09-01-2006
A runlevel is a software configuration of the system which allows only a selected group of processes to exist. The processes spawned by init process for each of these runlevels are defined in the /etc/inittab file. Runlevels 0, 1, and 6 are reserved. Other runlevels are dependent on how your particular distribution has defined them, and they vary significantly between distributions.
Most users run X from runlevels 3 or 5 (for linux?). Runlevel 3 places your system in multi-user mode with full networking capabilities. The machine will boot to a text-based login prompt with all necessary preconfigured services started. Most servers are run in runlevel 3, as X is not necessary to provide any services utilized by most users. Runlevel 5 is similar to 3, except that it automatically starts X and provides a graphical login screen. Many workstation users prefer this method, because it never forces them to see a command prompt.
You can add commands that must be executed in appropriate runlevel in some file/under some directory (e.g. /etc/rc.d/rc5.d)
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RUNLEVEL(8) runlevel RUNLEVEL(8)
NAME
runlevel - Print previous and current SysV runlevel
SYNOPSIS
runlevel [options...]
OVERVIEW
"Runlevels" are an obsolete way to start and stop groups of services used in SysV init. systemd provides a compatibility layer that maps
runlevels to targets, and associated binaries like runlevel. Nevertheless, only one runlevel can be "active" at a given time, while systemd
can activate multiple targets concurrently, so the mapping to runlevels is confusing and only approximate. Runlevels should not be used in
new code, and are mostly useful as a shorthand way to refer the matching systemd targets in kernel boot parameters.
Table 1. Mapping between runlevels and systemd targets
+---------+-------------------+
|Runlevel | Target |
+---------+-------------------+
|0 | poweroff.target |
+---------+-------------------+
|1 | rescue.target |
+---------+-------------------+
|2, 3, 4 | multi-user.target |
+---------+-------------------+
|5 | graphical.target |
+---------+-------------------+
|6 | reboot.target |
+---------+-------------------+
DESCRIPTION
runlevel prints the previous and current SysV runlevel if they are known.
The two runlevel characters are separated by a single space character. If a runlevel cannot be determined, N is printed instead. If neither
can be determined, the word "unknown" is printed.
Unless overridden in the environment, this will check the utmp database for recent runlevel changes.
OPTIONS
The following option is understood:
--help
Print a short help text and exit.
EXIT STATUS
If one or both runlevels could be determined, 0 is returned, a non-zero failure code otherwise.
ENVIRONMENT
$RUNLEVEL
If $RUNLEVEL is set, runlevel will print this value as current runlevel and ignore utmp.
$PREVLEVEL
If $PREVLEVEL is set, runlevel will print this value as previous runlevel and ignore utmp.
FILES
/run/utmp
The utmp database runlevel reads the previous and current runlevel from.
SEE ALSO
systemd(1), systemd.target(5), systemctl(1)
systemd 237 RUNLEVEL(8)