process, just after the device detections (needed for example to get at least the input devices needed for X recognized) but before the filesystems are checked or mounted. Rhgb is started from /etc/rc.sysinit as soon as device detection is done, and if the binary is found. At that point only the root filesystem is mounted, read-only and we are about running fsck checks on the filesystems. If /usr is not present of if rhgb can't start X the start is retryed later once the filesystems have been mounted. To make sure X is started on an environment where it is being configured properly we check that the init default from /etc/inittab is 5 or that that init level was passed on the command line. X and rhgb needs a writable filesystem to export communication channels with the boot process. To do this a small memory temporary filesystem is mounted under /etc/rhgb/temp first. To also allow communication at the X level, the network loopback device lo0 is also started and initialized prior to running X. Once both the filesystem and lo0 are ready /usr/bin/X11/X is started using vt8 and display :1 (not :0 to allow smoothing the transition from the temporary X server to the one started when going multiuser). Once X is sucessfully started, the graphic code instanciates the simple display with the progress bar and the detailed view using a vte widget. If a specific /etc/rhgb/xorg.conf configuration file for X under rhgb is provided then it will be passed to the X command line to allow a specific configuration for X boot.
  • Start console terminals, load keymap, system fonts and print console greeting: mingetty, setsysfonts
    The various virtual console sessions can be viewed with the key-stroke:
    • RHEL6: ctrl-alt-F2 through F7. F1 is reserved for the GUI screen invoked in run level 5.
    • Older systems: ctrl-alt-F1 through F6. F7 is reserved for the GUI screen invoked in run level 5.
  • Mount /proc and start device controllers.
  • Done with boot configuration for root drive. (initrd) Unmount root drive.
  • Re-mount root file system as read/write
  • Direct kernel to load kernel parameters and modules: sysctl, depmod, modprobe
  • Set up clock: /etc/sysconfig/clock
  • Perform disk operations based on fsck configuration
  • Check/mount/check/enable quotas non-root file systems: fsck, mount, quotacheck, quotaon
  • Initialize logical volume management: vgscan, /etc/lvmtab
  • Activate syslog, write to log files: dmesg
  • Activate PAM
  • Activate swapping: swapon

Local system boot processes can be placed in file: /etc/rc.d/rc.local

The system will then boot to the runlevel set by the directive initdefault.

Also see:

Linux init Run Levels:

SysV init and systemd both support the notion of run levels but manage them in different ways. SysV init uses symbolically linked start/stop init scrips located in directories defining the services to be run (/etc/rc.d/rc[0-6].d/) while systemd manages the services with "cgroups".

Typical run levels include:
  • 0 — Halt
  • 1 — Single-user text mode
  • 2 — Not used (user-definable)
  • 3 — Full multi-user text mode
  • 4 — Not used (user-definable)
  • 5 — Full multi-user graphical mode (with an X-based login screen)
  • 6 — Reboot
Systemd run levels:

The default boot level is set by /etc/systemd/system/default.target which is a symbolic link to the file defining the appropriate run level target.

Fedora 15-19:
  • Set run level 3 (multi-user with networking and console only login):
    rm /etc/systemd/system/default.target
    ln -sf /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target
  • Set run level 5 (graphic mode):
    rm /etc/systemd/system/default.target
    ln -sf /lib/systemd/system/runlevel5.target /etc/systemd/system/default.target
The run level targets are defined as runlevel0.target, runlevel1.target, runlevel2.target, runlevel3.target, ...

Ubuntu 12.04+: (Upstart: Mix of SysV init and systemd. Not all services have been migrated)
  • Run level defined by the services in directory /etc/systemd/system/multi-user.target.wants/
    ls -l /etc/systemd/system/multi-user.target.wants/
    This will show symbolic links to service files defined in /lib/systemd/system/

Runlevel "3" will boot to text or console mode and "5" will boot to the graphical login mode.

Runlevel Scripts Directory
(Fedora: /lib/systemd/system/)
0 runlevel0.target , poweroff.target shutdown/halt system
1 runlevel1.target, rescue.target Single user mode
2 runlevel2.target Multiuser with no network services exported
3 runlevel3.target , multi-user.target Default text/console only start. Full multiuser
4 runlevel4.target Reserved for local use. Also X-windows (Slackware/BSD)
5 runlevel5.target, graphical.target XDM X-windows GUI mode
6 runlevel6.target, reboot.target Reboot
Note that the user friendly name is just a symbolic link to the runlevel name:
# ls -l /lib/systemd/system/runlevel3.target
lrwxrwxrwx. 1 root root 17 May 24 10:55 /lib/systemd/system/runlevel3.target -> multi-user.target

Ubuntu names the directories with an additionaol suffix: ".wants". For example multi-user.target on Fedora would be named multi-user.target.wants for an Ubuntu system.

SysV run levels:

The Linux boot process has six states of operation of which "0" is the shutdown state and "3" and above are fully operational with all essential processes running for user interaction. Upon system boot the LINUX system /sbin/init program starts other processes by performing the following:

  • Init will bring up the machine by starting processes as defined in the /etc/inittab file.

  • The computer will be booted to the runlevel as defined by the initdefault directive in the /etc/inittab file.
    In this example a runlevel of "5" is chosen. Runlevel "5" will boot the system into GUI mode using XDM and X-Windows. Booting to runlevel "3" (often called console mode) is often used by servers which do not need a graphical user interface. If booted to init level "3" one can promote the run level with the command [root prompt]# init 5. See the more detailed explanation of run levels below.

    The inittab file will allow you to capture key sequences (ctrl-alt-del), start dial in internet connections etc.

  • One of these process started by init is /sbin/rc. This script runs a series of scripts in the directories /etc/rc.d/rc0.d/, /etc/rc.d/rc1.d/, /etc/rc.d/rc2.d/, etc

  • Scripts in these directories are executed for each boot state of operation until it becomes fully operational. Scripts beginning with S denote startup scripts while scripts beginning with K denote shutdown (kill) scripts. Numbers follow these letters to denote the order of execution. (lowest to highest)

Runlevel "3" will boot to text or console mode and "5" will boot to the graphical login mode ( "4" for slackware)

Runlevel Scripts Directory
(Red Hat/Fedora Core)
0 /etc/rc.d/rc0.d/ shutdown/halt system
1 /etc/rc.d/rc1.d/ Single user mode
2 /etc/rc.d/rc2.d/ Multiuser with no network services exported
3 /etc/rc.d/rc3.d/ Default text/console only start. Full multiuser
4 /etc/rc.d/rc4.d/ Reserved for local use. Also X-windows (Slackware/BSD)
5 /etc/rc.d/rc5.d/ XDM X-windows GUI mode (Redhat/System V)
6 /etc/rc.d/rc6.d/ Reboot
s or S
Single user/Maintenance mode (Slackware)
Multiuser mode (Slackware)

One may switch init levels by issuing the init command with the appropriate runlevel. Use the command "init #" where # is one of s,S,0,1,3,5,6. The command telinit does the same.

The scripts for a given run level are run during boot and shutdown. The scripts are found in the directory /etc/rc.d/rc#.d/ where the symbol # represents the run level. i.e. the run level "3" will run all the scripts in the directory /etc/rc.d/rc3.d/ which start with the letter "S" during system boot. This starts the background processes required by the system. During shutdown all scripts in the directory which begin with the letter "K" will be executed. This system provides an orderly way to bring the system to different states for production and maintenance modes.

If you installed all demons (background processes), Linux will run them all. To avoid slowing down your machine, remove unneeded services from the start-up procedure. You can start/stop individual demons by running service init scripts located in the /etc/init.d/ directory:

  • /etc/rc.d/init.d/ (Red Hat/Fedora) Also /etc/init.d/ which is linked to /etc/rc.d/init.d/
  • /etc/init.d/ (S.u.s.e.)
  • /etc/init.d/ (Ubuntu / Debian)
and issuing the command and either the start, stop, status, restart or reload option i.e. to stop the web server:
  • /etc/init.d/httpd stop

Use the command ps -aux to view all process on your machine.

TIP: List state and run level of all services which can be started by init: chkconfig --list
service --status-all | grep running (Red Hat/Fedora Core based systems)

GUI tool: /usr/X11R6/bin/tksysv

Run Level Commands:

Init Script Activation:
SysV init:

Adding a script to the /etc/rc.d/rc#.d/ directory with either an S or K prefix, adds the script to the boot or shutdown process. The scripts are run in numerical order. S20abc is run before S30xyz. The extensibility to the boot and shutdown procedures of the operating system is one of the strengths of UNIX. The orderly sequential initiation of processes can be coordinated for dependent processes. The orderly shutdown of processes is often required of complex programs such as databases. This is how it is done. Individual processes may be monitored, shutdown and started at any time using these scripts. i.e. /etc/rc.d/rc2.d/httpd start. The modifiers start, stop or status may be used.

The start/stop/status scripts actually reside in the directory:

  • /etc/rc.d/init.d/ (Red Hat/Fedora) Also /etc/init.d/ which is linked to /etc/rc.d/init.d/
  • /etc/init.d/ (S.u.s.e. and Ubuntu / Debian)
and are linked to the appropriate directories. These links may be created or destroyed using the chkconfig command. i.e. chkconfig --del httpd will remove the web server from the startup and shutdown process. Inversely chkconfig --add httpd will add it to the startup/shutdown process by generating links from the script in /etc/rc.d/init.d/ to the appropriate /etc/rc.d/rc#.d/ directory. For more information see the LINUX manual page on init.

Ubuntu 14.04:

Init scripts are found in /etc/init/

List services which are to be turned on at boot time: grep "start on" /etc/init/*

Also see man upstart-events to view events which can be configured.

System Services:

System Service Description
anacron Run jobs which were scheduled for execution while computer was turned off. Catch up with system duties.
arpwatch Keeps track of IP address to MAC address pairings
atd Run scheduled batch jobs.
autofs automounts file systems on demand.
bluetooth, pand, hidd, dund Bluetooth netwoork support.
crond Job sheduler for periodic tasks.
gpm Allows console terminal cut and paste. (Non X-window consoles)
https Apache web server.
iptables Firewall rules interface to kernel.
keytable Loads selected keyboard map as set in /etc/sysconfig/keyboard
kudzu New hardware probe/detection during system boot.
lpd or cups Network printer services.
microcode_ctl Uploads microcode to kernel and ultimately to the Intel Pentium processor. (Hardware specific.)
mysqld Database services
named DNS name services (Bind)
network Active network services during system boot. Required for network connectivity.
nfs Network file system. Unix file sharing services. Also uses services: nfslock, portmap, rpcgssd, rpcidmapd, rpcsvcgssd
nscd Password and group lookup services for use with network authentication (NIS, LDAP,...).
ntpd Network Time Protocol time synchronization services.
random Random number generation tool used for encryption.
rawdevices Enables raw IO. Useful for Oracle and software which utilizes this for high speed disk access.
smb SAMBA: MS/Windows PC file sharing services
syslog System log file facility.
ypbind NIS file sharing/authentication infrastructure service.
yppasswd NIS file sharing/authentication infrastructure service.
ypserv NIS file sharing/authentication infrastructure service.
xfs X-Windows font server.

Recommended basic services: anacron, ard, autofs, crond, gpm, iptables, keytable, kudzu, microcode_ctl (Intel32 hardware only), network, random. syslog
Graphics Workstation - add: xfs
File Server for PC clients - add: smb
Print Server - add: lpd or cups (hplip - HP Linux Imaging and Printing)
File server Linux/Unix clients - add: nfs, netfs, nfslock, portmap, ypbind, yppasswd, ypserv; NFSv4 add: rpcgssd, rpcidmapd, rpcsvcgssd
Web Server - add: httpd, tux, xinetdi, sshd

GUI configuration tools:

GUI tools can help you configure the appropriate services to start and provide a description of each service available:
  • Fedora/RHEL/CentOS: /usr/bin/system-config-services
    (Also /usr/sbin/serviceconf)
  • Red Hat 8.0/9.0: /usr/bin/redhat-config-services
  • Ubuntu / Debian:
    • bum (Boot Up Manager) (GUI image)
    • /usr/bin/services-admin (GUI image)
    • /usr/sbin/sysv-rc-conf (console program - see below)
Red Hat Configuration GUI:
  • Fedora/RHEL/CentOS: /usr/bin/system-config-services
  • Red Hat 8.0/9.0: /usr/bin/redhat-config-services

Red Hat/Fedora Core text console services selection tool: /usr/sbin/ntsysv

Debian/Ubuntu 12.04, 14.04: sysv-rc-conf
(Install: aptget install sysv-rc-conf)

This command can also be used in command line mode to control services:
  • sysv-rc-conf --list : List services
  • sysv-rc-conf service-name on : Start service
  • sysv-rc-conf service-name off : Stop service

Init Script:

A single copy of the script is located in the directory: /etc/rc.d/init.d/script-name (Red Hat/Fedora) or /etc/init.d/script-name (Ubuntu / Debian).
Use the command chkconfig to generate soft links to the appropriate directories for the various run levels.

Example: Red Hat
# Startup script for program
# chkconfig: 345 85 15     - start or stop process definition within the boot process
# description: Description of program
# processname: process-name
# pidfile: /var/run/process-name.pid

# Source function library.      This creates the operating environment for the process to be started
. /etc/rc.d/init.d/functions

case "$1" in
        echo -n "Starting  process-name: "
        daemon  process-name                 - Starts only one process of a given name.
        touch /var/lock/subsys/process-name
        echo -n "Shutting down process-name: "
        killproc process-name
        rm -f /var/lock/subsys/process-name
        rm -f /var/run/process-name.pid      - Only if process generates this file
        status process-name
        $0 stop
        $0 start
        echo -n "Reloading process-name: "
        killproc process-name -HUP
        echo "Usage: $0 {start|stop|restart|reload|status}"
        exit 1

exit 0

The bash script functions daemon, killproc and status can all be found in the script /etc/rc.d/init.d/functions. (Red Hat/Fedora distributions)

Example: Ubuntu 14.04
# Provides:          apache2
# Required-Start:    $local_fs $remote_fs $network $syslog $named
# Required-Stop:     $local_fs $remote_fs $network $syslog $named
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# X-Interactive:     true
# Short-Description: Start/stop apache2 web server
# Description:       Start the web server and associated helpers
#  This script will start apache2, and possibly all associated instances.
#  Moreover, it will set-up temporary directories and helper tools such as
#  htcacheclean when required by the configuration.

DESC="description goes here"

case "$1" in
        log_daemon_msg "Starting $DESC" "$NAME"
        initctl start provess-name
        case "$RET_STATUS" in
                        [ $RET_STATUS = 1 ] && log_warning_msg "Server was already running"
                        exit 1
        log_daemon_msg "Stopping $DESC" "$NAME"
        initctl stop provess-name
        case "$RET_STATUS" in
                        [ $RET_STATUS = 1 ] && log_warning_msg "Process was not running"
                        exit 1
See examples in /etc/init.d/ for examples onm how to support start, stop, status, restart, reload, etc.

The script must be executable to work. (chmod +x script-name).

The script may be used to start and stop processes. i.e.:

  • /etc/rc.d/init.d/httpd restart
    (Ubuntu / Debian / S.u.s.e.: /etc/init.d/apache2 restart)
  • /etc/rc.d/init.d/httpd stop
  • /etc/rc.d/init.d/httpd start
OR use the Red Hat/Fedora core based service command:
  • service httpd restart
  • service httpd stop
  • service httpd start

Note that two lines in the script enable the chkconfig command to control the script for the boot and shutdown process.

# chkconfig: 345 85 15 
# description: Description of program

When added to the boot process using the "chkconfig --add script-name" command the start order/priority will be set to 80 while the stop/shutdown order will be set to 15. The process will be added to runlevels 3, 4 and 5. This is enabled by generating links from the location of the script (/etc/rc.d/init.d/) to the directory for the appropriate run level: /etc/rc.d/rc#.d/. The file name in the run level directory will reflect if it is used for boot (starts with an "S") or shutdown (starts with a "K")

Upstart init:

Upstart init config files (Ubuntu 14.04 example): /etc/init/*
SysV init files are still in /etc/init.d/*.
The two systems cohabitate on Ubuntu 12.04 and 14.04.

File: /etc/init/service-name.conf
description     "service-name"

start on runlevel [2345]
stop on runlevel [!2345]

respawn limit 10 5
umask 022

expect stop

console none

pre-start script
    test -x /usr/sbin/service-name || { stop; exit 0; }

    mkdir -p -m0755 /var/run/service-name
end script

exec /usr/sbin/service-name -D
Systemd specific commands:

Fedora examples: (Note Ubuntu uses the command initctl with the same arguments)

  • Start a specified service: systemctl start name.service
  • Stop a specified service: systemctl stop name.service
  • Restart a specified service: systemctl restart name.service
  • Show status of a specified service: systemctl status name.service
  • Enable a specified service to be started upon boot: systemctl enable name.service
  • Disable a specified service to NOT be started upon boot: systemctl disable name.service
  • Check if a specified service is to be started upon boot: systemctl is-enabled name.service; echo $?
    The command "echo $?" shows the return code of the previous command:
    • 0: enabled
    • 1: disabled
  • List all active boot services: systemctl list-units --type=target


The (Red Hat/Fedora/IRIX) chkconfig command generates and breaks links between the directory /etc/rc.d/init.d/ and the appropriate run level directory: /etc/rc.d/rc[0-6].d/ to control boot process initiation and process shutdown.

     chkconfig [--level <levels>] <name> <on | off | reset >
     chkconfig --list
     chkconfig --list [name]
     chkconfig --add <name>
     chkconfig --del <name>
     chkconfig --level 0123456 <name> off


  • chkconfig --level 345 httpd on - forces apache to be invoked for run levels 3, 4 and 5.
  • chkconfig --add httpd - Start the web server daemon upon system boot.
  • chkconfig --del sendmail - Do not start the sendmail daemon upon system boot.
  • chkconfig --list - List all services and init levels.
  • chkconfig --list | grep on - List all services to be started upon system boot.

Using chkconfig to administer xinetd processes.

  • chkconfig wu-ftpd on - Turn on FTP service managed by xinetd.
  • chkconfig ipop3 off - Turn off POP3 service managed by xinetd.
This will reconfigure the appropriate xinetd file (in directory /etc/xinetd.d/) and restart the xinetdprocess.

Also see: chkconfig - Linux man page.

Related Commands:

service Display status of system services.
Example: service --status-all
Help: service --help

Also see: service - Linux man page.

Links and resources: