System V

Although Ubuntu uses Upstart, the knowledge of System V is, I think, very important, because we still find it on several distributions such as CentOS. Although the latter dates back to several mid-80s, it does very well, so much so that upstart, which is very recent, is inspired by System V. As mentioned, it uses the runlevels described earlier, the configuration file is: / etc / inittab plus it uses the directories /etc/rc0.d, /etc/rc1.d, /etc/rc2.d, /etc/rc3.d, /etc/rc4.d , /etc/rc5.d, /etc/rc6.d, /etc/init.d/

Here is an example of the configuration file:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
id:3:initdefault:

# System initialization.
si::sysinit:/etc/rc.d/rc.sysinit

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

# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now

# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3

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

# Ask root password even in single user mode
~:S:wait:/sbin/sulogin

Each line is structured as follows 4 fields delimited by colon, they are of the form:
Code: levels: action: process

• “Code” is a unique identifier for each entry, either a number or a keyword composed of characters and / or numbers: we have already seen “id” for the entry corresponding to the level of execution by default, there are others.

• “Levels” is a list of execution levels to which the entry will apply, if this list is empty, it is equivalent to indicating all levels.

• “Action” specifies how the process indicated by the “process” field should be treated when entering one of the levels in the “levels” list. Possible actions are:

  • • wait: start the process and wait for it to finish before going to the next inittab entry

    • respawn: start the process and restart it automatically when it finishes

    • ounce: start the process if it is not already started, do not wait for it to finish

    • initdefault: Specifies the default operating level

    • sysinit: the process must be done at system startup, before any action entry

Explanation of the configuration file:

• line 9 : id:3:initdefault:
  This defines the runlevel by default when the system is initialized, in this case the runlevel is 3 so multi-user and all the services EXCEPT the GUI, if we wanted to change the initialization level we would only have to change the value 3.

• line 12: si :: sysinit: /etc/rc.d/rc.sysinit
  Initialization script, the latter will be executed regardless of the runlevel defined, to really know what it does nothing like read the script, here is an example of this rc.sysinit file. Although the contents and the details can vary the goal is always the same initialize the system by loading modules of the nucleus, to activate points of assembly, to activate systems of security (SeLinux, crypto, …)

• line 14 to 20: l3: 3: wait: /etc/rc.d/rc 3
  At this point we have a functional system with modules for USB support, important editing points are realized, active encryption modules, … However no services or resources were started, in other if you wanted that the service we activate automatically at start this will realize that now. We are at the stage of starting the services according to the runlevel. In the example here, for level 3, the system will call the script /etc/rc.d/rc passing argument 3, you’ll guess it’s the runlevel number. Here is an example of the script: rc.
  The script will browse the directory /etc/rc3.d/ to find files that start with the letter S for Start, for each file found it will run the script with the start argument, here is an example of the contents of the directory:

  initialization script list rc3.d

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

  user@host:~$ ls /etc/rc3.d/ K01dnsmasq K35winbind S00microcode_ctl S13portmap S28autofs S95anacron K01KICKSTART K50netconsole S03sysstat S14nfslock S30nscd S95atd K02avahi-dnsconfd K50snmptrapd S03vmware-tools S15mdmonitor S50hplip S95jexec K02NetworkManager K69rpcsvcgssd S04readahead_early S18rpcidmapd S50snmpd S97yum-updatesd K02oddjobd K73ipmi S05kudzu S19rpcgssd S55sshd S98avahi-daemon K05conman K73ypbind S07iscsid S22messagebus S56rawdevices S98graylog2-server K05saslauthd K74syslog-ng S08ip6tables S23setroubleshoot S56xinetd S98graylog2-web-interface K05wdaemon K80kdump S08iptables S25bluetooth S58ntpd S98puppet K10cups K85mdmpd S08mcstrans S25netfs S64mysqld S99firstboot K10psacct K87multipathd S10network S25pcscd S65puppetmaster S99local K10tcsd K88wpa_supplicant S11auditd S26acpid S80sendmail S99nagios K15svnserve K89dund S12restorecond S26haldaemon S85gpm S99smartd K20nfs K89netplugd S12syslog S26hidd S85httpd K24irda K89pand S13cpuspeed S26lm_sensors S85mongod K25squid K89rdisc S13irqbalance S26lvm2-monitor S90crond K35vncserver K99readahead_later S13iscsi S27elasticsearch S90xfs

  Files starting with S (start) will start while files starting with K (Kill) will be stopped when this level of execution is done, to remember it is possible to move from one level to another. In order to allow the proper start sequence of the services, for example that the network starts before the ssh server, a number follows the start flag. Here is an example for the init script of ssh: S55sshd.

• line 23: ca :: ctrlaltdel: / sbin / shutdown -t3 -r now
  Regardless of the execution level, if the CTRL + ALT + DEL keyboard sequence is the command: / sbin / shutdown -t3 -r now is done in other words the reboot system. Commanding this line will disable the feature.

• line 26: 1: 2345: respawn: / sbin / mingetty tty1
  Here is a case of respawn, if we are in the runlevel 2,3,4 or 5 the system will make sure that the application / sbin / mingetty is in function, even if we stop the process by killing the latter process will restart.

• line 31: x: 5: respawn: / etc / X11 / prefdm -nodaemon
  this is very similar however this only applies for the runlevel 5

Once all the lines are processed and all the services run the server is available, the process at line 12 and especially at line 17 will be the longest because that is where all the work is done.

So in summary:

1. The system is booting through the BIOS

2. The BIOS loads the boot loader (GRUB)

3. The user chooses his kernel and the initrd associated with it

4. The initrd is loaded, the kernel modules are loaded and the root (/) is mounted

5. Initialization is done by / sbin / init

   1. Reading the / etc / inittab file

   2. Running the initialization script /etc/rc.d/rc.sysinit

   3. Running Resource Startup and Service /etc/rc.d/rcRunlevel_Number

   4. Terminals activation: / sbin / mingetty tty1