An introduction to services, runlevels, and rc.d scripts
Wednesday, January 11 2006 @ 02:59 PM EST Contributed by: sde
What's the first thing that you do once you've logged onto Linux? Is it to manually start up a processes such as Apache or MySQL, or even start your network connection? Or do you have to stop applications that have started up without your telling them to, and which are overloading your machine? If you have unwanted processes starting at boot time, or find yourself starting necessary services manually, let's make your life a little bit easier by introducing you the world of Linux services.
Let's start by looking at how the system is set up, and in particular at the directory /etc/rc.d. Here you will find either a set of files named rc.0, rc.1, rc.2, rc.3, rc.4, rc.5, and rc.6, or a set of directories named rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d, and rc6.d. You will also find a file named /etc/inittab. The system uses these files (and/or directories) to control the services to be started.
And what is a runlevel? You might assume that this refers to different levels that the system goes through during a boot up. Instead, think of the runlevel as the point at which the system is entered. Runlevel 1 is the most basic configuration (simple single user access using an text interface), while runlevel 5 is the most advanced (multi-user, networking, and a GUI front end). Runlevels 0 and 6 are used for halting and rebooting the system.
