The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-styleinitialization. Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine[citation needed], may be used. S is sometimes used as a synonym for one of the levels. Only one "runlevel" is executed on bootup - run levels are notexecuted sequentially, i.e. either runlevel 2 OR 3 OR 4 is executed, not 2 then 3 then 4.
"Runlevel" defines the state of the machine after boot. Different runlevels are typically assigned to:
- single-user mode
- multi-user mode without network services started
- multi-user mode with network services started
- system shutdown
- system reboot
The exact setup of these configurations will vary from OS to OS, and from one Linux distribution to another. For example, runlevel 4 might be multi-user, GUI, no-server on one distribution, and nothing on another. Note the difference in the Red Hat and Slackware distributions charted in this article. However, "runlevels" do commonly follow patterns described in this article. When installing Linux, it is best to consult that distribution's available user guides.
In standard practice, when a computer enters runlevel zero, it halts, and when it enters runlevel six, it reboots. The intermediate runlevels (1-5) differ in terms of which drives are mounted, and which network services are started. Default runlevels are typically 3, 4, or 5. Lower run levels are useful for maintenance or emergency repairs, since they usually don't offer any network services at all. The particular details of runlevel configuration differ widely among operating systems, and also among system administrators.
The runlevel system replaced the traditional /etc/rc script used in Version 7 Unix.
Linux
The Linux operating system can make use of runlevels through the programs of the sysvinit project. After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.
[edit]Linux Standard Base specification
Conforming implementations are not required to provide these exact run levels or give them the meanings described here, and may map any level described here to a different level which provides the equivalent functionality.[1]
| ID | Name | Description |
|---|---|---|
| 0 | Halt | Shuts down the system. |
| 1 | Single-User Mode | Mode for administrative tasks.[2][3] |
| 2 | Multi-User Mode | Does not configure network interfaces and does not export networks services.[4] |
| 3 | Multi-User Mode with Networking | Starts the system normally.[5] |
| 4 | Not used/User-definable | For special purposes. |
| 5 | Start the system normally with appropriate display manager. ( with GUI ) | As runlevel 3 + display manager. |
| 6 | Reboot | Reboots the system. |
^ = The additional behavior of this runlevel varies greatly. All distributions provide at least one virtual terminal. Some distributions start a login shell as thesuperuser; some require correctly entering the superuser's password first; others provide a login prompt, allowing any user access.
^ = In some cases, runlevels 2 and 3 function identically; offering a Multi-User Mode with Networking.
[edit]Debian Linux
Debian, as well as most of the distributions based on it, like early Ubuntu, does not make any distinction between runlevels 2 to 5.
| ID | Description |
|---|---|
| S | Only run on boot (replaces /etc/rc.boot) |
| 0 | Halt |
| 1 | Single-User mode |
| 2-5 | Full Multi-User with console logins and display manager if installed |
| 6 | Reboot |
[edit]Ubuntu
Ubuntu 6.10 (Edgy Eft) and later contain Upstart as a replacement for the traditional init-process, but they still use the traditional init scripts and Upstart's SysV-rc compatibility tools to start most services and emulate runlevels.
[edit]Red Hat Linux and Fedora
| Code | Information |
|---|---|
| 0 | Halt |
| 1 | Single-User mode |
| 2 | Multi-user mode 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 |
[edit]SUSE Linux
SUSE uses a similar setup to Red Hat:
| ID | Description |
|---|---|
| 0 | Halt |
| 1 or S | Single-User mode |
| 2 | Multi-User mode without networking |
| 3 | Multi-User mode, console logins only |
| 4 | Not used/User-definable |
| 5 | Multi-User mode with display manager |
| 6 | Reboot |
The services that run under a specific runlevel can be modified with YaST | System Services (runlevel), insserv command or withchkconfig command like the Red Hat based distributions.
[edit]Slackware Linux
Slackware Linux uses runlevel 1 for maintenance, as on other Linux distributions; runlevels 2, 3 and 5 identically configured for a console (with all services active); and runlevel 4 adds the X Window System.
| ID | Description |
|---|---|
| 0 | Halt |
| 1 | Single-User mode |
| 2 | Unused but configured the same as runlevel 3 |
| 3 | Multi-User mode without display manager |
| 4 | Multi-User mode with display manager |
| 5 | Unused but configured the same as runlevel 3 |
| 6 | Reboot |
[edit]Arch Linux
| ID | Description |
|---|---|
| 0 | Halt |
| 1 | Single-User (Maintenance Mode) |
| 2 | Not used |
| 3 | Multi-User |
| 4 | Not used |
| 5 | Multi-User with X11 |
| 6 | Reboot |
[edit]Gentoo Linux
| ID | Description |
|---|---|
| 0 | Halt |
| 1 or S | Single-User mode |
| 2 | Multi-User mode without networking |
| 3 | Multi-User mode |
| 4 | Aliased for runlevel 3 |
| 5 | Aliased for runlevel 3 |
| 6 | Reboot |
[edit]Unix
[edit]System V Releases 3 and 4
| ID | Description |
|---|---|
| 0 | Shut down system, power-off if hardware supports it (only available from the console) |
| 1 | Single-User mode, all filesystems unmounted but root, all processes except console processes killed |
| 2 | Multi-User mode |
| 3 | Multi-User mode with RFS (and NFS in Release 4) filesystems exported |
| 4 | Multi-User, User-definable |
| 5 | Halt the operating system, go to firmware |
| 6 | Reboot |
| s, S | Identical to 1, except current terminal acts as the system console |
[edit]Solaris[5]
| ID | Description |
|---|---|
| 0 | Operating system halted; (SPARC only) drop to OpenBoot prompt |
| S | Single-User mode with only root filesystem mounted (as read-only) |
| 1 | Single-User mode with all local filesystems mounted (read-write) |
| 2 | Multi-User mode with most daemons started |
| 3 | Multi-User mode; identical to 2 (runlevel 3 runs both /sbin/rc2 and /sbin/rc3), with filesystems exported, plus some other network services started. |
| 4 | Alternative Multi-User mode, User-definable |
| 5 | Shut down, power-off if hardware supports it |
| 6 | Reboot |
[edit]HP-UX
| ID | Description |
|---|---|
| 0 | System halted |
| S | Single-User mode, booted to system console only, with only root filesystem mounted (as read-only) |
| s | Single-User mode, identical to S except the current terminal acts as the system console |
| 1 | Single-User mode with local filesystems mounted (read-write) |
| 2 | Multi-User mode with most daemons started and Common Desktop Environment launched |
| 3 | Identical to runlevel 2 with NFS exported |
| 4 | Multi-User mode with VUE started instead of CDE |
| 5, 6 | Not used/User-definable |
[edit]FreeBSD, OpenBSD and NetBSD
The BSD variants don't use the concept of run levels, although on some versions init(8) provides an emulation of some of the common run levels.
[edit]AIX
AIX does not follow the System V R4 (SVR4) run level specification, with run levels from 0 to 9 available, as well as from a to c. 0 and 1 are reserved, 2 is the default normal multi-user mode and run levels from 3 to 9 are free to be defined by the administrator. Run levels from a to c allow the execution of processes in that run level without killing processes started in another.
| ID | Name | Description |
|---|---|---|
| 0 | reserved | |
| 1 | reserved | |
| 2 | Normal Multi-User mode | default mode |
How to Change Runlevels
by bobjensen — last modified Oct 27, 2009 08:24 AM
— filed under: Post-Install Solutions
Many people get confused when trying to boot in to a runlevel other than runlevel 5, for example runlevel 3, disabiling the GUI front end with which most users are familiar. Hopefully this howto will help answer the questions "How do I disable X" or "How do I boot without X" or even "How do I get to single user mode."
Requirements
One requirement of this document is to have an understanding of what runlevels are,how changing runlevels will impact your system, and what services may or may not be started. In general this is beyond the scope of this document but I will give a quick rundown of things as listed in /etc/inittab.
Runlevels
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 or Admin
5 - X11
6 - reboot (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 or Admin
5 - X11
6 - reboot (Do NOT set initdefault to this)
The most commonly used runlevels in Fedora Core are 0, 1, 3, 5 and 6. As you can see you use runlevels everytime you shutdown (runlevel 0) or reboot (runlevel 6) your machine. Most users boot to runlevel 5 with X11 support and a GUI login. Somehowever, boot to Full multiuser mode (runlevel 3) and may choose to "startx" manually once logged in. Single user mode (runlevel 1) can be very handy if you have forgotten your root password or are having trouble booting for any number of reasons. Full multi user mode (runlevel 3) is desirable when you are having trouble with X11 and would like to repair your xorg configuration or when running a server where you do not want the added overhead of having X running.
Doing the work
We will discuss ways to change your runlevel in this howto. First how to change yourrunlevel "permanently." Second how to change your current runlevel and finally how tochange the runlevel at boot time.
Editing /etc/inittab to change your default runlevel
- Open /etc/inittab in your favorite editor (I use nano).
su -c 'nano -w /etc/inittab'
- On about line 18 you will see a line like the one shown below.
id:5:initdefault:
- You simply change the "5" in this case to the runlevel you desire. Save the file and exit.
Changing your current run level
- You can change runlevels using the telinit or init commands as in the example below.
su -c 'telinit 3'
Changing your runlevel at boot time. (aka Grub Method)
The Grub menu is hidden by default- Reboot then press and hold a key until the grub menu appears
- Choose your desired kernel and press "a" to modify the kernel arguments.
- Simply add a 3 to the line for runlevel 3 or 1 for single user mode and Press "enter". After pressing enter press "b" to boot with the changes you have made. (NOTE: If you are having booting issues you might want to remove "quiet" and "rhgb.")
- Log in and do the damage you desire. If you are booting to single user you do not need to login you will simply be presented with a prompt.
- Logout, change runlevels or reboot as needed.
Linux Find Out Current Run Level Command
Type the following command:$ who -r
Sample outputs:run-level 2 2011-10-12 05:38
Linux Change Run Level Command
Use the init command to change rune levels:# init 1Runlevel And Its Usage
The Init is the parent of all processes with PID # 1. Its primary purpose is to create processes from a script stored in the file /etc/inittab file. This file usually has entries which cause init to spawn gettys on each line that users can log in. A runlevel is nothing but a software configuration of the Linux 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 as follows:- Runlevel 0 is halt
- Runlevel 1 is single-user
- Runlevels 2-5 are multi-user (some distro uses RUN level 5 to start X [KDE/Gnome])
- Runlevel 6 is for rebooting system
For example, typing the init 3 command will move system to run level 3:# init 3
On most Linux server system default run level is 3 and on most Linux Desktop system default run level is 5. The default run level is defined by the initdefault line at the top of /etc/inittab file under CentOS / Fedora / Redhat / RHEL / Debian Linux. To change the default run level, edit /etc/inittab file, and edit entry initdefault:# vi /etc/inittab
Set initdefault to 5, so that you can boot to X next time when Linux comes up:id:5:initdefault:
Save and close the file. Reboot the system to see changes:# reboot
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