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In this lesson, we’ll cover how to manage runlevels with SysV init, configure boot targets, and perform system shutdown or reboot. Controlling which services start or stop is essential for Linux administration—whether you’re running web servers, mail daemons, or network services.

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SysV init and Runlevels

On SysV-based systems, /sbin/init (PID 1) manages services through predefined runlevels (0–6). Each runlevel corresponds to a different system state:
RunlevelDescription
0Halt (shutdown)
1, SSingle-user mode (no networking), maintenance
2Multi-user mode without NFS (custom on some distros)
3Full multi-user mode with networking (console login)
4Unused/reserved (user-defined)
5Graphical multi-user mode (desktop environment)
6Reboot
The image is a text-based explanation of SysVinit runlevels, detailing their purposes and functions, such as system shutdown, single-user mode, and multi-user modes. It also mentions the role of /sbin/init in managing runlevels and services.
Runlevels and their associated services are defined in two places:
  • /etc/inittab: Specifies which scripts or processes to start at each runlevel
  • /etc/init.d/: Contains the actual service scripts

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Common /etc/inittab Actions

ActionDescription
sysinitRun once during system initialization (ignores runlevels)
bootRun at boot time, but init does not wait for completion (ignores runlevels)
bootwaitRun at boot time—init waits until it finishes (ignores runlevels)
waitRun when entering listed runlevels—init waits for it to complete
respawnAlways restart the process if it terminates
ctrlaltdelTriggered on SIGINT (CTRL+ALT+DEL)
initdefaultSets the default runlevel (values 1–5, not 0 or 6)
The image is a text-based explanation of SysVinit configuration files and runlevels, detailing the purpose of /etc/inittab and /etc/init.d/, along with descriptions of boot actions.

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Editing /etc/inittab

Before making changes, back up the file: 
sudo cp /etc/inittab /etc/inittab.bak
Open it in your favorite editor: 
sudo vi /etc/inittab
A typical configuration might include: 
# Set default runlevel to 3 (multi-user, console login)
id:3:initdefault:

# System initialization scripts
si::sysinit:/etc/init.d/rcS

# Single-user mode login
~:S:wait:/sbin/sulogin

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

# Handle CTRL+ALT+DEL
ca::ctrlaltdel:/sbin/shutdown -r now

# Virtual consoles for runlevels 2 and 3
1:23:respawn:/sbin/getty tty1 VC linux
2:23:respawn:/sbin/getty tty2 VC linux

# Serial console on runlevel 3
S0:3:respawn:/sbin/getty -L 9600 ttyS0 vt320
After editing /etc/inittab, reload init’s configuration without rebooting:
sudo telinit q

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Init Scripts and Service Directories

Service scripts live in /etc/init.d/, while each runlevel directory in /etc/rc*.d/ contains symlinks: 
ls /etc/rc*.d
# rc0.d/  rc1.d/  rc2.d/  rc3.d/  rc4.d/  rc5.d/  rc6.d/
Within each rcN.d directory, file prefixes determine actions:
PrefixOperation
SnnStart service when entering N
KnnStop service when entering N
Example for runlevel 3: 
ls /etc/rc3.d
# K01networking  S01apache2  S02ssh  ...

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Checking and Changing Runlevels

  • Show current and previous runlevels: 
    runlevel
# Output: N 3
  • Switch to single-user mode (runlevel 1): 
    sudo telinit 1
  • Reboot using runlevel 6: 
    sudo telinit 6
  • Halt using runlevel 0: 
    sudo telinit 0
Switching runlevels will start or stop multiple services. Always save your work and notify other users before changing to runlevels 0, 1, or 6.

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Further Reading

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