A runlevel is one of the modes that a Unix -based operating system will run in. Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. Conventionally, seven runlevels exist, numbered from zero to six.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.
Run Level |
Mode |
Action |
0 |
Halt |
Shuts down system |
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 |
Undefined |
Not used/User-definable |
5 |
X11 |
As runlevel 3 + display manager(X) |
6 |
Reboot |
Reboots the system |
Init scripts
Init (short for initialization) is the program on Unix and Unix-like systems that spawns all other processes. It runs as a daemon and typically has PID 1.
The /etc/inittab file is used to set the default run level for the system. This is the runlevel that a system will start up on upon reboot. The applications that are started by init are located in the /etc/rc.d folder. Within this directory there is a separate folder for each run level, eg rc0.d, rc1.d, and so on.
chkconfig
The chkconfig tool is used in Red Hat based systems (like CentOS) to control what services are started at which runlevels. Running the command chkconfig –list will display a list of services whether they are enabled or disabled for each runlevel.
# chkconfig --list filelimits 0:off 1:off 2:on 3:on 4:on 5:on 6:off syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off gpm 0:off 1:off 2:on 3:on 4:on 5:on 6:off cpanel 0:off 1:off 2:off 3:on 4:on 5:on 6:off kudzu 0:off 1:off 2:off 3:on 4:on 5:on 6:off ntpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off netfs 0:off 1:off 2:off 3:on 4:on 5:on 6:off network 0:off 1:off 2:on 3:on 4:on 5:on 6:off netplugd 0:off 1:off 2:off 3:off 4:off 5:off 6:off rawdevices 0:off 1:off 2:off 3:on 4:on 5:on 6:off ipchains 0:off 1:off 2:on 3:on 4:on 5:on 6:off iptables 0:off 1:off 2:on 3:on 4:on 5:on 6:off crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off anacron 0:off 1:off 2:on 3:on 4:on 5:on 6:off cups 0:off 1:off 2:on 3:on 4:on 5:on 6:off xfs 0:off 1:off 2:on 3:on 4:on 5:on 6:off xinetd 0:off 1:off 2:off 3:on 4:on 5:on 6:off httpd 0:off 1:off 2:off 3:on 4:off 5:on 6:off
Single User mode is a mode that a multi-user system (like a Linux server) can be booted into the operating system as a superuser. Booting a system into this mode does not start networking, but can be used to make changes to any configuration files on the server. One of the most common usages for single-user mode is to change the root password for a server on which the current password is unknown.
Runlevels are an important part of the core of the Linux operating system. While not something the average administrator will work with on a daily basis, understanding runlevels gives the administrator another layer of control and flexibility over the servers they manage.
References
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