What is Run Levels in Linux

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The “run level” term is refer to a way of process in one of the operating system of computer that execute initialization in V- style of Unix System. Usually, seven run levels are present, numbered from 0 to 6.
“Run level” explains the status of the machine or appliance after boot. Different run levels are normally defined to:

single-user mode
multi-user mode with no any network services started
multi-user mode having network services started
system shutdown
system reboot

Linux:
The Linux is an operating system that can be used of run levels by the application of sysvinit project. After the boot of Linux kernel, the init application reads the file the/etc/inittab to verify the action for every run level. If the user identifies other value as a parameter of kernel boot, the system will try to start the default run level.

Debian Linux:

0= Halt

1= single-User mode

2-5 =Full Multi-users with console logins and show manager if installed

6 =Reboot

Red Hat Linux and Fedora:

0= Halt

1= single-User mode

2=Multi-users style console logins only without networking.

3= Multi-User mode, console logins only

4=Not used/ User-definable

5= Multi-User mode, with display manager as well as console logins (X11)

6=Reboot

Slackware Linux:

0= Halt

1= single-User mode

2=not used to configured the similar as run level 3.

3=Multi-User with no show or display manager

4=Multi-User mode having display manager.

5= not used to configured the similar as run level 3.

6=Reboot

Arch Linux:

0= Halt

1= single-User maintenance mode

2=Not Used

3=Multi-User

4=Not Used

5=Multi-user with X11
6=Reboot





Gentoo Linux:

0= Halt

1= single-User mode

2=Multi-user mode with no networking

3=Multi-user mode

4=Aliased for run level 3

5=Aliased for run level 3

6=Reboot

UNIX:

System V Run levels

0= Shut down system, power-off if hardware leads it (only available from the console)

1= Single-User mode, all systems of file decreased but root, all methods except console procedures killed

2=Multi-user mode

3=Multi-User mode with RFS (and NFS in Release 4) file systems exported

4= Multi-User, User-definable

5= Halt the operating system, enter to firmware

6=Reboot

Solaris run levels:

0= Operating system stopped; SPARC only) release to Open Boot timely

1= Single-User mode having all local file systems increases (read-write)

2= Multi-User mode having most daemons started

3= Multi-User mode; identical to 2 (run level 3 runs both /sbin/rc2 and /sbin/rc3), with file systems exported, plus some other network services in progress.

4= Another Multi-User mode, User-definable

5= Alternative Multi-User mode, User-definable

6=Reboot

Changing Run Levels of Linux:

If user is moving to high run levels, then user may create further services accessible to user, whereas taking step to lower run level will trigger daemons to become not accessible. On the creation of server, run level 3 is the usually utilized and hardly ever modified. Nevertheless, several tasks of administrative need the manager or administrator to run system toward run level 1 such as single user mode.

Init command is used to modify run levels:
#init1
Run level and process

Run level 0 is halt
Run level 1 is single-user
Run levels 2-5 are multi-user (some uses RUN level 5 to start X [KDE/Gnome])
Run level 6 is for rebooting system
To move toward run level 3, user needs to write init 3

Typing init 3 will move system to run level 3:




source:http://wifinotes.com/