A runlevel is a software configuration of the system which allows only a selected group of processes to exist. The processes spawned by init for each of these runlevels are defined in the /etc/inittab file. Init can be in one of eight runlevels: 0-6 and S or s. The runlevel is changed by having a privileged user run telinit, which sends appropriate signals to init, telling it which runlevel to change to.

Runlevels 0, 1, and 6 are reserved. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to get the system down into single user mode. Runlevel S is not really meant to be used directly, but more for the scripts that are executed when entering runlevel 1. For more information on this, see the manpages for shutdown(8) and inittab(5).

Runlevels 7-9 are also valid, though not really documented. This is because "traditional" Unix variants don't use them. In case you're curious, runlevels S and s are in fact the same. Internally they are aliases for the same runlevel.

After init is invoked as the last step of the kernel boot sequence, it looks for the file /etc/inittab to see if there is an entry of the type initdefault (see inittab(5)). The initdefault entry determines the initial runlevel of the system. If there is no such entry (or no /etc/inittab at all), a runlevel must be entered at the system console.

Runlevel S or s bring the system to single user mode and do not require an /etc/inittab file. In single user mode, a root shell is opened on /dev/console. ONLY When entering single user mode, init reads the console's ioctl(2) states from /etc/ioctl.save. If this file does not exist, init initializes the line at 9600 baud and with CLOCAL settings.

When init leaves single user mode, it stores the console's ioctl settings in this file so it can re-use them for the next single-user session.

When entering a multi-user mode for the first time, init performs the boot and bootwait entries to allow file systems to be mounted before users can log in. Then all entries matching the runlevel are processed.

After it has spawned all of the processes specified in /etc/inittab, init waits for one of its descendant processes to die, a powerfail signal, or until it is signaled by telinit to change the system's runlevel.

When one of the above three conditions occurs, it re-examines the /etc/inittab file.

New entries can be added to this file at any time. However, init still waits for one of the above three conditions to occur. To provide for an instantaneous response, the telinit Q or q command can wake up init to re-examine the /etc/inittab file.

When init is requested to change the runlevel, it sends the warning signal SIGTERM to all processes that are undefined in the new runlevel. It then waits 5 seconds before forcibly terminating these processes via the SIGKILL signal.

Note that init assumes that all these processes (and their descendants) remain in the same process group which init originally created for them. If any process changes its process group affiliation it will not receive these signals. Such processes need to be terminated separately.

/sbin/telinit is linked to /sbin/init. It takes a one-character argument and signals init to perform the appropriate action. The following arguments serve as directives to telinit:

0,1,2,3,4,5 or 6 -- tell init to switch to the specified run level.

a,b,c -- tell init to process only those /etc/inittab file entries having runlevel a,b or c.

Q or q -- tell init to re-examine the /etc/inittab file.

S or s -- tell init to switch to single user mode.

U or u -- tell init to re-execute itself (preserving the state). No re-examining of /etc/inittab file happens. Run level should be one of Ss12345, otherwise request would be silently ignored.

It is possible to pass a number of flags to init from the boot monitor (e.g. LILO).

Init accepts the following flags:

Init listens on a fifo in /dev, /dev/initctl, for messages. Telinit uses this to communicate with init. The interface is not very well documented or finished. Those interested should study the initreq.h file in the src/ subdirectory of the init source code tar archive.

Init reacts to several signals:

Examples of programs for specified run level taken from the /etc/inittab file:

id:3:initdefault:
            

By default the system must boot to run-level 3.

Sysinit:

si::sysinit:/etc/init.d/rcS
            

The run-level specified here is single-user mode and once reached it calls the "sysinit", which runs all the scripts in /etc/init.d/rcS.

As defined in Debian as /etc/init.d/rcS which then runs all the /etc/rcS.d/S* scripts and then symlinks to /etc/init.d/* and /etc/rc.boot/* (depreciated)

A typical /etc/rc3.d/ directory

3:3:wait:/etc/init.d/rc 3 
            

When changing run levels to run level 3 or down from run level 3, use the scripts just once in the directory file /etc/init.d/rc 3. The scripts starting with an S* (Start) are used at bootup time and the scripts starting with a K* (Kill) are used at shutdown time.

Below are some of the files that would be in this directory /etc/rc3.d:

Each time a process terminates, init records the fact and the reason it died in /var/run/utmp and /var/log/wtmp, provided that these files exist. (See full desciption on Processes Chapter 6).

The file /etc/inittab contains the background programs that used to keep the system running. One of these programs is one getty process per serial port.

co:2345:respawn:/sbin/getty ttyS0 CON9600 vt102 respawn
            

Re-run the program if it dies. We want this to happen so that a new login prompt will appear when you log out of the console.

/sbin/getty ttyS0 CON9600 vt102 
            

In this case, we're telling the program called getty to connect to /dev/ttyS0 using the settings for CON9600.

This is a line that exists in /etc/gettydefs. This entry represents a terminal running at 9600bps.

The terminal is a later-model VT102 (later than vt100).

The getty program lives in /sbin and is used by /etc/inittab to call the /etc/gettydefs file as follows:

Define co in gettydefs:

# Serial console 9600, 8, N, 1, CTS/RTS flow control
co# B9600 CS8 -PARENB -ISTRIP CRTSCTS HUPCL # B9600 
SANE CS8 -PARENB -ISTRIP CRTSCTS HUPCL #@S @L login: #co
            

This means that the console is a serial console running at 9600 baud rate, 8 bits, No parity, 1 stop bit and carrier and receive flow control set up.

If the line does not manage to handshake then it refers to the end of line label of where to try next, in the case above it is looking at "co" again.

If you check the man pages you can find out all the labels that you can use in a gettydefs, definition.

As long as you define each label in the inittab correctly and then follow it up with a corresponding entry in the gettydefs file you can redefine any handshake between, terminals, consoles, printers and any other serial devices.

Each configuration line has the syntax:

<label>#<initial_flags>#<final_flags>#<login_prompt>#<next_label>
            

The <label> is referred to on the getty command line.

The <next_label> is the definition used if a RS-232 Break is sent. As the console is always 9600bps, this points back to the original label.

<initial_flags> are the serial line parameters used by getty.

<final_flags> are the serial line parameters set by getty before it calls login. You will usually want to set a 9600bps line, SANE terminal handling, eight data bits, no parity and to hang up the modem when the login session is finished.

The <login_prompt> for serial lines is traditionally the name of the machine, followed by the serial port, followed by login: and a space. The macro that inserts the name of the machine and the serial port may vary.

Dialling in from a modem

When using a modem, you will have top investigate and ultimately use the uugetty program. This program does file lock checking.

Modem entries in /etc/gettydefs

38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #19200 19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #9600 9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #2400 2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #1200 1200# B1200 CS8 # B1200 SANE -ISTRIP HUPCL #@S @L @B login: #300 300# B300 CS8 # B300 SANE -ISTRIP HUPCL #@S @L @B login: #38400

Add the following line to your /etc/inittab, so that uugetty is run on your serial port (use the information pertaining to your environment):

1:456:respawn:/sbin/uugetty ttyS1 F38400 vt100

The first lines in the preceding extract are typical for the system console. They set many initial and final flags that control how the console behaves.

console# B19200 OPOST ONLCR TAB3 BRKINT IGNPAR ISTRIP IXON IXANY PARENB ECHO ECHOE ECHOK ICANON ISIG CS8 CREAD # B19200 OPOST ONLCR TAB3 BRKINT IGNPAR ISTRIP IXON IXANY PARENB ECHO ECHOE ECHOK ICANON ISIG CS8 CREAD #Console Login: #console

	Note
	This entry would be one line in the /etc/gettydefs file, but because of display and printing issues we have entered linebreaks.

Xconn bound to screen 11
                

setenv DISPLAY remotename:11
                

DISPLAY=remotename:11

export DISPLAY