The idea behind SysV init runlevels revolves around the idea that different systems can be used in different ways. For example, a server runs more efficiently without the drag on system resources created by the X Window System. Or there may be times when a system administrator may need to operate the system at a lower runlevel to perform diagnostic tasks, like fixing disk corruption in runlevel 1.
The characteristics of a given runlevel determine which services are halted and started by init. For instance, runlevel 1 (single user mode) halts any network services, while runlevel 3 starts these services. By assigning specific services to be halted or started on a given runlevel, init can quickly change the mode of the machine without the user manually stopping and starting services.
The following runlevels are defined by default under Red Hat Enterprise Linux:
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0 — Halt
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1 — Single-user text mode
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2 — Not used (user-definable)
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3 — Full multi-user text mode
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4 — Not used (user-definable)
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5 — Full multi-user graphical mode (with an X-based login screen)
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6 — Reboot
In general, users operate Red Hat Enterprise Linux at runlevel 3 or runlevel 5 — both full multi-user modes. Users sometimes customize runlevels 2 and 4 to meet specific needs, since they are not used.
The default runlevel for the system is listed in /etc/inittab. To find out the default runlevel for a system, look for the line similar to the following near the top of /etc/inittab:
id:5:initdefault:
The default runlevel listed in this example is five, as the number after the first colon indicates. To change it, edit /etc/inittab as root.
Warning
Be very careful when editing /etc/inittab. Simple typos can cause the system to become unbootable. If this happens, either use a boot diskette, enter single-user mode, or enter rescue mode to boot the computer and repair the file.
For more information on single-user and rescue mode, refer to the chapter titled Basic System Recovery in the Red Hat Enterprise Linux Deployment Guide.
