Runlevel
From Wikipedia, the free encyclopedia
 |
It has been suggested that this article or section be merged with init. (Discuss) |
The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six, though up to ten, from zero to nine, may be used.
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. 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 slightly among system administrators.
The runlevel system replaced the traditional /etc/rc script used in Version 7 Unix.
Contents |
[edit] Standard runlevels
| ID | Name | Description |
|---|---|---|
| 0 | Halt | Halts—or shuts down—the system. |
| S | Single-User Mode | Does not configure network interfaces or start daemons. |
| 6 | Reboot | Reboots the system. |
^ = Almost 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.
[edit] 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.
On most systems users can check the current runlevel with either of the following commands:
$ runlevel # as root
$ who -r # as any user
[edit] Typical Linux runlevels
Most Linux distributions, in addition to the standard runlevels, define the following additional runlevels:
| ID | Name | Description |
|---|---|---|
| 1 | Single-User Mode | Does not: configure network interfaces, start daemons, or allow non-root logins. |
| 2 | Multi-User Mode | Does not: configure network interfaces or start daemons. |
| 3 | Multi-User Mode with Networking | Starts the system normally. |
| 4 | Unused | in special purpose |
| 5 | X11 | As runlevel 3 + display manager. |
^ = The additional behavior of this runlevel 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 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. See also the Debian FAQ on booting.
| ID | Description |
|---|---|
| 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[5] and Fedora
Red Hat as well as most of its derivatives uses runlevels like this:
| ID | Description |
|---|---|
| 0 | Halt |
| 1 | Single user |
| 2 | Not used/User definable |
| 3 | Full multi-user, console logins only |
| 4 | Not used/User definable |
| 5 | Full multi-user, with display manager as well as console logins |
| 6 | Reboot |
Which services are started in which runlevels can be managed with the chkconfig tool, which keeps its configuration settings under /etc/rc.d/. /sbin/chkconfig --list lists all the services controlled by chkconfig and whether they are on/off for each runlevel. Setting a service A controlled by chkconfig, for levels X, Y and Z is as simple as /sbin/chkconfig --level XYZ A
[edit] SUSE Linux
SUSE uses a similar setup to Red Hat:
| ID | Description |
|---|---|
| 0 | Halt |
| 1 | Single |
| 2 | Full multi-user with no networking |
| 3 | Full multi-user without display manager |
| 4 | Not used/User definable |
| 5 | Full multi-user with display manager |
| 6 | Reboot |
[edit] Slackware Linux[6]
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 |
| 2 | Full multi-user NO display manager |
| 3 | Full multi-user NO display manager |
| 4 | Full multi-user with display manager |
| 5 | Not used/User definable |
| 6 | Reboot |
[edit] Gentoo Linux[7]
| ID | Description |
|---|---|
| 0 | Halt |
| 1 | Single |
| 2 | Multi-user, no network |
| 3 | Full multi-user with display manager |
| 4 | Aliased for runlevel 3 (Full multi-user with display manager) |
| 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-defined |
| 5 | Halt the operating system, go to firmware |
| 6 | Halt the system, reboot to default runlevel |
| s, S | Identical to 1 (Single-user mode, all filesystems unmounted but root, all processes except console processes killed) except current terminal acts as the system console |
[edit] Solaris
| ID | Description |
|---|---|
| 0 | Operating system halted; (SPARC only) drop to OpenBoot prompt |
| S | Single-user with only root filesystem mounted (as read-only) |
| 1 | Single-user mode with all local filesystems mounted (read-write) |
| 2 | Multi-user with most daemons started. |
| 3 | multi-user, 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, user-defined |
| 5 | Shut down, power-off if hardware supports it |
| 6 | Reboot |
[edit] HP-UX
| ID | Description |
|---|---|
| 0 | System halted |
| S | Single-user, booted to system console only, with only root filesystem mounted (as read-only) |
| s | Single user, identical to S except the current terminal acts as the system console |
| 1 | Single-user with local filesystems mounted (read-write) |
| 2 | Multi-user with most daemons started and Common Desktop Environment launched |
| 3 | Multi-user, nearly identical to runlevel 2 with NFS exported |
| 4 | Multi-user with VUE started instead of CDE |
| 5, 6 | user-defined |
[edit] AIX[8]
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 multiuser 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 multiuser mode | default mode |
On AIX you can check the current runlevel with the following command:
$ who -r
[edit] See also
[edit] External links
- Runlevel Definition - by The Linux Information Project (LINFO)
