Run Levels
From: https://en.wikipedia.org/wiki/Runlevel
Runlevel
From Wikipedia, the free encyclopedia
A runlevel is a mode of operation in the computer operating systems that
implements Unix System V-style initialization. Conventionally, seven runlevels
exist, numbered from zero to six. S is sometimes used as a synonym for one of
the levels. Only one runlevel is executed on startup; run levels are not
executed one after another (i.e. only runlevel 2, 3, or 4 is executed, not more
of them sequentially or in any other order).
A runlevel defines the state of the machine after boot. Different runlevels
are typically assigned (not necessarily in any particular order) to the
single-user mode, multi-user mode without network services started, multi-user
mode with network services started, system shutdown, and system reboot system
states. The exact setup of these configurations varies between operating systems
and Linux distributions. For example, runlevel 4 might be a multi-user GUI
no-server configuration on one distribution, and nothing on another. Runlevels
commonly follow the general patterns described in this article; however, some
distributions employ certain specific configurations.
In standard practice, when a computer enters runlevel zero, it shuts off,
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
runlevels are useful for maintenance or emergency repairs, since they
usually offer no network services at all. The particular details of runlevel
configuration differ widely among operating systems, and also among system
administrators.
In various Linux distributions, the traditional /etc/rc script used in the
Version 7 Unix was first replaced by runlevels and then by systemd states on
most major distributions.
Standard runlevels
| ID
| Name
| Description
|
| 0 | Off | Turns off the device.
|
| 1 | Single user mode | Does not configure network interfaces or start
daemons.[a]
|
| 6 | Reboot | Reboots the device.
|
Linux
Although systemd is, as of 2016, used by default in most major Linux
distributions, runlevels can still be used through the means provided by the
sysvinit project. After the Linux kernel has booted, the /sbin/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.
Linux Standard Base specification
Systems conforming to the Linux Standard Base (LSB) need not provide the
exact run levels given here or give them the meanings described here, and
may map any level described here to a different level which provides the
equivalent functionality.[1]
| LSB 4.1.0
|
| ID
| Name
| Description
|
| 0 | Off | Turns off the device.
|
| 1 | Single-user mode | Mode for administrative tasks.[2][b]
|
| 2 | Multi-user mode | Does not configure network interfaces and does not
export networks services.[c]
|
| 3 | Multi-user mode with networking | Starts the system normally.[1]
|
| 4 | Not used/user-definable | For special purposes.
|
| 5 | Full mode | Same as runlevel 3 + display manager.
|
| 6 | Reboot | Reboots the device.
|
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.
| Slackware Linux runlevels[3]
|
| ID
| Description
|
| 0 | Off
|
| 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 (X11 or a session manager)
|
| 5 | Full mode
|
| 6 | Reboot
|
Gentoo Linux
| Gentoo Linux runlevels[4]
|
| ID
| Description
|
| 0 | Off
|
| 1 or S | Single-user mode
|
| 2 | Multi-user mode without networking.
|
| 3 | Multi-user mode
|
| 4 | Aliased for runlevel 3
|
| 5 | Full mode
|
| 6 | Reboot
|
Debian GNU/Linux
| Debian GNU/Linux runlevels[5]
|
| ID
| Description
|
| 0 | Off
|
| 1 | Recovery mode
|
| 2,3,4 | Partial mode
|
| 5 | Full mode
|
| 6 | Reboot
|
Unix System V Releases 3 and 4
| System V runlevels
|
| ID
| Description
|
| 0 | Off
|
| 1 | Single-user mode, all filesystems unmounted but not 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 | Go to firmware
|
| 6 | Reboot
|
| s, S | Identical to 1, except current terminal acts as the system console
|
Solaris
Starting from Solaris 10, SMF (Service Management Facility) is used instead
of SVR4 run levels. The latter are emulated to preserve compatibility with
legacy startup scripts.[6]
| Solaris runlevels
|
| ID
| Description
|
| 0 | On SPARC systems, it returns to firmware, while on x86 systems, it
shuts off the machine. If the user presses Enter, the system reboots.
|
| S | Single-user mode with only root filesystem mounted (as read-only) --
Solaris 10+: svc:/milestone/single-user
|
| 1 | Single-user mode with all local filesystems mounted (read-write)
|
| 2 | Multi-user mode with most daemons started – Solaris 10+:
svc:/milestone/multi-user
|
| 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. -- Solaris 10+: svc:/milestone/multi-user-server
|
| 4 | Alternative Multi-user mode, User-definable
|
| 5 | Off
|
| 6 | Reboot
|
HP-UX
| HP-UX runlevels
|
| ID
| Description
|
|
| 0 | Off
|
|
| 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
|
AIX
AIX does not follow the System V R4 (SVR4) runlevel specification, with
runlevels from 0 to 9 available, as well as from a to c (or h). 0 and 1 are
reserved, 2 is the default normal multi-user mode and runlevels from 3 to 9
are free to be defined by the administrator. Runlevels from a to c (or h)
allow the execution of processes in that runlevel without killing processes
started in another.
| AIX runlevels
|
| ID
| Name
| Description
|
| 0 | | reserved
|
| 1 | | reserved
|
| 2 | Normal Multi-user mode | default mode
|
The S, s, M and m runlevels are not true runlevels, but are used to tell the
init command to enter maintenance mode. When the system enters maintenance
mode from another runlevel, only the system console is used as the
terminal.
See also
Notes- all systems use runlevel 1 for this purpose. This mode is intended to
provide a safe environment to perform system maintenance. Originally this
runlevel provided a single terminal (console) interface running a root login
shell. The increasing trend towards physical access to the computer during
the boot process has led to changes in this area.
- additional behavior of runlevel 1 varies greatly. All distributions provide
at least one virtual terminal. Some distributions start a login shell as the
superuser; some require correctly entering the superuser's password; others
provide a login prompt, allowing access to any registered user.
- In some cases, runlevels 2 and 3 function identically, offering a multi-user
mode with networking.
References
External links