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What does changing run-levels in RedHat require?

An administrator might change the run-level from graphical to multi-user if there will be no monitor attached to the Linux machine. Known as a "headless" machine, Linux computers without a monitor have no need for a graphical system since the monitor consumes resources.

What is a run-level?

A run-level is a system state that is defined by the services listed in /etc/rc.d.
Typically, advanced administration of a machine is done by switching run-levels (or state) to perform tasks such as minimal resource usage, run-level 3, shutdown run-level 0, or maintenance. Maintenance is
  1. run-level or
  2. single user mode, as it is commonly known.

Unix/Linux run-level comparison chart

If you happen to work with AIX, Solaris, HP-UX, Ubuntu, Fedora, Free BSD and/or Red Hat Enterprise Linux, then it can get a bit confusing figuring out which run-level does what. Please refer to the Wikipedia reference at the bottom of this article to get more information on cross platform run-level comparisons.

System Start−up and Shutdown

During system start−up, a series of scripts are run to start the services that you need. These include scripts to start network interfaces, mount directories, and monitor your system. Most of these scripts are run from subdirectories of /etc/rc.d. The program that starts most of these services up when you boot and stops them when you shut down is the /etc/rc.d/rc script. The following sections describe run−level scripts and what you can do with them.

Starting run−level scripts

As previously mentioned, the /etc/rc.d/rc script is a script that is integral to the concept of run levels. Any change of run level causes the script to be executed, with the new run level as an argument.
Linux runs in three different states: 1, 3, and 5 (referred to as run levels). Each run level is used to perform different functions. At Level 1 (also called single-user mode), Linux operates with a minimum of processes so that you can make configuration changes and debug problems.

Three Different States

Level 3 is essentially the same as Level 5 except that Level 3 does not run the X server, it is nongraphical. You typically run servers that do not need to run graphical applications, such as word processors, at Level 3. Level 5 is the default for personal workstations. You can use GRUB (Grand Unified Bootloader) to select a different Linux run level. When GRUB appears, press e for edit. Three lines appear. Press the down-arrow key to select the line that begins with kernel. Press the e key again, append either 1, 3, or 5, at the end of the line and press the Enter key. Next, press the b key and your computer boots into the specified state.