Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Scaricare il pdf (Pagina 73)

Chapter 3. Boot Process, Init, and Shutdown 73

• Scripts used to bring up and down ISDN interfaces, such as ifup-isdn and ifdown-isdn

• Various shared network function scripts which should not be edited directly.

For more information on the network-scripts directory, see Chapter 13

• rhn — This directory contains the conﬁguration ﬁles for the Red Hat Network Registration

Client, the Red Hat Update Agent Conﬁguration Tool, the Red Hat Update Agent, and the Red

Hat Update Agent panel applet as well as the systemid and GPG keys. No ﬁles in this directory

should be edited by hand. For more information on the Red Hat Network, see the Red Hat Network

website at the following URL: https://rhn.redhat.com/.

3.4. Init Runlevels

The idea behind operating different services at different runlevels revolves around the fact that dif-

ferent systems can be used in a different ways. Some services cannot be used until the system is in a

particular state, or mode, such as ready for more than one user or has networking available.

There are times in which you may want to operate the system at a lower mode, such as ﬁxing disk

corruption problems in runlevel 1, when no other users can possibly be on the system or leaving a

server in runlevel 3 without an X session running. In these cases, running services that depend upon

a higher system mode to function does not make sense because they will not work correctly anyway.

By already having each service assigned to start when its particular runlevel is reached, you ensure

an orderly start up process and can quickly change the mode of the machine without worrying about

which services to manually start or stop.

Generally, Red Hat Linux operates in runlevel 3 or runlevel 5 — both full multi-user modes. The

following runlevels are deﬁned in Red Hat Linux:

• 0 — Halt

• 1 — Single-user mode

• 2 — Not used (user-deﬁnable)

• 3 — Full multi-user mode

• 4 — Not used (user-deﬁnable)

• 5 — Full multi-user mode (with an X-based login screen)

• 6 — Reboot

The default runlevel for a system to boot into and stop is conﬁgured in /etc/inittab. For more

information on /etc/inittab, see Section 3.2.3.

Feel free to conﬁgure runlevels 2 and 4 as you see ﬁt. Many users conﬁgure those runlevels in a way

that makes the most sense for them while leaving the standard runlevels 3 and 5 alone. This allows

them to quickly move in and out of their custom conﬁguration without disturbing the normal set of

features at the standard runlevels.

If your machine gets into a state where it will not boot due to a bad /etc/inittab or will not let you

boot into single-user mode.

If you are using LILO, you can enter single-user mode by typing linux single at the LILO boot:

prompt.

If you are using GRUB as your boot loader, you can enter single-user mode using the following steps.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 73